DISCLAIMER Throughout this publication, reference is made to a number of pesticides (herbicides, fungicides and insecticides) which have been used either commercially or experimentally in poplar production. It should be emphasised that with any pesticide application the relevant legislation (Health and Safety at Work Act 1974; Poisonous Substances in Agriculture Regulations 1984; Food and Environment Protection Act 1985; Control of Pesticides Regulations 1986; Control of Pollution Act 1974; Plant Protection Products (Sustainable Use) Regulations 2012 etc.) and the Product Label Recommendations must be adhered to.

1. INTRODUCTION

1.1. POPLAR (Populus spp.)

Poplar, of the genus Populus, comprises around 35 species of trees. Poplars belong to the Salicaceae family along with the poplars. The Latin name Populus, refers to the fact the Romans tended to plant poplar trees around their most popular public meeting places. Poplars are some of the most productive tree species, along with poplar, which are native to the temperate climates of the northern hemisphere.

Poplars are fast growing, relatively short-lived trees. They tend to live around 50-70 years; however, black poplar (Populus nigra) can live up to 200 years. They are widely distributed throughout the northern temperate regions, ranging from Europe, to Asia, through to North America and even northern Africa. Poplar are riparian species and can grow well in nutrient depleted soil. Poplar leaves alternate on the way up the stem and are oval to heart-shaped, with serrated edges. Poplars are dioecious plants, meaning there are male and female trees. They display flowers in drooping catkins before the leaves emerge in spring, utilising wind pollination. The fruit capsules contain many tiny seeds dressed in fluffy seed hairs. The seeds fall in great quantities in late summer, utilising their cottony seed hairs for wind dispersal. Due to the sexual dimorphism of poplar trees, they have shown great potential for hybridisation and thus plant breeding programmes.

Poplar hybrids have previously been favoured in the UK for their rapid and vigorous growth by the match stick making industry, nowadays, their fast-growing nature is utilised to make cardboard boxes, crates, paper, and veneer. What is more, poplars display very vigorous shoots emission, and thus easy vegetative multiplication. Hence, more recently poplar has been grown for biomass production under the woodland creation grant schemes.

Poplar is grown in a cultivar of formats, including short rotation coppice (SRC), short rotation forestry (SRF), to conventional single stem forestry along with windbreaks in most parts of the UK. SRF consists of planting a site and then felling the trees when they have reached a size of typically 10-20 cm diameter at breast height. SRC employs coppicing, cutting the trees back to a stool to promote the vigorous growth of multiple stems, on a regular cycle of 2-5 years. Both SRC and SRF utilise the high productivity of a young plantation.

Poplar’s ability to be grown on marginal land, easy hybridisation, soil regenerative properties, along with its desirable growth traits: fast growth, straight stems, high cellulose content and coppicing susceptibility, make it an attractive crop for biomass production.

2. PRE-PLANTING

2.1. SITE SELECTION

Poplar will grow well in most soil types. However, site selection depends on what the grower is trying to achieve: timber production, windbreak, preventing soil erosion, livestock shelter, or phytoremediation of contaminated soil. Poplar has a wide range of uses and therefore selection of the correct site and variety is essential.

2.1.1. SUITABLE SOILS

Poplar will grow in a range of soil types with some cultivars tolerating different pH and soil types better than others. Poplar will thrive in slightly acidic and alkali soils; however, optimum growth is observed between pH 6.0-8.0. Poplar will tolerate a wide range of soil textures provided there is good drainage and no pooling of water. Light clay, sand, loam, and humus soils are all preferable for poplar. Heavy soils and those with inadequate drainage will not provide the ideal oxygenation at the root zone for poplar to thrive. However, poplars are highly adaptive. Base-rich loamy soil with a water table 1-1.5m below the surface in summer is ideal for poplar growth (Jobling, 1990). Acid sphagnum peats are unsuitable and light sands and gravel heaths can prove too dry and acidic.

With a pH below 5.0 a reduction in the vigour of black poplar and its hybrids is observed. Whereas balsam and aspen poplars fare somewhat better. On the contrary, balsam poplars, in particular, show reduced vigour in alkaline soils and are susceptible to chlorosis (Jobling, 1990).

It’s always a good idea to carry out soil analysis to find out the soil type, structure, and depth. If you know the site and if it has been cropped with cereals in the past, then any poor areas will show up the same as the cereal crops. Depth of soil is important for stability and retention of moisture and all these factors will affect the growth rate and productivity of the site. Ideally, if planting short rotation coppice or forestry, the more cropping history you have of a site will help with the selection of cultivars and allow you to improve the site prior to planting if required.

2.1.2. WATER AVAILABILITY

Like any other SRC or SRF crop, water availability is a major determinant of productivity and cellulose production. Poplar can be planted on very wet ground which will result in the area becoming drier. Additionally, poplar roots display a strong hydrotropic nature, hunting for water, thus, drains and any existing land drains will be affected by root growth. Poplar roots grow very fast, reaching as deep as 2.5 m in 2 years.

Poplars prefer easy draining soils that are consistently moist. However, waterlogged soils are unsuitable as they do not like to remain saturated for extended periods. A perfect site would have well drained soils to a good depth 1.5-2.0 m then gravel with moving water underneath. Optimum water availability leads to maximise growth rates. Other options are to flood irrigate or, at planting laydown trickle tape to supply water directly to the tree roots.

While poplar trees require access to a constant supply of water, they do not tolerate waterlogged soils as the roots require adequate oxygenation. Poplar should not be considered in areas where the water table never falls below 50-60 cm, or in stagnant areas. Areas with high reed growth are indicative of high water-content and may not be suitable for poplar cultivation (Jobling, 1990).

Surprisingly, poplar will grow on sand and planting on sand dunes to help prevent erosion has been done in the UK. However, growth rates are greatly reduced. Yet, this goes to show the highly adaptive nature of the tree’s water usage.

2.1.3. TEMPERATURE

Poplar will grow best in warm conditions with long days and plenty of water. In the UK poplar grow well, but with our reduced sunlight we do have up to a 20% reduction in growth rates compared to France or Spain. Poplar will grow towards the light and so spacing is very important to maximise growth rates and timber production per hectare. Poplar in the UK grows well and with our temperate maritime climate the risk of extreme temperature issues is reduced.

Poplars are hardy, frost-tolerant trees. However, cold weather reduces growth rates and should be taken into consideration when choosing a site. Within the UK poplar can be found within all nations including Scottish highland areas. Poplar can display die back when frosted but will regrow. Hence, frost pockets should be avoided if possible or frost tolerant cultivars should be considered. It is imperative to consider the selection of cultivars, along with the design and factors of the site before planting starts. The trees are most vulnerable to the cold when new growth is occurring in the spring with later snap frosts posing a serious threat. Cultivars that are later flushing are preferable in exposed or colder sites.

2.1.4. SITE ACCESS

Site access depends on the end use of the woody biomass product. Single stem forestry and SRF sites must allow for the access of large harvesting machinery and extraction equipment. Slopes exceeding 13% will provide difficulty for harvesting machinery, particularly in wet conditions, and should be avoided. It should be noted, harvesters for single stem trees can cope with wet and sidling ground and the timber can be removed for chipping at the biomass plant. Transporting cordwood timber is more efficient over long distances than chipped material. SRC will require an adapted forage harvester that are very similar to traditional agricultural harvesters on a more regular basis than SRF and forestry poplar, thus, hard tracks to the site are advised. If the site is flat and easily accessed, then SRC would be better suited, allowing conventional harvesting and chipping into biomass. Whereas, in less easy to access areas, SRF poplar or single stem forestry would be advised.

2.1.5. SUITABLE LAND AREA

Understanding your market is imperative before deciding on which type of plantation to start. In addition, the quality of the chip / ash content / moisture content and volume of biomass to be stored are all factors that need to be considered. This then affects the spacing and crop rotation 2, 4, 8 or, 20-year crop cycle.

It should be noted that if it is desired to plant poplar for use in a single 5 kW household wood stove, then only around 0.18 ha would be required. However, on a commercial scale the size of a site will be subject to the economies of scale and feasibility of the project. Additionally, consideration should also go into the other resources available on site or within the estate. Are the biomass plantations just supplementing other crops being grown and what is the long-term plan for the business.

2.1.6. LOCATION IN THE LANDSCAPE

Poplar trees planted for forestry can grow up to 30-50 m tall with a dense foliage cover, thus creating a novel 3D structure which can alter the very feel and nature of a landscape. SRF and SRC plantations will not grow as tall, however, 4 year old trees can be up to 8m tall also obscuring views and creating a new environment.

• SRC poplar at the end of a 3-4 year growing cycle can be up to 8m tall and SRF poplar can reach up to 20 m in 8 years, and therefore create a 3D mass in the landscape which arable crops do not. Poorly planned plantations have the potential to adversely affect the rural landscape. However, well-designed, and carefully sited plantations could bring small but important landscape improvements.
• Siting in the landscape may well be constrained by existing enclosure patterns. Where these are well developed with hedgerows and trees, the problem is limited because sight lines are short. In addition to the deciduous nature of the crop, diversity is created with cultivar mixtures and harvesting patterns.
• Because of the small-scale production and use patterns of SRC and SRF poplar, it is unlikely to be a dominant landscape feature in any particular area.
• If enclosure patterns are weak, sight lines long, or topography flat, plantings should provide interlocking blocks with organic rather than geometric shapes. Additionally, in a large landscape, SRC and SRF plantings should be in scale and link up, if possible, with existing woodland to give visual and environmental benefits.
• If the extent of planting in any landscape is greater than the field pattern, it should conform to the overlying landform rather than be in larger rectangular blocks.
• SRC and SRF development is likely to be sited in landscapes which are already in agricultural use and should not impinge on landscapes of special interest or scarcity.
• SRC and SRF should not be planted on or adjacent to sites of historical importance or where it would obscure natural landscape features. Power lines will require consultation with the utility company involved, remembering that mature coppice can reach up to 8m before harvest and SRF plantations even taller.
• Sites with specific designations such as Areas of ‘Outstanding Natural Beauty’ or Areas of ‘Special Scientific Interest’ will also require consultation with the regulatory bodies concerned.
• Overhead power lines may pose a risk to farmers, contractors, or anyone in the vicinity of machinery involved in site preparation, planting, or husbandry of poplar coppice. The Health and Safety Authority has published a booklet, ‘Guidelines for safe working near overhead electricity lines in Agriculture - Health and Safety Authority’ to ensure safety when a farming activity is carried out in the vicinity of overhead power lines.
• SRC poplar in the 3-4 years prior to harvesting will reach a height that is very close to overhead power line heights and SRF plantations can reach even higher prior to its first harvest. This can (1) create a serious electrical risk to anyone in the vicinity of harvesting machinery, (2) interfere with the safe operation of the power line. Contact the utility company before planting for advice if planting directly under an overhead power line is planned.

2.2. SITE PREPARATION

Once the site has been assessed, soil sampling undertaken and site layout has been determined, site preparation can begin. This should be a meticulous process as the lifespan of a plantation will be upwards of 20 years. The greater amount of care taken in site preparation can reduce costs associated with establishment and maintenance of the site later on in its life.

Ground works required for the infrastructure used throughout the life of the project should be done at this time. Clearing ditches, site access improvement, any boundary fencing, or waterway protection needed along with minimising any risk from obstacles or ground unsuitable for planting should all be done before plants go into the ground.

2.2.1. PRE-PLANTING PEST CONTROL

For SRC all ground vegetation needs to be removed prior to planting. This can be done by mechanical means or by spraying before the seedbed is completed. For SRC poplar it is important that this phase is carried out effectively, particularly on old pasture land where the presence of perennial weeds such as docks and nettles are more likely. Herbicide application is required when weeds are still actively growing using a translocated (systemic) herbicide (glyphosate 360-400 g/l product at 4-5 l/ha) in early autumn, just before autumn ploughing (if locally permitted). If docks, clover, or other legumes have a high weed presence then fluroxypyr and triclopyr can be used (e.g. Doxstar Pro - both ingredients 100 g/L at 3 l/ha). If necessary, the translocated herbicide can be re-applied just prior to ploughing and cultivating in spring. For stubble/arable, cover crop and pasture ground a translocated (systemic) herbicide (e.g. glyphosate - 360 g/l product at 4-5 l/ha) should be applied in early spring followed by ploughing.

Where sites have excessively heavy vegetation present, consideration should be given to cutting and removal of the vegetation to allow for effective weed control. If this is necessary, sufficient time should be allowed for re-growth to allow for active herbicide uptake.
SRF is a less intensive system with less plants going in usually as taller rods able to outcompete weeds. Although still requiring weed management, SRF requires less input than SRC. Thus, initial weed control should take place across the whole site (glyphosate 360 g/l product at 4-5 l/ha). Alternatively, if there is little weed pressure spot spraying or band spraying could be advised. Spot spraying is where the individual planting locations are sprayed and band spraying is spraying where the row of stems will be planted. These methods can be cheaper and have less of an environmental impact. Where there is excessively heavy vegetation present, for instance sites with bracken, time should be allowed for effective control. It is critical to work with the best local advice to manage vegetation control.

2.2.2. SOIL TESTING AND MANAGEMENT

Soil testing should be undertaken before planting. The pH and Soil Nitrogen Supply (SNS), phosphorus §, potassium (K) and magnesium (Mg) indices should be screened for and can be adjusted during the site preparation stage if needed. There are multiple soil testing companies which can offer services with in-depth soil analysis and reporting. Even field level analyses can be provided to help carry out targeted nutrient application.

Based upon the results of the soil tests, nutrient or lime application, if necessary, can be carried out to adjust the levels to the optimal conditions for poplar. It should be noted, unnecessary fertiliser application may only encourage weed competition during establishment.

Additionally, soil microbial biomass and community profiling can be employed. Mycorrhizal fungi are imperative to the successful establishment and pest control of poplar and analysis can give an idea as to whether supplements may be required for when planting commences.
What is more, baseline soil analysis can be used for identifying soil carbon stores.

https://ahdb.org.uk/knowledge-library/soil-and-forage-testing-companies

2.2.3. PLOUGHING AND SUB-SOILING

For SRF, shallow ploughing is the desirable way forward when creating new woodland as it is typically cheaper, faster and provides a greater weed suppression. However, this is only applicable when creating new woodland because root structures and stumps from previous trees will provide too much of a challenge for ploughing. For example, when transitioning from arable or grassland to woodland. If planting where ploughing is not feasible, the land should ideally be mounded to produce a clear planting area. Sub-soiling or ripping at a minimum depth of 25 cm will significantly improve establishment, increasing the air volume within the soil working against compaction, or settling, to allow for the white roots to spread laterally and anchor with initial taproot development. It is important for lateral roots to develop freely to provide structural support to the tree. The root systems tend to be high in developing mycorrhizal fungal activity, which specifically helps establishment and initial growth of the young trees. Generally, there is ample available fungi within a replanted woodland context, but as with all tree establishment, added mycorrhizal content is important with new tree planting in fields and arable sites.

For SRC, a minimum of ten days after herbicide application is required before the site can be ploughed. A minimum plough depth of 20-25 cm will be required to allow for insertion of the cuttings.

On free working soils, the site can be ploughed, and power harrowed, and a stale seedbed prepared 6 weeks before planting. The germinated weeds can then be sprayed off prior to planting using glyphosate (360 g/l product at 2.0 l/ha). On heavy clay soils, this approach is not practical, and the site should be power harrowed as close to planting as possible. It may be necessary to remove stones after power harrowing to avoid interference with mechanical planters. Additionally, spraying can be avoided, and mechanical weed removal can be carried out on heavy clay soils. Although potentially not as effective as chemical removal, poplars have large leaves and can reach canopy closure after 2-3 years which will allow plantations to outcompete weeds once established.

2.2.4. EXCLUDING VERTEBRATE HERBIVORES

Rabbits, hares, and deer, where they are present in sufficient numbers, can be very destructive in new and establishing coppice plantations. These herbivores should be excluded, if necessary, with appropriate fencing. This is an expensive operation and where necessary, will represent the single largest cost in site preparation.

For SRC, fencing may be temporary in nature as an established coppice is less susceptible to economic damage. Netting is generally used with the lower portion buried or turned horizontally to deter rabbits from burrowing underneath. Machinery is now available to plough in wire netting, and this substantially reduces the cost. Electric mesh fencing has proved satisfactory, but it must be kept weed free to prevent shorting out. The cost of fencing to exclude deer is prohibitively expensive, hence, alternative methods can be utilised. Planting of deer deterrent energy grass species such as Arundo, miscanthus, or switch grass, bordering SRC poplar could reduce browsing from deer. Additionally, Garlic Barrier (1-2 l/ha) can be applied when the crops have freshly emerged on the whole plantation. If grazing pressure is more severe, Grazers (250-500 ml/ha) could be applied to deter rabbits.

For SRF, biodegradable tree guards can be applied on individual stems along with fencing around the site. Additionally, hybrid cultivars can play a role in reducing grazing. It should be taken into consideration that fencing will be one of the largest costs and is only necessary during the establishment phase. Genotypes with a rough bark and low branch biomass appear to be less affected by deer grazing. Additionally, where applicable, stalkers can be employed to control numbers of vertebrate grazers on site.

2.3. CULTIVAR SELECTION

Poplars are fast growing and present a great opportunity for increasing performance through breeding programmes. Variety improvement programmes are always ongoing and generally revolve around hybrid breeding exploiting heterosis. Heterosis refers to the phenomenon that progeny of crosses between species or diverse varieties of a species, perform better than both parents of the cross (Birchler et al., 2010).

The most successful hybrids in Europe have been derived through crossing natural varieties and based on seed collection from the natural population. Followed by selecting the best genotypes for each different population, intraspecific breeding and controlled cross breeding. Then selection of high performing hybrids followed by testing them in experimental plantations. Genetic material is collected from the temperate regions of the northern hemisphere from the USA and Canada through to Europe and across to China. The poplar trees have very different characteristics and through breeding, timber quality and growth rates can be greatly enhanced.

2.3.1. CULTIVARS

2.3.1.1. BELGIAN CULTIVARS

The varieties below have been planted extensively in the UK.

• Gaver (Populus deltoides x Populus nigra)
  Diameter at breast height (DBH) of 6-8 cm at 5 years.
  Branchy cultivar with small leaves that is tolerant to high altitudes and exposure.
  Moderately susceptible to rust.
• Gibecq (Populus deltoides x Populus nigra)
  DBH of 6-7 cm at 5 years.
  Branches are upright.
  Tolerant to high altitudes and exposure.
  Moderately susceptible to rust.
  The timber quality of Gaver and Gibecq is high. The growth is not quite as rapid as other cultivars but is still impressive. They are sometimes planted on the edge of the plantation to provide some shelter to faster growing but less wind tolerant cultivars.
• Muur (Populus deltoides x Populus nigra)
  Very fast growing with high yields; suitable to most soils, but better in well drained soils.
  Suitable for 5 year rotations.
  Good resistance to most diseases.
  Drought tolerant but somewhat more susceptible to frost.
• Vesten (Populus deltoides x Populus nigra)
  DBH of 6-7 cm at 5 years.
  Suitable to most soils, but better in rich and medium drained soils.
  Suitable for 2-3 year rotations.
  Good resistance to most diseases.
• Oudenburg (Populus deltoides x Populus nigra)
  Very fast growing with high yields.
  DBH 8 cm at 5 years.
  suitable to most soils, but better in well and medium drained soils.
  Suitable for 2-3 year rotations.
  Good resistance to most diseases.
• Griminge
  Outstanding, vigorous cultivar with large leaves.
  DBH of 8-10 cm at 5 years.
  Grows well on marginal land and even sand dunes.
  Suitable as a fodder crop. Resistant to rust.
  More prone to snap in high winds.
  AF18 poplar cultivar in 3rd year- (never harvested)
  Some other Belgian varieties are also available. These have been widely planted in Germany but there is only one trial in the UK, in Buckinghamshire.

2.3.1.2. ITALIAN CULTIVARS

The varieties below have been planted extensively in the UK.

• A4A (Populus x canadensis)
  Outstanding, vigorous cultivar with large leaves.
  DBH of 8-10 at 5 years.
  The high degree of branching makes it slightly more difficult to harvest.
  Good as a fodder crop and as a nursery crop for poultry.
  Drought tolerant but moderately susceptible to rust.
  More prone to snap in high winds
• AF2 & AF7
  Siblings.
  Both are adaptable cultivars that can tolerate exposure, high altitudes, and drought.
  DBH of 8 cm at 5 years.
  Resistant to rust.
• AF2 is more suitable for biomass production.
• AF8 (Populus x generosa)
  This cultivar can be managed as a coppice with 7-8 year rotation.
  DBH of 9-10 cm at 5 years.
  Can tolerate some exposure, high altitudes and drought.
  More prone to snap in high winds.
  Moderately susceptible to rust.
• AF13, AF16 and AF18 (all Populus x canadensis)

2.3.2. LIST OF SUPPLIERS

The following companies are actively involved in selling poplar material:

Poplar Tree Company (Herefordshire, UK)
Supplies rods and cuttings of Belgian and Italian varieties.
www.poplartree.co.uk

Crops for Energy (Bristol, UK)
based in Bristol. Is a partner of PTC and provides an easier way for web surfers to get access to the same varieties as the C4E website has far superior SEO rankings.
www.crops4energy.co.uk

Energy Crops Consultancy
based in Harrogate provide poplar varieties as part of an integrated planting and management service.
www.energycropsconsultancy.com

Eadha Enterprises
sell native aspen trees (Populus tremula) as cell grown plants. These are particularly well suited for SRF planting in Scotland.
www.eadha.co.uk/what_we_do/specialist_tree_nursery

Agraligna
based in Northern Germany. Supplies the Max varieties that are grown extensively across Germany.
www.agraligna.com

Alasia Green Forest
www.alasiagreenforest.it

Biopoplar
supply Italian varieties
www.biopoplar.com

2.4. PLANTING MATERIAL

On well cultivated land smaller poplar material can be used, for example, 25cm cuttings or 40 cm setts can be used to enable better establishment. Again, every site will present different challenges, however, there are many options for cropping density and in the case of poplar cultivar choice, growth rates and end market use can help in the deciding which planting material will give the best results.

2.4.1. PLANTING ROD & QUALITY

Similarly to SRC willow, dormant hardwood cuttings are the planting stock of choice for establishing poplar plantations. Vegetative propagation material is derived from vigorous stool beds which provide annual growth and the production of one-year-old shoots. These shoots are harvested to provide 1.5-2 m rods. The rods can either be sectioned into cuttings, which can root when planted in the field with no further treatment. Or provided as ~2 m rods that can be inserted into an automatic step planter which cuts the rods into 20-25 cm cuttings and plants into the soil, all in one motion. Depending on your intended use and access to planting machinery you can select your desired cutting length, and usually these can be provided for by the suppliers.

Standard planting material used:

• Single stem 20 year plus plantation
  • 1.5 m rod
• 5 year rotation
  • 1.5 m rod
• 2 year rotation
  • 25 cm cutting/40 cm sett
• 2 year rotation (high intensity)
  • 25 cm cutting

An effective system utilised to store dormant 1.5-2m rods and cuttings, is through cold storage at 0-4°C and wrapping in plastic, this keeps the rods or cuttings dormant and stops them from drying out. Rods should be kept cold stored until the day of planting. Establishment is dependent on planting material quality. Thus, it is imperative to identify rod, sett, and cutting quality before planting.
Planting rods should be:

• Harvested during dormancy
• Vigorous to ensure adequate carbohydrate reserves to sustain the cutting before establishment.
• Sufficiently mature (lignified) to prevent deformation on insertion into the prepared ground.
• Not show any discoloration or wrinkling of the surface, indicating dehydration.

2.4.2. PLANTING MIXTURES

Planting mixtures of varieties and cultivars is imperative for the establishment of poplar due to the direct correlation between genotypic diversity and disease susceptibility. The less diverse a field is, the more susceptible to rusts and other biotic diseases a plantation will be. Although this is not currently practised for poplar it would be advised due to biotic stresses representing a major threat to plantations. Additionally, with the potential increase in biomass crop land-use, a knock-on effect will be that disease pressure will also increase. To further this, climatic change is also expanding the latitudinal ranges of pathogens.
Successful plantations have been established, grown to maturity and harvested in the UK by the Poplar Tree Company using the following cultivar mixing methods:

Single stem plantations and Short Rotation Forestry:

• In smaller sites (≤100 plants) 5m x 5m spacing, all single cultivar.
• In larger sites a mix of cultivars is recommended to reduce susceptibility to rust and other diseases/pests. Trees planted in blocks of ~100 with each neighbouring block containing a different cultivar.

Short Rotation Coppice:

• Random mixtures of poplar varieties are recommended to reduce susceptibility to disease and pests. A mixture of willow with poplar is another option to increase resilience to pests while providing improved biodiversity.
• Not only do mixed plantations help mitigate the damage caused by diseases, but they can also enhance the production and thus yields of a plantation. However, there is very little literature suggesting this in poplar. What is more, the environmental and ecological benefits are vast in comparison to monoclonal plantations. Thus, it’s imperative to increase genotypic diversity on plantations to future proof the cropping system over its long lifespan.

2.4.3. PLANT BREEDER’S RIGHTS

All approved registered commercial varieties are protected by EU and/or UK Plant Breeders’ Rights. Plant breeders’ rights are rights reserved for breeders of new varieties of plants, giving the breeder exclusive control over the propagating and harvested material of new plant varieties in accordance with EC Regulation No. 2100/94. For the case of poplar, if the breeder has registered the plant, they have rights over the propagating rods/setts/cuttings and are thus usually the only suppliers of the material, unless there is some form of legally binding agreement between the breeder and propagator. Hence, it is illegal to self-use or sell propagation material harvested from crops you have grown. However, there is a minor derogation, which allows the gapping up of establishing crops with the material produced at cutback. Generally, therefore, cuttings will be produced by specialist growers in nursery beds and supplied as 1 year old rods for mechanical planting. It should also be noted, acts done by third parties for non-commercial, private, or experimental purposes or for the purpose of breeding a new variety are not prohibited by PBRs.

2.4.4. PLANTING MACHINERY

Depending on layout and size of the site, planting may be carried out by hand or by using machinery. Similar to willow, step planters can be used to plant for SRC at high density while rod planters can be used to plant at lower densities for SRF and single stem plantations.
Lignovis GmbH (https://www.lignotech.de/) produces planting equipment specifically for Poplar and Willow. For high density planting (>10,000 stands/ha) the ‘LignoPlanter-C’ can be used to plant cuttings of length 20-30 cm. Lower densities (2,500 to 5,000 stands/ha) can also be achieved. Planting of up to 10 ha/day can be achieved depending on ground conditions and site shape.

The LignoPanter-R by Lignovis GmbH is used to plant 1-2 m poplar rods at a density of 1,500-3,000 stands/ha (suitable for SRF and single stem). Depending on soil conditions and rod length, a depth of 50-70 cm can be chosen.

Lignovis also produces a step planter that takes poplar rods which are cut to size (15-25 cm) before insertion into the soil. Densities of 10,000-14,000 stands/ha can be achieved which is suitable for SRC.

In addition to equipment made specifically for poplar, equipment designed for willow planting can also be used. The Salix Maskiner Step Planter has become the industry standard for establishing willow coppice. This step planter is also fed with rods which are cut to size before insertion into the ground. Planting by hand can be carried out in areas where access is restricted or ground conditions do not allow for heavy machinery.

3. PLANTING

Planting, along with site preparation, are critical to the long-term goals of a plantation and play a key role in the long-term profitability of the site. Poplar stands, SRF and SRC, can last for over 30 years, thus, understanding the site and end use is very important. Therefore, planting should only commence after the careful planning and preparation of the site.
It may be necessary to soak your cuttings, setts, or rods for 24 hours prior to planting (this should be confirmed with your supplier). Planting is then carried out by machine or hand, depending on the size of the rods to be planted, size of the site and how much site preparation can be done. For example, for sites that are difficult to plough and spray, longer rods will be used and the end goal will be SRF not SRC.

3.1. TIMING

For SRC poplar, planting can begin after the last spring frosts which are dependent on area, climate, and latitude. Depending on your intended end use of the crop, planting can be delayed to allow for the safety of avoiding any snap frosts, however if the land is likely to dry out in late spring into summer there is a risk of high losses. Maximising the length of the growing season can increase yields for the first year of production, however, frost can damage young plants and cause stool deaths.

Planting of single stem poplar and SRF can commence from November through to April as the larger setts and rods used are more tolerant to frost. However, there is a risk of early spring frosts destroying new growth and causing stool death.

If planting for short rotations, then you want to maximise the length of the first growing season. However, if you are planning on a longer rotation, then the first year’s growth is not as important and can be planted later in the season making sure to avoid frosts and maximise establishment.

3.2. PLANTING DENSITY

On well cultivated land smaller poplar material can be used, for example, 25cm cuttings. On trickier sites where weed presence is high and cultivation is not possible 1.5-2m rods can be used to facilitate better establishment. Again, every site will present varying challenges but there are many options for planting densities. In the case of poplar cultivar choice, growth rates, and end market use can aid in the decision for which cropping system will give the best return.

Cropping system and spacing to be used:

• Single stem 20+ year plantation
  • 5 x 5 m
• 5 year rotation
  • 3 x 3/2 m (clone dependent)
• 2 year rotation
  • 3 m x 60/50 cm clone selected
• 2 year rotation (high intensity)
  • 1.5 m x 40 cm

Rotation / cropping intensity will depend on the desired end market and the quality of the wood required for its end use. The longer the crop cycle planned, the greater the spacing between plants.

20-year timber can be used for high quality construction materials, veneers, and other desirable wholesale woody products, however, will also produce a better chip with less bark content, more evenly sized, and greater overall quality. Shorter rotations will produce lower quality wood with higher bark content, this would mainly be used for chipping for combustion to produce renewable energy, wood for pulp and paper, biofuels and biochemicals.

3.3. PLANTING DEPTH

Planting depth is essential for successful establishment. Standard planting depths practised are:

• For 25 cm cuttings, insert the cutting around 22-23 cm into the soil with only 2-3 cm of the tip showing.
• For a 40 cm sett you should plant 20 cm deep with 20 cm emerged.
• For a 1.5 m long rod you want ~30-50 cm pushed into the soil with 1-1.2m of rod still above ground.
  Additionally, planting depth can be altered for end use or due to varying land types. For example, if utilising poplar for phytoremediation of riverbanks, then you may want to insert a longer 1 or 2 year old rods and push them a metre or more into the soil. Alternatively, if there is a low water table and you want to give your rods a better chance at obtaining and outcompeting weeds for water, hence, longer rods inserted deeper into the soil will establish better.

3.4. TREE GUARDS & MULCHING

Reaching canopy closure as soon as possible is a key factor to maximising returns from your plantation. Weeds, which compete with your newly planted rods or cuttings, can reduce the chances of establishment and increase the length of time to obtain canopy closure. Both biodegradable mulch films and biodegradable mulch matts are good options for reducing weed pressure. Film mulches are more compatible with SRC systems whereas mulch mats with 30-45 cm centres are best for SRF systems. Additionally, tree guards can discourage browsing from rabbits and deer. However, if pre-planting operations and subsequent timed weeding interventions are carried out, guards and mulch mats are beneficial, but not necessary.

4. POST PLANTING ESTABLISHMENT

Management of the poplar plantation after planting is vital to maintaining the health of a plantation up until canopy closure. Once canopy closure has occurred, there becomes less stress from competing weeds due to the established roots system and ability to shade out weeds. However, there is still a need for the crop to obtain nutrients from the soil. Depending on the site’s previous land use, for example, sites that were previously intensively cropped may need a nutrient addition.

Additionally, once the canopy has closed and competition is removed as an issue for crop output, irrigation can be utilised to increase productivity. Irrigating sites is not common in the UK, however, is a necessity in some areas of mainland Europe where summers are hotter and drier than the UK. Methods that are used are flood irrigation and tape irrigation. This can be carried out any time after planting if needed. Water is a key limiting factor for poplar growth.

4.1. NUTRITION & FERTILISATION

Soil sampling should be undertaken prior to planting and every 3-5 years after that to determine whether nutrient interventions are required. Poplars are efficient in nutrient utilisation, and many sites may not show yield responses sufficiently high enough to justify the investment of fertilisation. This may be down to planting on soils already containing enough available nitrogen. As with any crop, a sufficiently high response to fertilisation must be achieved to validate the expense of applying nutrients to cropping systems. If the soil is extremely degraded, improving soil prior to planting can result in increased crop growth which can be sustained over many years. Growth rates can be increased by 40%, however, nutrient application is tightly regulated by local environment agencies and water authorities and can only be done under a licence.

Application of treated sewage on a large scale can only be undertaken prior to the planting of poplar plantations. Methods such as direct injection of nutrients can be carried out after harvesting, especially in short rotation systems. It should be noted that applying fertilisers prior to canopy closure will only increase the competitive pressure from fast growing weeds, thus affecting establishment and yields of the first harvests for SRC systems. Nutrient application will require licences.

4.2 GAPPING UP

However rigorously procedures were followed during planting and establishment there will always be gaps where a cutting was missed by the planter or rods failed to establish. This can be assessed at the end of the first growing season. Cuttings that have not been established in the plantation can be replaced through gapping up. Push 1.5 metre rods into the gaps, leaving at least 1m protruding. This will allow the gapped-up poplar to compete with those already established around it.

4.3 PRUNING & THINNING

Longer rotation forestry can benefit from pruning to improve timber quality. This depends on the final market requirements. Pruning reduces the knots in the wood and can produce a clean timber material. By removing side branches the main stem will also increase in size, thus timber quality is increased along with volume. With close planting and good management, you will be left with tall, high-yielding, high-quality timber that is easy to harvest.

Thinning gives more cropping options and opens multiple markets for the plantation owner. Poplar can be planted at a close spacing (4 x 2 m) and thinned over the next 10 years to (8 x 8 m). This allows the site manager to select the best trees to maintain strong growth rates over the lifespan of the cropping system. The thinned material can be harvested and sold as wood chip. This also gives an additional income stream over the crop cycle and allows selection and maintenance of quality within the plantation.

4.4. WEED CONTROL

Weed control is essential in the first couple of years until canopy closure occurs. Poplars are intolerant of shading so it is important to control the growth of competing vegetation early in the establishment of the plantation (Isebrands and Richardson, 2014). Canopy closure leads to the shutting out of competitors through established root systems and above ground foliage cover after around 2 years. Canopy closure is effective at closing out weeds in poplar due to their large leaves. What is more, reaching closure is expedited through careful planning, and pre- and post-planting management. If this is all meticulously carried out, successful establishment means less or no inputs are required after canopy closure.

4.4.1. CHEMICAL CONTROL

SRF follows traditional forestry practices, newly planted trees on previously grassed sites should be spot or band sprayed to reduce weed competition and maximise initial growth. Due to the fast-growing nature and large leaves of poplar this will only be required for the first two years of establishment, equating to 2-3 applications of herbicide in total. In this time the trees will have reached canopy closure and are capable of outcompeting weeds for sunlight, available water, and nutrients.

Spot spraying is recommended as the most cost-effective and environmentally responsible method of weed control, this must be undertaken by a certified professional contractor in line with the Control of Pesticides Regulations 1986 (COPR). The spot should ideally be a 1m x 1m square sprayed around the tree using an herbicide chemical such as glyphosate. Minimising the area of application lowers costs and has perceived environmental benefits from reduced herbicide use.

SRC follows traditional farming practices more closely. Post planting, an application of a pre-emergent residual herbicide mix to keep the crop clean during the establishment phase is required. These residual herbicides should be applied within 14 days after planting, it is essential that this is done before any growth begins. High volume (500 l/ha) applications should be used to give good surface coverage of the herbicide. The new crops should be monitored weekly during the first 3 months, so if any weed issues arise action can be taken immediately and effectively.

If weeds become a problem during establishment, as can be the case particularly on ex-grassland sites where the seed bank in the soil is greater than on ex-arable sites, there is only a very limited range of contact herbicides available for over-spraying the established crop. Additionally, these are highly specific herbicides with a limited weed spectrum. Other herbicides can be applied as directed sprays using an inter-row guarded sprayer. However, this is a skilled operation and should only be undertaken with advice as poplar is highly susceptible to the herbicides used. Spot treatment of small areas of troublesome perennial weed (docks and nettles) can be undertaken with appropriate herbicides using guarded knapsack sprayers or weed wipes.

Additionally, after harvesting SRC poplar there arises another opportunity to spray. This needs to be a specific herbicide which does not target the crop and be timed accordingly as to not damage the crop. Generally, after canopy closure occurs herbicidal inputs become redundant.

Herbicide use should be considered carefully when creating a forest management plan as there are instances where the application of herbicide may not be as beneficial and there can be detrimental environmental effects of using herbicides.

4.4.2. MECHANICAL CONTROL

Mechanical weeding with specific machinery can also be used to aid establishment and reduce competition. Inter-row weeders can be used in SRC systems. If weeds become a problem after planting, inter-row weeding can occur up until the new shoots reach ~80cm (depending on the machinery). Additionally, inter-row weeding can take place after the crop has been harvested if weeds begin to grow again in the spring.

Methods that have been utilised in orchard systems could be utilised in older SRF systems, such as mounted row crop cultivators which combine stars which break up the soil and finger cultivators which remove weeds. These finger weeders have rubber fingers that can disturb the soil and weeds but will not damage the woody stems of the tree. These allow for the management of weeds and grasses that grow close to the trunks of the newly establishing trees without the need for chemical input.

4.5. DISEASE & PESTS

Newly emerging pests and diseases pose the main threat to agriculture in our changing climate. In poplar plantations, this risk has increased because of the mass uptake of monoclonal plantations, with only a few different genotypes planted. What is more, there is a further push to plant more plantations in less favourable climates for renewable energy sources. Additionally, climatic change has led to the expansion of the latitudinal ranges of pests and diseases. For example, overwintering mountain pine beetles die off in extreme cold spells in Canada. High mortality rates during these cold spells control the populations of beetles thus not allowing levels to reach a damaging threshold. Nowadays, cold spells are less frequent, less cold and do not last as long, thus mortality rates in the pine beetle do not reach the heights they previously did leading to greater populations of beetles and greater losses of pine trees. This narrative is also true for fungal pathogens of trees.

Poplar breeding programmes can combat the onslaught of pests and diseases. There has been progress breeding new poplar genotypes and it continues to be one of the main objectives in the breeding programs of the genus.

Broadly, diseases and pests can be divided into the categories of: Insect pests, Bacterial diseases, and fungal diseases.

4.5.1. INSECT PESTS

A large number of insect species grow on poplar, of which only a few are of importance. The most common pest for poplar is the small poplar leaf beetle (Phyllodecta vitellinae, and P. vulgatissima). This beetle is generally only an important pest when present in very large numbers. The larval stages of these beetles feed on the underside of the leaf causing death of surrounding tissue, leaving irregular darkened patches on the upper leaf surface. Damage by the small poplar leaf beetles can be of importance in the stool bed during the first year’s growth. New poplar growth is particularly at risk if there are nearby mature poplar which are infected with beetles.

The large poplar leaf beetle (Chrysomela populi) is particularly damaging to aspen poplar. Larval and mature stages feed on the whole leaf and cause shoot dieback following removal of foliage. The adult is brick red and similar in appearance to a ladybird, while the larvae is whitish with black spots and a black head.

Larvae of the white-satin moth (Leucoma salicis) can cause complete defoliation of large trees, feeding from April to June.

The poplar shoot borer (Gypsonoma aceriana) can cause forking of young crops by damaging the leading shoot. The larvae feed from May to June.

Insect pests can be controlled through the use of insecticides such as lambda-cyhalothrin adhering to local rules and suppliers’ safety precautions

4.5.2. BACTERIAL DISEASE

Xanthomonas populi – the cause of bacterial canker in poplar. First noticeable in late spring as slits form on the bark of twigs. A whitish bacterial slime is exuded from the slits and the surrounding bark begins to decay. Cankers can form on the main stem and eventually encircle it, killing the tree. The main method of dealing with bacterial cankers is through the planting of resistant cultivars.

4.5.3. FUNGAL DISEASE

Poplar leaf-rust (Melampsora larici-populina) is the main fungal pathogen associated with poplar. Infection leads to premature defoliation and significant loss in biomass production. The most effective method to mitigate the spread of leaf rust is to use a diverse mixture of poplar varieties in plantations. The use of fungicides is not recommended, in line with willow, as poplar is a high volume, low value crop and the necessary repeated fungicide applications could not be sustained economically.

5. HARVESTING

Poplar growing within the UK has a lot of factors that affect how we grow and where we grow timber. There are many different cropping plans along with harvesting methods with some being tried and tested and new concepts and machinery being developed every year. It is very much worth looking at work done in the area you are looking to grow timber and in the current climate and mixed species approach is a must. But poplar is a very versatile timber and can be made to fit the system that suits both your market / site and management plan.

5.1. TIMING

Harvest takes place between November and March after leaf-fall and before leaf set when the sap has withdrawn to the roots which reduces moisture content of the wood.

5.2. HARVESTING METHODS AND MACHINERY

5.2.1. THINNING OF SRF POPLAR

Thinning of trees can take place to encourage the growth of the remaining trees when cropping poplar in an SRF system. This is best judged by a silviculturist, however, a standard indication is when the canopies of the trees begin to touch. Usually this is between 5-8 years after planting, if planted 4 x 2 m spacing. This can be carried using the methods below.

5.2.2. SINGLE STEM HARVESTING

5.2.2.1. CLEAR-FELL HARVESTING FOR CHIP OR TIMBER

Using a harvester head and removing branches and cutting the trees to lengths gives you a clean and easily handled material that can be transported easily. In this format you can transport the timber more efficiently and more volume can be moved by road transport.

5.2.2.2. WHOLE TREE HARVESTING OF MATURE TREES

This can be done by cutting the whole tree at ground level and either leaving the timber to dry onsite or stack at the edge of the site to dry. This means all the wood on site is used and leaves a clear site ready for re-stocking or natural regeneration.

The quality of the chip may reduce but nothing is left so the volume will increase. Transport will be increased as the volume of wood after chipping will also greatly increase.

5.2.3. HARVESTING SRC POPLAR

5.2.3.1. DIRECT CHIP HARVESTING

Direct chip is the most efficient and commonly used method for harvesting SRC poplar. Harvesting can take place every 2-4 years. SRC poplar is cut and chipped in one process, with the chip blown into high-side silage trailers for transport to storage or end user. Removal of above ground growth leaves a clean field ready for new shoots to develop from the stumps. Chipped material will need to be dried to prevent deterioration. Drying requirement can be minimised by ensuring correct timing of the harvest, after leaf drop when the sap has withdrawn to the roots.

5.2.3.2. BIOBALER

The Biobaler (Anderson Group, Canada) can be used to produce dense poplar bales which can be stacked in fields and allowed to dry naturally. This removes the need for forced drying which is required for chipped biomass. Bales can be transported using conventional equipment and chipped at the destination. The Poplar Tree Company has successfully used baling on several sites in the UK but the technology is not common.

6. POST HARVEST

6.1. YIELD

6.1.1. SRF POPLAR

Yields for SRF poplar will depend on a range of factors such as site conditions, tree spacing, and management practices. Generally, yields of 20-25 m3 of wood/ha/yr are expected from SRF Poplar plantations in the UK.

However, it is important to note that yields can vary widely depending on local conditions and specific management practices. For example, good soil preparation, proper weed control, and regular pruning can all help to increase yields. Additionally, selecting appropriate poplar varieties for the site conditions can also help to improve yields.

6.1.2. SRC POPLAR YIELDS

The yields for SRC poplar grown in the UK can vary depending on a range of factors such as the variety of poplar used, the planting density, soil type, climate, and management practices. Generally, yields of 10-18 tonnes of dry matter per hectare per year are expected from SRC poplar plantations in the UK.

However, it is important to note that yields can vary widely depending on local conditions and specific management practices. For example, good soil preparation, proper weed control, and regular pruning can all help to increase yields. Additionally, selecting appropriate poplar varieties for the site conditions can also help to improve yields.

In general, SRC poplar is a high-yielding crop, which can produce more biomass per unit area than many other crops.

6.2. DRYING & STORAGE

Timber can be stacked roadside and left to dry. In the UK we do have an issue with timber moisture and drying wood naturally for biomass use. By storing wood within the woodland, it is possible to dry timber and chip on site with a moisture content below 30%. This means no forced drying is required and the energy conversion of the wood is improved.

Storage and drying material in the UK is an issue and in many cases forced drying is required for end use, but with good management can be avoided.

Woodchip is more difficult to dry naturally as it is harvested and directly chipped as a standing crop. Drying in chip piles can be done but the chip will self-heat and reduce calorific value. This also has risks of combusting and so in many cases will be dried indoors on a drying floor.

6.3. TRANSPORTATION

Method of transportation will depend on the manner of growing and harvesting. If transporting long distances, it is better to transport wood ‘in the round’ (as whole logs, with brash removed) for chipping or processing at the destination. Whole logs have a higher bulk density than chipped wood therefore less volume is taken up at equal weight.

Woodchip from direct chip harvesting will require transport to drying facilities using lorries or tractors with trailers depending on biomass yield.

For the transport of round wood (e.g. from single stem forestry) there is a legal requirement to adhere to maximum limits for axle weights as set out by the DVSA. Hauliers need access to weighing equipment (either weighbridge or lorry mounted device) to determine gross vehicle weight or load weight. The device may be on the vehicle or the device loading the vehicle. Best practice guidelines exist for the transport of timber which should be adhered to.

6.4. SITE RESTORATION

When a poplar plantation has reached the end of its life, the site can be restored to either grass or arable production or replanted with new higher yielding varieties of poplar. Mechanical removal of poplar may prove more difficult than removal of willow due to the formation of lar taproots which require removal by excavator or a longer period of time for natural decay.