Salix spp.

General info

Willow is one of the most popular short rotation coppicing species for energy crop utilisation, it is a pioneer species that establishes well in new environments with rigorous juvenile growth. It comprises over 400 species worldwide with around 10% of these being deciduous tree species and most being multiple stemmed trees and shrubs.

Due to popularity and success, there are >50 willow cultivars registered with the Community Plant Variety Office (CPVO) with more the 25 commercially available in the UK. Commercial varieties provide benefits including increased rust resistances and increased yields compared to wild varieties due to targeted enhancements. Key willow species for bio-energy production in the UK tend to originate from northern temperate species as they can largely deal with the range of temperatures faced in the UK.

Cultivation and agronomy

Willow can be harvested on 2–5-year rotations at densities up to 15,000 plants per hectare depending on the productivity of the site and the environmental conditions. Planted willow can remain viable for 15–30 years and has a rapid growth rate achieving 6–8 metre heights between each rotation of cutting (depending on rotation year strategy employed).

Generally, willow is adaptable to a range of soils and conditions, however dry sandy soils may struggle to supply sufficient moisture for efficient growth, and organic peat heavy soils may cause too much competitive weed impacts to make the growth strategy efficient. Willow coppices need more water than any other conventional agricultural crop therefore areas of high rainfall are extremely beneficial.

Despite the temperate adaptability of most willow species for higher productivity rates it is recommended that plantation sites be below 100m above sea level. It has been shown that Willow harvesting yields equated to 5–14 oven dried tonnes ha-1 y-1 across several studies, with 1 hectare supply of willow woodchip providing energy roughly equivalent 4,500 litres of home heating oil (depending on moisture levels of willow biomass and productivity of a given hectare per year etc).

Commercially, willow propagation involves the planting of winter-dormant stem cuttings in the spring. Mechanical planting is performed whereby cut stem sections from willow whips are inserted into prepared soils. Following the first growing season cutbacks are performed to around 10cm above ground level to encourage the formation of multi stemmed stools for increased productivity benefits.

Populus spp
P. nigra, P. deltoides, P. maximowiczii and P. trichocarpa, P. tremula

General info

Poplar (Populus spp.) belong to the family Salicaeae and are native to the Northern hemisphere. Poplar is among the fastest growing of temperate trees and is therefore of considerable interest as a bioenergy crop.The plant grows to a height of 15–50m with a trunk diameter of 2.5m. Once established, poplar can beharvested every 2-5 years over a life span of over 20 years. Poplar is a multipurpose crop with several environmental benefits such as reducing erosion, phytoremediation of contaminated soils, windbreaks, and biodiversity.

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Cultivation and agronomy

Poplar is suited to most soil types including clay, sand, loam, and humus soils. It grows particularly well on well drained and fertile loam with a wide range pH of 5-8. Poplar can be grown as a Short Rotation Forestry(SRF) or Short Rotation Coppice (SRC). The plant can grow up to about 1.5m in height during the first yearand from 3-5m for each of the following years depending on the cultivar, site conditions, spacing andmanagement practices. For large scale plantations, it is recommended to plant mixtures of poplar varieties with about 100 trees of a single cultivar in each block, and different cultivars in neighbouring blocks.

If considering mechanised harvesting, plant at a spacing of 0.6m apart in twin rows with 0.75m between rowsand a 1.5m alley between each twin-row. Poplar can be planted in densities of 1,500 up to 1,800 trees per hectare for bioenergy purposes. Yields of 20-25m3 of wood/ha/yr is achievable in the UK. Young growth is attractive to deer, hare, and rabbits. Appropriate fencing may be required in areas where these animals areprevalent. Common pests includes the small poplar leaf beetle (Phratora vitellinae), and the large leaf beetle (Chrysomela populi).

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