Details on many biomass related subjects can be found here.
Biomass fuel simply means living or recently dead organic matter that is used to make energy. In this case, wood from fast growing plants is commonly used to heat homes and to power generators. Coppicing trees for a renewable source of wood is an ancient practice, the only thing that is special about modern, hybrid willows and hybrid poplars is that they grow so quickly and can be harvested frequently, hence the plantation is called a Short Rotation Coppice (SRC). Alder, Hazel, Sweet Chesnut and Ash are all suitable for biomass as well, although they aren't as vigorous as the hybrid willows and poplars.
Willow is the crop that is used on large scale plantations. Poplar isn't grown for commercial bio fuel, but it is still a good choice for small scale plantations, especially if you want to combine the crops to improve biodiversity. On very fertile, sunny sites, poplar can outperform willow. Yields from willow should be around 8-10 dry tonnes per hectare, according to the studies sited here.
Where to grow Biomass Wood Fuel
Willows will do very well in wet soil. In general, any moist soil that is slightly acidic to neutral is ideal. Dry sites aren't suitable. A sunny position is necessary for the best yields.
Preparing the Site
1. Weeds: This is vital - you must destroy all the weeds and grass in the site before planting, using a glyphosate weed killer. Then, you must destroy any new weeds until your crops have grown enough to cast good shade over the soil.
2. Soil: Dig an inspection pit to see the condition of the topsoil and subsoil. If the soil is compacted, deep ploughing is recommended. It is best to do this in a dry autumn period and then leave the soil exposed to frost for the winter. Before planting, power harrow the soil to make a fine tilth. Rotovating is not as good, because it tends to dry the soil.
Planting Distances for Willow & Poplar Biomass
The forestry commission recommends the twin row planting method. Each block of twin rows consists of two parallel rows 75cms apart, with the plants 59cms apart along each row. Each of these twin row blocks is 150cms apart from the next row block. This planting scheme allows 15,000 plants per hectare, which is very close to the optimal planting density.
Planting Bio fuel
Willow and Poplar hybrids are delivered in short, unrooted cuttings (called setts) for easy planting - you just need to push them into the soil. Large scale planting is done with machines based on cabbage planters.
Planting should be done towards the end of the winter season, which is generally from mid-February in the South and in March in the North. The failure rate of setts planted earlier in the winter is higher, especially if the winter is either very wet or unseasonably dry.
Leatherjackets (crane-fly larvae): If the area was grassy before it was ploughed and made ready for planting, there could be a problem with Leatherjackets attacking the roots of the new crop. One application of insecticide, before planting, is advised if there is a risk. In subsequent years, the crane flies will not lay new eggs.
Rodents: The best protection against rabbits & hares is a fence that has been dug into the soil. The fence can be temporary to save money, as the crop is only at risk of serious damage in the first year or two.
Willow: At the end of the first year, cut the stems right down to a foot or so. This will encourage a denser stool of stems for cropping in the future.
Poplar: Poplar tends to product fewer extra stems after cutting back, so it is common not to cut it back until the first harvest.
When to Harvest Willow & Poplar
Harvest your crop soon after all of the leaves have fallen. Harvesting is on a 2-4 year cycle, depending on how the site affects the vigour of the plants.
Willow is harvested more often than poplar. After the cut-back at the end of the first year, willow is usually harvested every 2 years. If the plants aren't at peak performance, they can be harvested every 3 years.
Poplar is less bushy than willow and takes longer to "capture" the site (i.e. absorb as much sunlight as possible). It is usually harvested every 4 years, starting from the planting date, but it can be harvested every 3 years if the site is excellent.
After Cutback & Harvesting Weed Control
When you cut back the willow crops at the end of the first year and either crop at harvesting, you must use a residual herbicide to destroy the weeds that will try to take advantage of the sunlight reaching the soil.