Short Rotation Coppice (SRC) Willow
Short Rotation Coppice (SRC) is the practice of planting woody crops at high density, which are harvested every two to three years. While there are a number of candidate crops, willow is particularly well suited to Irish climatic conditions. Willow produces vigorous juvenile growth when it is coppiced, that is, cut back to ground level. The coppiced stools that remain after harvest re-sprout to form multiple stems and can give dry matter yields in excess of 10 t/ha/yr depending on the site and the climate.
Willow will grow in most agricultural soils with a pH of 5.5 - 7.0. Medium to heavy clay soils with good aeration and moisture retention are ideal, although it must be possible to cultivate to a depth of 25cm to enable mechanical planting. Light sandy soils may have a problem with moisture retention and heavily organic soils should be avoided because of difficulties with initial weed control.
Willow is a water demanding crop and needs soils with good water retention. Optimum growth is achieved with annual rainfall of 900 - 1,100 mm. Willow will thrive in wet soils but will not tolerate water logged anaerobic soils. However, soil moisture and structure may have implications for harvesting machinery.
The production site should be less than 100m above sea level and have slopes of less that 13 percent.
It is essential to have hard access to the plantation, particularly for the movement of harvesting machinery in the winter. The root systems of the willow will support the harvesters during cutting, but hard access is vital for the removal of cut willow from the site.
A minimum area of two hectare blocks is recommended. Smaller blocks make it difficult for planting and harvesting. Furthermore it is expensive to rabbit fence small areas if required.
SRC willow will blend into the landscape in most situations. However as it can grow to a height of 5 - 6m by harvest this needs to be taken into consideration. Neighbours will need to be consulted.
SRC willow will potentially be in the ground for a minimum of twenty years, so thorough and careful site preparation is essential.
In September, there should be an application of herbicide (4.0 - 5.0 l/ha) while vegetation is still actively growing. If the site has excessively heavy vegetation, it should be cut and removed. Allow sufficient time for regrowth to allow herbicide uptake. On grassland and set-aside sites, an application of 3.0 l/ha Dursban will be needed for leatherjacket control. A minimum of ten days after treatment the site should be ploughed. Ploughing should be to a depth of at least 25 cm.
If the site is suitable, it can be ploughed and power harrowed in mid-March, six weeks before planting. Any germinating seed can then be sprayed off using 2.0 l/ha Glyphosate. If the site has a heavy clay soil, it will be necessary to power harrow as close to planting as possible. Additionally, it may be necessary to lift large stones which have the potential to interfere with the planting machinery.
Rabbit and hare fencing
Rabbits and hares can cause significant economic losses, especially during establishment. If there is a rabbit and hare population, then the crop needs to be protected using an appropriate rabbit fence.
There are two willow breeding programmes in Europe developing new improved varieties for SRC production. The Swedish programme is controlled by Svalof Weibull AB. Commercially available varieties from the Swedish Programme include; Tora, Sven, Torhild, Tordis, Olof, Gudrun and Inger. The European Breeding Programme, which is now based at Rothamsted Research in England, has released varieties including Nimrod, Resolution, Discovery, Endeavour, Beagle and Terra Nova.
It is important to note that all of these varieties are protected by Plant Breeders’ Rights, which means that it is illegal to produce propagation material for self-use or sale. However, it is permitted to produce small numbers for gapping up of established crops, from material produced at cutback. Cuttings will therefore have to be obtained from a specialist grower or supplier and will be supplied in the form of one-year old rods for mechanical planting.
For reasons of disease management, it is vitally important that a mixture of varieties is planted. The mixture should contain at least six components, of which at least two should come from either the Swedish or the European Breeding programmes. The importance of using genotype mixtures cannot be overemphasised. Mixtures will reduce the impact of any disease or insect attack; they will reduce the disease pressure on individual components and will ensure the long term viability of the plantation. Use of mixtures also increases yield.
Planting should take place from early spring (February/March) to late May depending on weather conditions. Cuttings are planted in double rows (0.75 m apart and 1.5 m between double rows). A spacing of 0.5 m between plants within rows will give a planting density of approximately 18,000 plants/ha. There are a number of types of planting machines but the industry standard has become the Step Planter. In ideal conditions and large fields, a planting rate of 6 - 8 ha/day can be achieved. In most situations, there will be an establishment rate of in excess of 90 per cent, giving a final plant density of at least 15,000 plants/ha. Where possible, the rows should be planted in parallel with the long axis of the field.
The crop is best harvested during the winter, December - February when there are no leaves on the plant. Two approaches are used for harvesting.
Direct chip harvesting
The crop is cut and chipped in a single pass, and blown into trailers for removal. The moisture content of fresh wood at harvest is around 50 per cent and so will require immediate artificial drying, which is normally carried out in a ventilated grain drying floor. Heated air (6 - 10ºC above ambient) is used to increase its water holding capacity. It is necessary to reduce the moisture content of the chip to less than 20 per cent and this can normally be achieved in three to six weeks.
Whole stem harvesting
The ‘Stemster’ harvester cuts the entire rod which can then be stacked, preferably on a hard surface. The rods will dry naturally to around 30 per cent moisture when they can be chipped for use in boilers. This method avoids the need for specialist drying equipment used in the direct harvesting. However, whole stem harvesting requires collection of the rods and their removal to the standing area. Higher power requirements are needed for chipping the drier rods and the chip produced tends to have a wider particle size.
Wood chip for heat
Willow wood chip is a high volume low density fuel, so it is very important from an economic perspective that the production site is close to where the chip will be utilised – normally no more than 20 miles. The simplest and most convenient method of energy recovery is combustion to produce heat. A wide range of boilers are available at a range of outputs to match many requirements. At the present initial stages of the development of the industry, it is essential to have established possible end use for the wood chip before planting.
Wood chip energy
In energy terms, SRC willow dry matter has an energy content of approximately 19 MJ/kg or 45 per cent of the energy in an equivalent volume of light fuel oil. This gives a mean annual production equivalent to 3300 - 5700 litres of oil/ha/yr. It is normally considered that willow wood chip is carbon neutral, having absorbed the same amount of carbon during growth as is released during burning. Additionally, there are significant carbon savings through the displacement of heating oil. It is estimated that 3,300 litres of domestic heating oil produces 8,355kg CO2. Burning wood chip will reduce the net CO2 emissions by an estimated 90 percent. Expressed another way, wood produces 7kg CO2 per GJ compared to heating oil which produces 79kg CO2 per GJ. Furthermore, biomass does not contain many of the other noxious chemicals that are released when fossil fuels are burnt, for example, sulphur, which can cause major environmental problems.
SRC willow has the potential for the bioremediation or biofiltration of both solid and liquid effluent wastes. Willow can take up large volumes of water and is efficient in the utilisation of nitrogen, and to a lesser extent phosphorous. The use of a plantation for the treatment of sewage sludges and effluents could significantly improve the economics of growing SRC willow. However, caution is required when considering bioremediation; note that:
- it is essential to have a full soil analysis to determine whether or not the site is suitable to receive the sludge / effluent
- a full analysis of the sludge / effluent is required in order to ensure that its application complies with all waste treatment legislation such as the Nitrates Directive and the Safe Sludge Matrix
- sludge will have to be pre-treated before application in order to bring about a pathogen kill
- approval must be obtained from the Department of the Environment before any waste material can be applied to the land
- sites where sludge / effluent is being applied need to be carefully monitored