Soil suitability
Willow is not a demanding species in terms of its site requirements and it will flourish on a wide range of soil types and environmental conditions, and in common with other crops, productivity will be determined by site fertility, the availability of water, light and temperature.
Most agricultural soils with pH in the range 5.0-7.0 will produce satisfactory coppice growth. However, light sandy soils, particularly in drier areas, will have a problem with moisture availability. Highly organic or peaty soils should be avoided as initial weed control, which is vital, will be extremely difficult. Medium to heavy clay loams with good aeration and moisture retention are ideal although they must have a capability of allowing a minimum cultivation depth of 200-250mm to facilitate mechanical planting.
Water availability
Willow coppice requires more water for its growth than any other conventional agricultural crop and hence requires a good moisture retentive soil. Areas with an annual rainfall of 900-1,000mm are best or areas where the crop has access to ground water. The crop can tolerate occasional inundation but this may have implications for harvesting. It has been calculated that willow coppice can use up to 1.0m litres per tonne of dry matter produced annually.
Temperature
Willow is in its native environment in a northern temperate zone. Consequently, temperatures in Ireland are unlikely to be an issue. However, elevated sites can result in exposure problems and a reduction in the number of growing days per year. Therefore, production sites should generally be below 100m above sea level.
Access
Harvesting is carried out from December to April and whilst the root system of the growing coppice will support the harvesting and extraction equipment on the coppice site, hard access to the site is required. Slopes in excess of 12o are difficult for harvesting machinery particularly in wet conditions and should be avoided.
Location in the landscape
Short Rotation Coppice (SRC) has more similarities with arable cropping than conventional forestry; it has a regular harvest pattern and its deciduous nature gives a seasonal diversity of texture and colour. Willow SRC at the end of a two-year growing cycle will be up to 8m tall and therefore creates a three dimensional mass in the landscape which arable crops do not. Poorly planted Willow SRC plantations have the potential to adversely affect the rural landscape. However, well-designed and carefully sited plantations could bring small but important landscape improvement. In most cases, with some thought, the establishment of SRC is likely to bring, at best, a significant improvement or, at worst, no detrimental effect to most mixed agricultural landscapes.
Willow SRC 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 appropriate utility company as mature coppice can reach 8m before harvest.
Weed control
Pre-ploughing
It is important that this is carried out effectively particularly on old pasture land where the presence of perennial weeds such as docks and nettles is more likely. A translocated (systemic) herbicide (e.g., Glyphosate at 4 litres/ha) should be applied to actively growing vegetation from 15 January . To allow the herbicide to fully translocate, a period of ten days post-herbicide application should be allowed before ploughing.
Post-planting
A mix of pre-emergence residual herbicides should be applied immediately after rolling, and no later than five days after planting, for broad spectrum early season weed control. The quality of the seedbed is critical to this operation. A good, fine seedbed will allow a seal on the soil surface to be created and help reduce the need for further herbicide applications. However, depending upon the weeds present it may be necessary to apply follow-up contact sprays for specific problems. The aim is to eliminate competition from the weeds to allow the crop to grow and develop to its maximum potential. See the Teagasc factsheet on weed control in willow based on active ingredients and given off-label approval by chemical companies.
Pest and diseases
Leatherjacket larvae
Leatherjacket larvae pose a threat, particularly on former grassland or long-term set-aside. If present, control is required in the form of a suitable insecticide sprayed shortly after planting, typically alongside the pre-emergence residual herbicide application.
Fencing
Normal stock fencing is required to prevent livestock from entering the crop. Such fencing may not be required around the actual willow plantation, if the wider perimeter is already securely fenced. Rabbit/hare fencing may be needed during the first growing season after planting, during which time the shoots develop. Such protection is not required after this initial establishment period or in subsequent cycles, due to the vigorous nature of the crop. Therefore, where required, consider using temporary rabbit and hare fencing that can be moved and redeployed elsewhere. If using this fencing, a 15cm outward-facing flap of fencing must be left at the bottom of the fence, and securely pegged down. Where present in small numbers, deer may cause localised grazing or bark stripping. Such loses can be absorbed by the vigorously growing crop. However, high deer numbers and the likelihood of heavy damage may necessitate deer fencing, or, due to the cost involved, may simply rule out the feasibility of growing SRC willow on that site.
Rusts
Melampsora rusts represent a serious threat to SRC willow crops, potentially affecting both productivity and survival. This threat is best countered by the use of willow varieties that have been specifically bred for resistance to this disease. Typically, a mixture of five-to-six varieties is included in the plantation, to allow adequate yield compensation should individual varieties loose productivity or die out due to increasing disease susceptibility. This measure also increases the genetic diversity within the crop, thereby reducing the selection pressure on the disease organism.
Cutback
The crop can reach a height of 2.0m or more during the first growing season. To initiate the development of multiple shoots, each stem is cut back at its base during the winter of the first year, before bud burst. Cutback is normally carried out using a finger bar mower. The cut itself should be clean and regular. The material ‘harvested’ at the cutback stage may have limited or no commercial value.
Harvesting
The first harvest is usually undertaken three to four years after planting, i.e., two to three years after cutback. Subsequent harvests are undertaken thereafter on a two-year cycle. Harvesting is carried out from November to the end of February, when foliage is absent and stem moisture content is at its lowest (approximately 55%). Careful planning is required to avoid excessive rutting and soil disturbance, particularly in localised wet areas. Stools should be cut as close to the ground as possible at each harvest, as this keeps stool height as low as possible throughout the life of the crop. Direct-chip harvesting is typically used, whereby the stems are cut and chipped by the harvester and blown into an accompanying trailer.
Whole rod harvesting systems are also becoming increasingly feasible.
