Willow biomass crops can be grown on marginal land using a coppice management system so that multiple harvests are generated from a single planting of genetically improved shrub willow varieties [21]. The system typically includes three-year rotations with one year of site preparation prior to planting. After the first growing season, the willow is coppiced and material is typically left in the field since first year production is very low, typically between 0.5 and 1.0 tons ha-1. The willow resprouts the following spring and produces multiple stems on each plant. The willow is left to grow for three years and then harvested during the dormant season. After harvest the willows resprout and grow for another three-year cycle. Up to seven three-year rotations are currently projected before the willow stools spread out and limit access for harvesting and chip collection equipment. Following the final harvest, the willow can be killed with herbicide and stools ground down and incorporated into the soil [21].

Effective establishment of perennial energy crops like willow is essential to their biologi­cal and economic success, so conducting proper site preparation is essential. Site preparation should begin with control of existing weeds using a combination of chemical and mechan­ical techniques. These activities should begin in the fall before planting if the field contains perennial weeds, which is often the case with marginal land, or after crops are harvested if the land is currently being used for the production of an annual crop. It is essential to control competing vegetation and prepare the soil before willows are planted in the spring. Improper or incomplete site preparation often results in strong weed competition during the establishment phase and has frequently been noted as one of the main limitations to successful establishment of willow biomass crops [6, 9, 22].

Willows are planted as unrooted, dormant hardwood cuttings at about 15 000 plants per ha-1 as early in the spring as the site is accessible with mechanized planters attached to farm tractors. Over the years several versions of planters have been developed and two of them, the Step planter and the Egedal (Figure 12.1), are currently being used in the United States. With both machines, one-year-old stems are fed into the planter, cut at an appropriate length (15-20 cm) and either actively inserted into the ground (Step planter) or placed into a slit opened by the planter (Egedal). Both machines are capable of planting around 0.8 ha h-1 when site preparation has been done properly and ground conditions are appropriate.

The use of dormant cuttings allows planting of selected varieties of genetically improved willow. In North America, varieties are generally planted in individual blocks with several different genetic varieties planted across a field to maintain diversity. In parts of the United Kingdom, where pressure from willow leaf rust (Melampsora spp.) is stronger, studies have shown that planting random mixtures of different willow varieties can maintain or even increase yields [23, 24]. Differences in growth rates, stem form, canopy width and other characteristics among varieties may influence the effectiveness of this approach if some external pressure, such as a disease or pest, is not a major influence on the system. Research is underway in North America to characterize willow varieties and to explore the potential benefits associated with mixed random planting designs.

Current recommendations for planting designs and densities for willow in North America are based on the double row system developed in Sweden and research from Europe largely based on the growth of S. viminalis [25] and trials in North America [26]. Research in

North America was based on a single variety, ‘SV1’ (Salix dasyclados), over multiple rotations with densities ranging from about 15 000 to 111 000 plants ha-1. The current recommended spacing for a double row system allows 1.8 m between each set of double rows, 0.75 m between individual rows, and 0.55 m between plants along each row. This results in a planting density of about 14 600 plants ha-1. However, recent studies with new willow varieties developed in New York suggest that their growth rate is rapid enough that there is no significant yield difference among planting densities ranging from 8800 to 17 500 plants ha-1 [27].

Following the first year of growth, the willows are cut back close to the soil surface during the dormant season to force coppice regrowth. This increases the average number of stems per stool from 1-4 to 8-13 depending on the variety [18] (Figure 12.2). After an additional three to four years of growth the stems are mechanically harvested during the dormant season after the willows have dropped their leaves so those nutrients are maintained in the system (Figure 12.3). In addition, most end users do not want foliage in

the biomass delivered to their facility because of its higher ash and nutrient content. The chipped material is then delivered to end users for conversion to bioenergy, biofuels and/or bioproducts. The plants will sprout again the following spring and are allowed to grow for another three or four years before being harvested. Projections indicate that the crop can be maintained for 7-10 rotations before the rows of willow stools begin to expand to the point that they are no longer accessible with harvesting equipment. At this point the crop can be replanted by removing the existing stools with herbicides after harvesting, followed by chopping using a heavy disk and/or grinding machine, and subsequently planting new cuttings that year or the following year.

Nutrient removal from willow biomass crops is limited because only the aboveground woody portion of the crop is harvested during the dormant season after the leaves have dropped and most nutrients have been translocated to the root system. Nutrients not translo­cated from the foliage are returned to the system in litterfall. For most soils in the region where willow is being deployed, the only nutrient addition that is recommended is nitro­gen, which is typically added at the rate of about 100 kgN ha-1 once every 3-4 years in the spring after the crop is harvested. However, recent research has indicated that for a number of sites in the northeast there was no yield response when nutrients either in the form of commercial fertilizers or organic amendments were applied to willow crops [28, 29]. Marginal agricultural soils in the northeast United States are typically limited by poor drainage and wet conditions rather than nutrient supply. Additionally, tight nutrient cycling in these systems [30] and relatively low nutrient removal rates in the woody biomass [31] are other factors that may further reduce the need for fertilization on a wide range of favorable sites for woody crops in the region. If regular nutrient additions are not required, willow systems will reduce production costs and greenhouse gas emissions, improve the net energy balance, and preserve or improve water quality in natural streams and waterways when compared to annual cropping systems.

A rapid growth rate is one of the attributes that makes shrub willows an appealing biomass crop. Yields in research plots of fertilized [32] or fertilized and irrigated [31] unimproved varieties of willow grown for three years have exceeded 27 oven-dried metric tonnes (odt) ha-1 yr-1. Due to costs associated with irrigation and the relatively low value for biomass, irrigation will not be used for most large-scale production operations. The exception with regard to irrigation is where willow biomass crops could be irrigated with wastewater as part of an overall nutrient management plan. Nonetheless, these studies set a benchmark for the potential production of willow biomass, with even higher yields being possible with improved genetic material from current breeding and selection programs.

First-rotation, non-irrigated research-scale trials across a range of sites planted between 1993 and 2007 with a range of willow varieties have produced yields of 6.9 odt ha-1 yr-1 [33]. Trials planted after 2005 that included new willow varieties developed in North America produced 9.2 odt ha-1 yr-1, an increase of 33%. Many of these trials involved the testing of a wide range of varieties, some of which were later determined to be unproductive and were eliminated as potential commercial varieties. If just the top five varieties in each trial are included, then the reported yield is 9.2 odt ha-1 yr-1 in the trials with older varieties and 11.2 odt ha-1 yr-1 when new varieties were included, a 22% increase. Second rotation yields of willow are typically higher than first rotation yields because the plant’s root system is already established and more of the carbon that is fixed by the plant in the second rotation can be allocated to aboveground growth. In one trial, second rotation yields increased by about 20% while third and fourth rotation yields were maintained and largely dependent on weather conditions [33].