Shrub willow is a perennial that can be grown as a sustainable bioenergy crop on marginal land. The stems are typically harvested every three or four years using a self-propelled forage harvester that cuts and chips the stems. Willow wood chips can be dried and pelletized or combusted directly to produce heat and/or electricity. Willow wood chips can also be used to produce biofuels, such as ethanol.
Shrub willows produce a dense cluster of stems on each plant, so they can be planted to form living snowfences, privacy hedges, riparian buffers or used to restore stream banks. Some varieties are planted as ornamentals and other varieties produce long, straight, flexible stems that can be woven to make baskets.
For more information about specific uses, refer to a series on the environmental applications of poplar and shrub willow below:
- Poplar for Biosolids Management
- Potential for a Poplar Industry Using Recycled Water
- Willow Buffers in Agricultural Systems
- A Willow Vegetative Cover in New York State
- Shrub Willows for Living Snow Fences
- Short-Rotation Coppice Willow in Ireland
These white papers are companion guides to a larger publication, A Roadmap for Poplar and Willow to Provide Environmental Services and to Build the Bioeconomy. The roadmap details how shrub willow and poplar can be grown to produce bio-based fuels and chemicals.
Willows are classified into three type – trees, shrubs, and dwarf alpine willows. Shrubs grow 15 to 25 feet (5 to 7 meters) tall, have multiple stems and re-sprout after being cut. Willow trees can grow much taller, have only one trunk and many species do not re-sprout well after being cut. It is the shrub willows that are planted as a bioenergy crops, due to their ease of harvest and reliable re-sprouting.
You can find a list of state and federally listed Introduced, Invasive, and Noxious Plants here.
There are no Salix species on the composite Federal and State Noxious Weeds list.
In a recently harvested genetic selection trial our breeding program, a disproportionate number of the highest yielding genotypes were triploid. Because they possess unbalanced numbers of chromosomes, triploids are essentially sterile and have been sought as non-invasive cultivars of otherwise invasive species. Since willows are dioecious (male or female plants), they must outcross to form seed, and although triploids have been shown to produce viable progeny through crosses with a diploid or tetraploid individual, this occurs at very low rates of success and those progeny are likely to suffer from aneuploidy, resulting in low fertility (Ramsey & Schemske 1998). While some willow species are classified as invasive in some locales (e.g. Salix nigra in Australia (Stokes 2008)), their establishment beyond the site of original planting is dependent on seed production (Stokes & Cunningham 2006). Vegetative propagation is highly localized, is not a good predictor of invasiveness in willow, and is highly dependent on riverine disturbance regimes (Stokes & Cunningham 2006; Stokes 2008). Another contributing factor to willow invasiveness is it ability for species hybridization, which should be minimal in triploid species hybrids. Willow seeds are very short-lived, have very strict establishment requirements, and suffer from very high rates of establishment year mortality (Dixon 2003), which further reduce their potential invasiveness.
Shrub willow is grown as a bioenergy crop on marginal land primarily in the United States, Canada, and northern Europe, although interest is spreading in Asia and portions of South America. In North America, shrub willow is grown primarily in the Northeast, upper Midwest and southern Canada. In Europe, willow is grown as a bioenergy crop primarily in Sweden, Denmark, Ireland, and the United Kingdom, with interest spreading to Poland, Germany, Estonia, and many other countries.
Although willow crops regularly produce high yields without the use of any pesticides, some pests and diseases can damage shrub willow plantings, reducing yield and making the plants more susceptible to other biotic and abiotic stresses. Most pests and diseases are moved by wind, rain, or animals from one shrub to another, but most pests and diseases are also very specific as to which host species they will feed upon or infect. Growing shrub willow in a monoculture, where only one variety or many closely related varieties within a species, are grown together, can provide a habitat conducive to major outbreaks of pests and diseases since the pathogens can easily move from one shrub to the next. Planting polyculture plantations, with many different genotypes, produces natural barriers to that movement, reducing the spread and ultimately reducing the damage pests and diseases can do in shrub willow plantations.
Shrub willow planting stock (dormant stem cuttings and whips) is available from nurseries in the United States, Canada and Europe. In North America, planting stock of varieties bred and tested by researchers at SUNY College of Environmental Science and Forestry and Cornell University are available through Double A Willow and its sub-licensees in Canada, AgroÉnergie and Pacific Regeneration Technology. Unrooted dormant cuttings must be kept frozen at 26 to 28 F, otherwise they will start to break bud and produce roots. Typically, cutting that are 6 to 8 inches (15 to 20 cm) in length are planted in early spring, after the threat of frost, but successful plantings have been established as late as early August.
One of the biggest advantages of willow is that it can be grown successfully on soils that are marginal for traditional crops. The best willow growth, however, occurs on sites with a large rooting volume and good aeration, water, and nutrient availability. Soil pH should be between 5.5 and 8.5 and depth should be at least 18 inches. The soil can range from sandy loam to silt or clay loams. Slopes in the fields should not be greater than 8% to accommodate the harvesting machinery during the dormant season when the field is wet or covered in snow. The site must also be cleared of weeds, as weed competition is the biggest issue for willow in the intial two growing seasons after planting. The range of latitudes where willow is grown in North America has extended from 38ºN in Missouri to 53ºN in Alberta, including USDA plant hardiness zones 6a to 3b, although there are differences in cultivar tolerance to extremes in summer and winter temperatures.
This will depend on the nutrient status of each particular field, but current research suggest that in marginal ag fields, yields of shrub willow can be improved with some input of fertilizer. However, it requires less fertilizer than corn, which currently produces the majority of biomass for biofuels. The amount of fertilizer required depends on the soil nutrient content: in fertile soils, especially in early rotations, there is less need for fertilizers and not even necessarily a beneficial response to them. Soils that have less initial nutrient content, especially phosphorus and nitrogen, will need fertilizers in the earlier stages of growth in order to maintain productivity. Usually, soils have enough nutrients that fertilizers are not recommended in the first growing season, but will need to be applied after harvest and before re-growth, to replace the nutrients removed from the field through harvest. Willow can form mycorrhizal associations to access nutrients in the soil and over time nutrients are cycled as leaves fall and are broken down in the leaf litter, so it is not clear if fertilizer added to mature stands will confer any yield benefit.
Dormant cuttings of 6-10 inches (15 to 25 cm) are collected from one-year-old stems in nursery beds, after leaves have fallen. The dormant cuttings are frozen at 26°-30°F (-4 to -2 C), and are shipped just prior to planting, and should not be refrozen after being thawed. They are planted in a double-row system with five feet between double rows, two and a half feet between rows, and two feet between plants within a row, using machinery such as the Nordic Biomass Step Planter or the Egedal Energy Planter. Once planted, the dormant cuttings will produce roots below-ground and shoots above. Weed control is very important in the initial planting stage, as weed competition is the biggest cause of failure for establishing shrub willow. Immediately following planting, a pre-emergent herbicide must be applied. After the first year of growth dormant stems are cut back to the ground, which will promote coppice regrowth of multiple stems per plant the following spring, allowing the crop to close the canopy by the end of the second growing season. Continued weed maintenance is usually required until the end of the second growing season.
The USDA Biomass Crop Assistant Program, BCAP, created in the 2008 Farm Bill, currently provides financial incentives from the Farm Service Agency for farmers to plant and harvest bioenergy crops for heat, power, bio-based products and advanced biofuels. As of 2013, there are eleven BCAP project areas; the 10th area covers nine counties in northern New York to support production of shrub willow for heat, power and electricity. Producers enter into 11-year contracts and can have up to 75% of the costs of establishment covered through the program. Biomass producers in BCAP project area also receive annual rental payments for 15 years for woody biomass, if certain conditions are met. Matching payments for up to two years for the harvest, storage and transport of shrub willow to a biofuel conversion facility may also be possible. For more information, visit the USDA BCAP website.
Willows should be ready for their first harvest three years after the first coppice, which is four years after planting, and can be done at any time during the winter, after leaf fall and before bud swell in early spring. Harvesting can be completed with up to one foot of snow on the ground if it is not packed, but must be suspended if the ground is too wet. The best harvesting conditions are when the ground is dry or frozen, which means night harvesting may be necessary. Target harvest rates are from 50 to 60 wet tons per hour, or approximately 2 ac per hr. Willows sprout vigorously in the spring following winter harvest and harvest can be repeated on a three- to four-year cycle. It is expected that six to seven harvest rotations can be obtained from a single planting before replanting; meaning a field of shrub willow can be harvested for over twenty years.
The relative costs and monetary benefits of shrub willow production are hugely dependent on individual factors. SUNY-ESF has developed a model called the EcoWillow budget model that allows users to calculate how yield, management, and cost factors influence the cash flow and internal rate of return of shrub willow. The categories addressed in the model include labor, travel, equipment, and supply costs; row length and turning times at the ends of the rows; and establishment and harvest costs. The model can be found on the SUNY-ESF website.
Shrub willow is a woody perennial crop producing biomass, which is mainly a lignocellulosic material. All biomass can be converted to renewable forms of energy via thermochemical or biochemical conversion. Depending on the conversion process used, willow can be used to generate heat and electricity and synthesize liquid transportation fuels. Conversion processes include:
- Combustion – the production of heat through burning the biomass directly
- Gasification – Incomplete combustion of the biomass producing a syngas. The syngas can be combusted to generate heat and power. Hydrogen gas can be isolated from the syngas for fuel cells. The syngas can also be converted to liquid fuels through the Fischer-Tropsch method.
- Pyrolysis – Thermochemical breakdown of biomass into liquid oils and gases.
- Liquifaction – Thermochemical breakdown of biomass in the presence of hydrogen to produce bio-crude oil.
- Biochemical conversion – The production of ethanol and other alcohols, such as butanol, via fermentation of the biomass.
Shrub willow crops are ideal for renewable energy due to the environmental benefits and the sustainability of the crop. They have a low nutrient and nitrogen demand compared to other crops. Since the crop is coppiced in the fall after leaf senescence, this allows for nutrient cycling back to the roots and for the production of new organic matter. They grow very well on underutilized, fallow land and can improve soil conditions and microbial diversity. Once the crop is established, very little maintenance is required and fertilizer applications are relatively low.
Ethanol can be produced from three main types of raw materials: sucrose-rich materials such as sugarcane and sugar beets; starch-rich such as corn grain; and cellulose and hemicellulose from lignocellulosic biomass. Ethanol production from lignocellulosic biomass is considered to be a “second generation” fuel source, produced from non-food based, dedicated energy crops. In contrast, “first generation” feedstocks are food-based crops such as corn. First generation feedstocks have led to some controversy over issues related to environmental impacts and competition for food. Since cellulose and hemicellulose (comprising over 50% of most plant biomass) are made up of sugars, they both can be sources of ethanol. Converting lignocellulosic material into ethanol involves four major steps: pretreatment, hydrolysis to simple sugars, fermentation, and product purification or distillation. The production of ethanol from corn requires similar steps to break down the starch into simple sugars for fermentation. However, the production of corn ethanol (moonshine) is a well known, established method dating back hundreds of years, whereas, ethanol from lignocellulosic biomass is a new process. Today, corn ethanol production facilities and commercial distilleries have an annual production rate of ethanol around 30 billion liters. While lignocellulosic ethanol is not a commercial reality yet, research, development and the deployment of pilot facilities at several locations around the nation are bringing the potential into focus.
The standard unit for the measurement of land is a hectare, which is 10,000 square meters, and also equal to 2.47 acres. The average yield of shrub willow in North America is 11.3 oven dry tonnes per hectare per year or 4.7 oven dry tons per acre per year (1 tonne/ha = 0.414 tons/ac or 1 ton/ac=2.242 tonnes/ha). The standard energy content in oven dried wood chips is 18 GJ (gigajoules) per tonne or 15.5 million Btu per ton. From one hectare of willow, 203 GJ is generated in oven dry wood equivalent each year or 77.9 million Btu per acre. This is equivalent to 56,388 kWh per hectare or 22,830 kWh per acre each year. The equivalent amount of energy in fossil fuels to that in oven dry tons harvested from 1 hectare per year would be roughly 33.3 barrels of oil, 1,678 gallons of gasoline, 1,924 therms of natural gas, 1,330 gallons of fuel oil, or 7.5 tonnes of anthracite coal (27 GJ per tonne). The equivalents from 1 acre of willow per year would be: 13.5 barrels of oil, 680 gallons of gasoline, 779 therms of natural gas, 538 gallons of fuel oil, or 3.3 tons of anthracite coal. With average electricity use in the US of 11,280 kWh per year, (in 2014) each hectare of willow can power approximately five households annually or equivalent to two households per acre. These values DO NOT account for differences in conversion efficiency for each fuel type, nor do they account for the difference in energy content of wet wood chips compared with oven dry wood.
One of the biggest environmental benefits of shrub willow is its carbon neutrality with regards to the process of growth and combustion. The carbon dioxide absorbed by the plant during its growth is equal to the amount released by its conversion into heat or electricity. As compared to burning strictly coal, co-firing with biomass crops provides a significant reduction in other air pollutants as well, namely SO2, NOX, and particulate emissions. Mercury emissions are also highly reduced when co-firing biomass, and the ash content of willow biomass in dry weight percentage is 2-3%. Relative to the current U.S. grid, significant pollution prevention occurs with biomass-generated electricity, though there are still emissions that occur throughout the process.
Dixon, M.D. (2003) Effects of flow pattern on riparian seedling recruitment on sandbars in the Wisconsin River, Wisconsin, USA. Wetlands 23: 125-39.
Ramsey, J. and Schemske, D.W. (1998) Pathways, mechanisms, and rates of polyploid formation in flowering plants. Annual Review of Ecology and Systematics 29: 467-501.
Stokes, K.E. (2008) Exotic invasive black willow (Salix nigra) in Australia: influence of hydrological regimes on population dynamics. Plant Ecology 197: 91-105.
Stokes, K.E. and Cunningham, S.A. (2006) Predictors of recruitment for willows invading riparian environments in south-east Australia: implications for weed management. Journal of Applied Ecology 43: 909-21.
Thammina et al. (2011) In vitro regeneration of triploid plants of Euonymus alatus ‘Compactus’ (Burning Bush) from endosperm tissues. Hortscience 46: 1141-47.