Environmental Engineering Reference
In-Depth Information
production for decades. Because willow stems exhibit weak dormancy and are capable of initiating
growth even at very cool temperatures, whips and cuttings must be stored frozen until just before they
are planted. However, exposure of whips or cuttings to temperatures below -12 to -16°C (10 to 3°F)
can cause tissue damage and reduce the viability of cuttings (Volk et al. 2004a). Whips and cuttings
should also be packaged in bags or in lined boxes so they do not dry out while in the freezer. Within
4-5 days of removal from the freezer to room temperature or above, the stem sections will start to
grow roots and the buds will break to form shoots. Refrigerated trucks are critical for the proper
transport of whips to the field and short-term on-site storage just before planting.
Immediately after planting, pre-emergence herbicides are applied to prevent the germination
of weed seeds. Pesticide regulations and product labeling vary tremendously depending on local
authorities, so growers need to be well informed about product applicability. In the United States,
some products that have been used in research and development trials include oxyfluorfen [1 lb
active ingredient (ai) per acre Goal 2XL, Dow AgroSciences] in combination with simizine (2 lb ai/
acre, Princep, Syngenta) or pendimethalin (2 lb ai/acre Prowl 3.3EC, BASF) (Wagner 2000). When
pre-emergence herbicides are not used or available, mechanical cultivation should be applied in a
timely and frequent manner throughout the season to prevent the establishment and growth of weeds.
Mechanical cultivation can also be applied during mid-season in fields where the pre-emergence
herbicide protection has begun to fail. Some postemergence herbicides have been used in research
trials to control weeds by spraying over the top of willow vegetation, including fluazifop-P-butyl
(FusiladeDX, Syngenta) for the control of grass weeds and clopyralid (Stinger, Dow Agrosciences),
which can control horseweed, ragweed, thistle, burdock, curly dock, and vetch. Further research
is needed to gain an understanding of varietal sensitivity to phytotoxic effects of each product,
to optimize application rates for varying soil types, and to gain legal labeling registration for
commercial use on willow bioenergy crops.
During the establishment year, willow plants will typically produce two to three stems that will
reach heights of approximately 2 m while at the same time building a below-ground diffuse root
structure. At the end of this first year when the plants are dormant, current management practices
call for the cutback of the stems, also termed “coppicing.” This is accomplished using a sharp
sickle bar mower attachment on a tractor or in smaller plots by hand using a gas-powered rotary
brush saw. Stems are cut within 4-8 cm of the ground, and that biomass is usually left on the field.
Coppicing stimulates vigorous sprouting of an increased number of shoots in the spring of the
second season. Some varieties can produce more than 40 shoots per plant in the year after coppice.
Coppicing also allows the grower the opportunity to mechanically cultivate between rows and/or
apply pre-emergence herbicide before willow bud break to accomplish improved weed management.
Standard practice is also to apply nitrogen fertilizer to the crop in the spring after coppice at a rate
of 80-120 kg N/ha (~100 lb N/acre). Solid fertilizer can be broadcast in the form of ammonium
sulfate or sulfur-coated urea, but other environmental benefits can be gained by using material from
a waste stream, such as animal manure, wastewater treatment plant biosolids, fermentation waste,
composted municipal yard waste, or other organic industrial waste products (Labrecque et al. 1998;
Adegbidi et al. 2003; Keoleian and Volk 2005). By the third or fourth growing season after coppice,
the stems have grown to heights of 7-9 m (Figure 28.4) with stem diameters averaging 2-7 cm
depending on varietal characteristics.
28.5 harvestInG and conversIon oF WIlloW BIomass
Willow is best harvested at the end of the third growing season after coppice which is typically the
fourth year of a newly established crop. The product of harvest is woody biomass, but the dimensional
specifications of the harvested and processed product will vary for each conversion application. The
logistics and energy input required for transport and drying are also critical considerations that
need to be matched with downstream conversion processes. Harvesting the stems after the plants
have entered dormancy has multiple advantages: the leaves and their significant accumulation of
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