Shrub Willow - Bioenergy Crop Plants - page 694

28.4 cultIvatIon oF shruB WIlloW For BIomass ProductIon
Although shrub willow research has often been associated with forestry, the agronomic practices
are much more akin to conventional agriculture of perennial plants. Proper and thorough site
preparation is critical for successful establishment of shrub willow bioenergy crops. A likely
scenario for the expansion of perennial bioenergy crop acreage is the conversion of underutilized and
abandoned farmland to the production of biomass. Often, these particular fields are underutilized
and abandoned for good reason—their characteristics make those sites unsuitable for annual crop
cultivation, including but not limited to difficulty in plowing of shallow, rocky, or clay soils; poor
drainage leading to periods of flooding; unbalanced pH of soil—usually acidic; and small or irregular
parcel size or shape, which complicates harvesting logistics. As agricultural fields fall out of regular
cycles of cultivation, they become prone to the establishment of annual and perennial weeds and
undesirable perennial plants. Many of these fields may be old pasture, low-value hay fields, or
abandoned agricultural fields that have reverted to fallow scrub land. On the basis of these typical
characteristics, the conversion of these fields back to cultivation, including the killing and removal
of existing vegetation, soil tillage, planting of bioenergy crops, and control of weed competition,
can be extremely challenging and prone to failure. The greatest challenge to the establishment
of perennial feedstock crops, especially on previously fallow land, is weed management. At its
worst, weed competition can cause total crop failure, which is extremely expensive considering
that establishment costs for willow are approximately $1,900-$2,400/ha (T. Buchholz and T. Volk,
personal communication). Poor weed management leads to proportionate decline in establishment
and yield per hectare for multiple rotations.
When planting willow bioenergy crops on land that has been in regular annual crop rotation, site
preparation in the spring is effective because the field is likely to be largely free of perennial weeds,
their seeds, or rhizomes. Proper site conversion from fallow land must be started in the late summer
and fall in anticipation of spring planting. This involves application of appropriate broad-spectrum
herbicides to kill the existing vegetation. For hay fields, it is best to cut, bale, and remove tall growth
a few weeks before herbicide application to ensure maximum effectiveness. Ideally, fields can be
cleared, plowed, and planted in mid-summer with a vigorous cover crop such as buckwheat or a
legume that can add fixed nitrogen to the soil. Although the standard practice has been to perform
moldboard plowing to break up any compaction or plow pan layer, it may be more efficient to use
ripper shanks followed by heavy disk harrows or zone tillage equipment, especially on rocky fields.
For fields that are prone to erosion, it is best to plant a winter cover crop, such as winter rye ( Secale
cereal L.) (Volk 2002). In the spring, any emerging perennial weeds should be killed with another
application of broad spectrum herbicide (such as glyphosate) before disk harrowing and leveling of
the field with a cultipacker-type roller.
Planting operations can begin as soon as possible after the threat of frost is over in the spring,
which is usually from late April to early June, depending on the local climate. Planting can continue
through early summer, as long as there is ample opportunity for rain, which is critical for willow
establishment. Obviously, the earlier that plants are established, the greater the opportunity for
growth during that first season. Stands of willow energy crops are established by the vegetative
propagation of unrooted stem cuttings and, as a result, are populated with clonally-derived
germplasm. For small plots, cuttings can be pushed into the soil by hand, but for commercial-scale
acreage, mechanical planting systems are used that have been specially designed for this purpose
(Figure 28.3). These planters use long pieces of 1-year-old stems, called “whips” or “switches,”
which are then cut progressively from the base of the whip into approximately 20-cm-long pieces
that are pushed into the soil as the planter moves across the field. Because whips are typically 2-3
m in length, a single stem can be used to establish 10-15 new plants. Willow planters have been
designed and built in Sweden and Denmark by Salixphere and Egedal, respectively, and attached
to a medium-sized (~125-140 hp) tractor. The standard models plant four rows simultaneously,