Environmental Engineering Reference
In-Depth Information
FIGure 28.3
Planting shrub willow using whips loaded into a mechanical four-row step planter.
although some six-row willow planters have been designed. Typically each person riding on the
back of the planter can load whips into two planting cartridges, thus a crew of two or three plus a
driver are needed for planting operations. For a single planter, the average rate of planting is 2-3
acres (1-1.5 ha) per hour or approximately 20 acres (~8 ha) per day with a total seasonal capability
of planting approximately 800 acres (~320 ha).
Planting density and row spacing have been optimized to attain maximal yield in a 3- to 4-year
harvest cycle while minimizing the cost of planting stock per acre (Kopp et al. 1997; Bergkvist and
Ledin 1998; Wilkinson et al. 2007). The usual standard for final planting density is approximately
15,000 plants/ha (~6,100 plants/acre) in a double-row arrangement with 0.6 m (24 in.) between
plants in a row and 0.76 m (30 in.) between rows, with 1.52-m (5-ft) alleys between the double
rows (Bergkvist and Ledin 1998; Abrahamson et al. 2002). This arrangement allows harvesting
equipment to straddle the double row and efficiently harvest two rows in a single pass. However,
many growers have noted that the closely spaced double rows make weed control by herbicide
application or mechanical cultivation more difficult than wider single-row spacing (F. Allard and
D. Rak, personal communication). As larger and larger harvesting machines arrive in the market, it
will be necessary to widen the spacing of the alleys so that the harvester tires will fit inside of the
rows adjacent to the double row being harvested.
Willow planting stock (whips and cuttings) is grown in dedicated nursery beds that are harvested
annually and may be irrigated and fertilized to optimize production. Between the time when the
plants have gone dormant and have shed their leaves, but have not yet broken bud in the spring, stems
are cut using single-row, whole-stem harvesters that saw the stems just above the ground and collect
them in a bunk or wagon. Subsequently, they must be processed by trimming off any branches and
removing any portions of stem that do not conform to standards for diameter and straightness. If
whips are too large in diameter or if there is curvature, they can jam in the planter, depending on the
model. If the diameter is too small, they may bend as they are being pushed into the soil. Depending
on the species, a single plant can produce as many as 10-12 stems that are at least 2 m in length
and of sufficient diameter such that 50-100 cuttings may be produced per plant. The plants in the
nursery beds will resprout the following spring and will continue to produce stems for annual cutting
