Establishing and Tending Poplar Plantations in the North-Central U.S.

Daniel Netzer, USDA Forest Service, North Central Forest Experiment Station, Rhinelander, Wisconsin

Edward Hansen, USDA Forest Service, North Central Forest Experiment Station
Grand Rapids, Minnesota

ABSTRACT

Cultural methods and equipment are described for successful establishment of poplar plantations in the north-central United States. Methods use both ground and aerial equipment including standard farm machinery, newer style compact tractors, "four-wheelers", and helicopters.

INTRODUCTION

Establishing successful plantations of hybrid poplars requires thorough site preparation, effective weed control until tree canopy shade prevents weeds from growing, and other specific care. When properly established, plantations are ready for harvest in 6 to 10 years with yields ranging from 6.7 to 13.5 metric tons of wood plus bark per ha per year (Hansen and others 1993).

In this paper, we describe the cultural methods and equipment used in establishing and caring for hybrid poplar plantations. Our recommendations on site preparation, planting material, planting, weed control, fertilization, and insecticide application stem from a program of short rotation intensive culture (SRIC) multidisciplinary research that began at the North Central Station's Forestry Sciences Laboratory in Rhinelander, Wisconsin, in 1971. In 1976, the program expanded to incorporate both developmental and basic research at the Harshaw Forestry

Research Farm near Rhinelander. Supplemental funding in 1977 from the U.S. Department of Energy further expanded this research. In 1986 a regional network of plantations was established in Wisconsin, Minnesota, North Dakota, and South Dakota in cooperation with the Northern States Power Company and the Department of Energy. These plantations continue to provide information on biomass yields, clonal performance, and cultural treatments across a wide range of soils, climate, and weed conditions.

SITE PREPARATION

(Figure 1. Moldboard plow to a depth of 25 cm or greater for good site preparation)

Poplar plantations in the north-central U.S. are typically planted on tilled agricultural land. Large agricultural farm tractors and implements are used for site preparation. Implements include large moldboard plows or field cultivators, discs, and harrows . On fields in sod, pasture or hay, apply glyphosate (Roundup) and plow the site by midsummer, and leave it fallow during the remainder of the season. On fields subject to erosion, seed a fall cover crop, leave vegetated strips, or use other conservation tillage practices. On fall-harvested fields, apply glyphosate if perennials are present after harvest and plow a minimum of 25 cm deep (Hansen and others 1984). Field cultivate or otherwise till 25 cm deep a day or two before planting in the spring. Do not use no-till unless required for erosion control (Hansen and others 1986).

PLANTING MATERIAL

Most planting material is 25-cm-long hardwood cuttings with diameters of 1 to 2 cm. The cuttings must have well-developed buds, no disease, and no bark damage. Store them in plastic bags slightly below freezing. Warm and soak the cuttings for 5 to 10 days before planting. Make small slits in the bags, leave cuttings in bags, and immerse three quarters of their length in water (Dickmann and others 1980). Keep the cuttings shaded at a temperature of 10 to 21 degrees C. Cuttings are ready to plant when the buds start elongating, showing some bright green around the bud scales, and just before roots begin to grow. If weather prevents planting, cuttings can be held for weeks by placing them in cold storage at 1 degree C or by covering them with crushed ice (Phipps and others 1983).

PLANTING

(Figure 2. Unrooted hardwood cuttings are machine planted with one bud above the ground surface)

Poplar plantations are normally planted between mid-April and early June. The best time is when soil temperatures reach 10 degrees C, or when corn is being planted locally (Hansen 1986). Tree spacing ranges from 2.5 x 2.5 m to 3 x 3 m (1730 and 988 trees per ha, respectively). The width of available tending equipment for access between tree rows often determines row spacing. Narrow row spacing has the advantage of allowing tree crowns to shade out competing weeds as early as year three or four. Wider spacings may require continued tending operations through year five or six, or even through the entire rotation. Soils should be moist; otherwise delay planting until after rain (Hansen and Phipps 1983). Keep cuttings wet while planting. Hand plant using a marked wire-core rope for a spacing guide, or machine plant with a tractor driver and two 1-row planters. Hand planting rates are 4 ha/day/person; machine rates are 16 ha/day/3-person-crew. Plant cuttings with one bud above the soil surface (usually about 2.5 cm of the cutting exposed) (Hansen and others 1991). When hand planting, push cutting into soil by hand if the soil is loose. If soil is firm, use a dibble to make a hole and insert the cutting. Eliminate air pockets by pushing the soil against the cutting. Do not pack the soil. If machine planting, be sure that soil tilth is adequate for firm, but not excessive soil packing around the cutting. Many planting machine designs have been used with variable success over the years to plant unrooted cuttings . Problems with machine planting most commonly occur in rocky soils, wet soils, or soils that have not been well prepared. Hand planting must be used if cross-cultivation is planned.

WEED CONTROL

(Figure 3. Preemergent herbicides are oversprayed immediately following planting)

Use a combination of selected herbicides and tillage equipment to control weeds. Immediately after planting, overspray the field with the preemergent herbicide linuron using boom application equipment covering multiple rows. These booms are also used to apply selective grass herbicides (when needed) like sethoxydim directly over the tree tops until midseason of the first year. High clearance spray equipment will allow you to spray multiple rows until the end of the first growing season, and sometimes early into the second growing season.

(Figure 4. Rotary hoes are used to control germinating weeds in light (sandy) soils)

Rotary hoes can be used to till newly established plantations on light (sandy) soils to control emerging weeds until the trees are 30 cm tall . Taller trees become entangled and damaged by the rotary hoe blades. Rotary hoeing can be very effective if weather renders a preemergent herbicide ineffective. It is most effective if done every 7 to 10 days as weeds germinate; larger weeds are not removed with this equipment. Also, if cuttings are planted flush with the ground, the tender growing shoots can be buried by soil.

(Figure 5. Narrow four-wheel drive tractors are used for between-row operations)

Once the trees are too tall to straddle, smaller equipment must be used. Small old tractors, or newer compact tractors can be operated in plantations with row spacing as close as 2.5 m. The newer compact tractors of about 30 horsepower have maximum widths of less than 1.5 m and four-wheel drive options. The compact tractor with implement is efficiently transported with a one-ton pickup truck and trailer .

Narrow light discs (1.8 m wide and less), 1.5-meter-wide mowers, fertilizer spreaders, and small sprayers are easily pulled with these tractors. The shallow rooting habit of poplars requires shallow tillage (2.5 to 5 cm) during weed control to avoid damaging root systems. Discs are very effective for between-row cultivation because they can be towed at a higher speed (6 km per hour) and they remove large weeds. But discs compact the soil more than from other types of cultivation equipment. Rototillers, set very shallow, provide the most complete control of small weeds, but their slow travel speed makes them prohibitively expensive for large areas. Shovel cultivators with a variety of configurations and shovel types can be effective if they are set shallow and the frame does not scrape the trees. Combinations of tillage equipment with sprayers can be used to both band herbicides within tree rows and cultivate between rows in a single operation.

FERTILIZATION

(Figure 6. Four-wheel ATV's are used to apply granular fertilizer between rows of plantations)

Most planting sites in the Midwest have adequate nutrients for poplars for at least the first one or two years (Hansen, in press). However, nitrogen may be needed on some sites by year three or later. Granular nitrogen fertilizer can be applied by either ground or aerial equipment. Ground application of granular nitrogen is done with cyclone-type applicators pulled by either a small tractor or a four-wheel ATV (four-wheeler) . Alternate rows are traveled because it is difficult to apply fertilizer uniformly to more than two rows per pass. Helicopters with cyclone-type spreaders apply fertilizers much more quickly and uniformly than ground application systems. However, cost can be a limiting factor of aerial application.

INSECTICIDE APPLICATION

Severe damage to poplar plantations can occur from outbreaks of cottonwood leaf beetle (Chrysomela scripta). Defoliation, as well as damage to the growing tips of young poplars, can significantly reduce growth and in some cases cause tree mortality. During the first growing season and occasionally early in the second growing season, pesticides can be applied with standard farm sprayer boom systems. As the trees get taller, fog systems pulled by small tractors can be used. However, at tree heights greater than 5 m, ground application systems become ineffective. In those situations, and even in younger plantations, insecticide application by either small airplane or helicopter can be effective and economically competitive.

SUMMARY

Many types of standard agricultural equipment are being used to establish and tend hybrid poplar plantations in the north-central U.S. Successful plantation establishment requires good site preparation followed by rigorous weed control during the first several growing seasons. Small compact tractors allow the tending of plantations with rows spaced as close as 2.5 m. At this spacing, the plantation canopy may shade out weeds in 3 or 4 years much sooner than at wider spacings where tree crowns may never completely shade out weeds through an entire 10-year rotation. No matter what equipment is used be sure to select good clones, plow deep for adequate site preparation, till shallow for good weed control, and KEEP THE WEEDS OUT!

REFERENCES

1. Dickmann, D.; Phipps, H.; Netzer, D. 1980. Cutting diameter influences early survival and growth of several Populus clones. Res. Note NC-261. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station. 4 p.
2. Hansen, E.A.; Netzer, D.A.; and Woods, R.F. 1986. Till superior to no-till for establishing hybrid poplar plantations. Tree Planters' Notes. 37(1): 6-10.
3. Hansen, Edward A. 1986. Planting date affects survival and height growth of hybrid poplar. Forestry Chronicle. 62(3): 164-169.
4. Hansen, Edward A.; Phipps, Howard M. 1983. Effect of soil moisture tension and preplant treatments on early growth of hybrid Populus hardwood cuttings. Canadian Journal of Forest Research. 13: 458-464.
5. Hansen, Edward; Netzer, Daniel; Rietveld, W.J. 1984. Site preparation for intensively cultured hybrid poplar plantations. Res. Note NC-320. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station. 4 p.
6. Hansen, Edward A.; Netzer, Daniel A.; Tolsted, David N. 1993. Guidelines for establishing poplar plantations in the North-Central U.S. Res. Note NC-363. U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station. 6 p.
7. Hansen, Edward; Tolsted, David; Tower, Matthew. 1991. Planting depth of hybrid poplar cuttings influences number of shoots. Res. Note NC-355. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station. 4 p.
8. Phipps, Howard M.; Hansen, Edward A.; Fege, Anne S. 1983. Preplant soaking of dormant Populus hardwood cuttings. Res. Pap. NC-241. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station. 8 p.

File posted on March 5, 1996; Date Modified: February 21, 1999