Hybrid poplars (Populus spp.) are among the fastest-growing trees in North America and are well suited for the production of bioenergy (e.g., heat, power, transportation fuels), fiber (e.g., paper, pulp, particle board, etc.) and other biobased products (e.g., organic chemicals, adhesives). With the exception of the more arid regions, hybrid poplars can be produced throughout most of the continental U.S.

Members of the willow family, hybrid poplars are closely related to cottonwoods and aspens; the hybrids themselves represent crosses among various cottonwood species. Early French explorers in North America brought home eastern cottonwood (Populus deltoides), which crossed naturally with poplars in Europe (P. nigra). Poplars were used as windbreaks around fields, and fast-growing varieties were selected by farmers. Hand-pollinated poplar hybrids were first produced in Britain in 1912, and many European countries established plantations after the Second World War, in response to shortages of timber. Some of the European varieties were reintroduced to North America during the early 20^th century. Commercial planting of hybrid poplar did not commence until the 1970s although commercial plantings of cottonwoods can be traced to the 1960s.

Work initiated in the early 1970s by the U.S. Department of Agriculture Forest Service in Wisconsin became a cornerstone of the hybrid poplar research coordinated by the U.S. Department of Energy's Bioenergy Feedstock Development Program (BFDP) beginning in 1979. Since then, the task of improving hybrid poplar has been conducted by a national consortium that involves government researchers from several agencies, universities, and the private sector. Research has focused on reducing costs by improving yields, increasing pest and disease resistance, and developing efficient management systems. The program also seeks to document production costs and determine environmental impacts associated with hybrid poplar production. Favorable market conditions together with improved clones and knowledge has resulted in successful planting of approximately 90,000 acres (36,400 hectares) of hybrid poplar and cottonwoods in the U.S. for fiber use. Commercial plantings have been established in the Pacific Northwest, the Midwest, the Lake States, and the southeastern U.S.

Hybrid poplars, when grown under short-rotation silviculture, can produce between 4 and 10 dry tons of wood per acre per year (8-22 metric tonnes per hectare per year) and achieve a height of 60 feet (20 m) in as little as six years. This compares to yields of less than 1 ton/ac/yr for native forests and 2.5 ton/ac/yr for managed pine plantations. Hybrid poplar stands are typically planted at wide spacings ranging from 8 x 8 feet to 12 x 12 feet (300-700 trees/acre; 750-1700 trees/ha) and depending on geographic location, soil type, and end-use needs, allowed to grow for 6-12 years before harvest. Although hybrid poplars are capable of resprouting from their rootstocks after harvest, reestablishment is recommended to take advantage of improved hybrids and to minimize potential insect and disease problems.

In order to grow at their maximum potential, hybrid poplars need careful management. Hybrid poplars are expected to be grown on agricultural cropland using standard production methods. As with any agriculturally produced crop, proper site preparation and preplanting weed control are essential. Sites with adequate water supply and well-drained, slightly alkaline soils (pH 5-7.5) such as medium-textured, sandy-loam soils are recommended for the production of hybrid poplars. Sites are plowed to a depth of 10 inches (25 cm) and either manually (3 acre/person/day) or mechanically (20 acre/day with three people) planted with 10 inch cuttings. Cuttings are pushed into the ground with just the top bud showing, according to a predetermined grid pattern.

In the first year, weeds must be controlled using herbicides and/or mechanical methods. Additional weed control may be needed in years 2 and 3, but once the canopy closes, weeds are shaded out and further weed control is generally not needed. Insecticides are applied if necessary to control cottonwood leaf beetle during the rotation. Fertilizer applications are minimal and are called for only if nitrogen levels in the leaves fall below 3 percent on a dry weight basis. Typically this means that one or two applications of nitrogen of up to 50 lbs/acre (56 kg/ha) are required during the entire production cycle. Harvest utilizes standard forestry equipment widely available in the U.S.

As perennial crops, production of hybrid poplars can offer substantial environmental benefits compared to annual row crop production. Chemical and fertilizer applications are considerably lower, lessening the potential for chemical runoff and leaching. Hybrid poplars, as buffer strips, also intercept runoff of nutrients from fields near streams, rivers and wetlands. As perennial cover, wind and water erosion over the life of the rotation is less than that with annual crops. Hybrid poplars also provide increased year-round habitat for birds and small mammals compared to annual row crops.

Hybrid poplars offer a new opportunity to diversify income and production on agricultural croplands. However, as a new crop, there are new risks and challenges compared to continued production of traditional agricultural crops. Before undertaking large-scale production, farmers should understand how hybrid poplar production will fit into their farming operations and what potential markets exist in their production regions. Farmers should consult with local county extension personnel or others knowledgeable about hybrid poplars to identify appropriate hybrids for the area, determine appropriate management practices, and estimate expected yields, production costs and market value for the crop.

With the pressing need for diversification and stability in U.S. agriculture, and the demand for new domestic sources of clean energy, poplars are a new crop whose time has come.

Research Coordinator for Woody Energy Crops: Dr. Gerald Tuskan, Fax: +1 (865) 576-8143

Oak Ridge National Laboratory, P.O. Box 2008, MS-6422, Oak Ridge, TN 37831-6422