Weed Control Strategies for SRIC Hybrid Poplar Plantations: Farmer’s Perspective

Bill Schuette and Chuck Kaiser, James River, Clatskanie, OR

Several speakers during this conference have mentioned the importance of weed control to their programs when growing SRWC species. Westvaco aptly demonstrated the need during the excellent field tour of both their river bottom and irrigated plantations. From a farmer’s perspective the whole topic can simply be stated as “Weeds are Bad”.

Why is weed control so necessary for short rotation woody crops such as hybrid poplar? One clue may be found in the ecological mechanisms which lead to establishment of native cottonwood stands. These stands are often established from seed and vegetative material deposited on freshly scarified riparian areas. Native stands are also subjected to repeated flooding that can limit herbaceous growth during the establishment period. Secondly, much silvicultural research has been conducted throughout the United States confirming the ecological effects of weed competition on hybrid poplar. A review of this research verifies that stand growth and yield can be improved significantly by controlling weeds during the early years of establishment. Figure 1 demonstrates the effect of four levels of weed control on a hybrid poplar stand in the Pacific Northwest. Finally, protection problems can be dramatically increased when weed control is not done in poplar plantations. Unwanted vegetation can become habitat for pests such as rodents, slugs and insects.

Most agronomic strategies for controlling weeds begin at site preparation and continue during the first two to three years of plantation establishment. The machines and methods for accomplishing this strategy may vary from region to region but usually consist of mechanical, chemical, and manual cultivation, or a combination of all three. Intensive site preparation begins with mowing or flailing to remove tall grasses and broad leaf weeds and to stimulate new growth. When new vegetation begins to flush, the field can then be sprayed with an emergent herbicide (glyphosate or 2,4-D) and allowed to sit until the weeds begin to breakdown. The field is then disked or ripped twelve to fourteen inches deep to prepare a proper seed bed for planting. Variations of this strategy include the sowing of cover crops between tree rows to stabilize soil or provide wildlife forage. This site preparation is essential for controlling existing weed rhizomes. It also stimulates the germination of the “seed bank” that has accumulated over the years especially if the ground had been in a less intensive cultivation regime prior to plantation establishment. At the same time, controlling the existing vegetation also acts to reduce production of new weed seeds. James River Corporation in the Pacific Northwest will prepare raised beds 24 inches wide by 18 inches high as a final operation in the fall prior to planting the following spring. This “hill”, besides providing a guide for hand planting also stimulates earlier spring growth by warming the soil faster and elevates the cutting out of detrimental field conditions such as standing water. Other operations will rip planting slits to mark tree rows and facilitate planting.

Herbicides are applied again just prior to planting often using combinations of emergent and pre- emergent chemicals. The emergent herbicide is needed to control winter annuals that have grown since the cessation of fall site preparation and the pre-emergent is used to control any spring annuals germinating from the existing “seed bank”. Herbicide applications are often applied as a band along the tree rows when mechanical cultivation is expected to follow during the growing season. Herbicide bands are typically between four and five feet wide to allow for a complete weed free zone around the trees and enable mechanical cultivators to remove the between-row vegetation in one pass. A dormant season application of both emergent and pre-emergent chemicals is often applied in one year old stands to control winter annuals that grow after mechanical and manual cultivation cease in the fall. Some herbicides are available for use during the growing season but often are restricted to highly selective chemicals which go after one type of weed. Normally these very expensive chemicals are used as a last resort when conventional methods have failed.

A combination of both chemical and mechanical operations are critical in the early stages of plantation establishment. Figure 2 represents a field study conducted by James River which evaluated options for plantation establishment in western Oregon. A combination of chemical and mechanical techniques provided the best survival and tree growth during the first year of establishment. Chemical and mechanical techniques together often provide the operator more options and more flexibility to adjust to seasonal conditions.

Mechanical weed control can take many forms. Typical between-tree cultivation is accomplished by either rototilling or disking. Both have their strengths and weaknesses. Rototilling gives you better weed control closer to the trees and provides a “soil mulch” for better moisture retention in heavier soils. However, it is much slower than disking and requires multiple sets of machinery to be able to cover a large acreage. Disking has the benefit of using less energy and covering more ground per day but can also dry out the soil. Disking has an added benefit that it can be used on ground having woody debris from post harvest site preparation and can handle higher amounts of vegetation without clogging. Other methods that have successfully been used on first rotation fields are within-the-row cultivators originally designed for the grape industry and manual cultivation (hoeing). Hoeing is sometimes needed to control weeds close to the first year trees when pre-emergent herbicides do not function as intended and mechanical cultivators risk damage. Cross cultivation can reduce some need for hoeing but requires that plantations be planted on a perfectly square spacing. Cross cultivation can increase the chance of mechanical injury. Mowing between trees has benefits when soil loss is a concern but often does not provide the same growth benefits as mechanical cultivation (Figure 1).

Weed control in hybrid poplar stands will continue until the stand has closed canopy and “captured” the site. This may take two to three years in the Pacific Northwest depending on the initial spacing, site quality and clone used. Some stands of eastern cottonwood may never fully close canopy allowing for a permanent understory to develop. Weed control in these stands will often occur until the trees are established and free to grow.

The farmer did a good job of controlling weeds during the first rotation, but now he has a post harvest field consisting of woody debris, limbs, broken pieces of trees, and a few hundred stumps to the acre. Unfortunately, most farmers did not count on this and are unprepared for handling these conditions. Can the same equipment be used as on the first rotation? Probably not. James River Corporation in the Pacific Northwest has chosen to treat second rotation fields as an agricultural operation but has altered the farm equipment to handle the debris and stumps. The large, four-wheel drive tractor used to pull the site preparation disk has had its agricultural tires replaced with forestry, skidder tires. One-half inch steel plates have been welded on to armor the under side of the tractor. Two plastic fuel tanks normally found on each side of the tractor have been replaced with one steel tank mounted over the rear drive wheels. The large break down disk used during the first rotation has been replaced with a heavier, tougher forestry type disk capable of handling stumps and woody debris. Another method that is being employed by both Crown Vantage at Fitler Managed Forest and Westvaco includes using a large bulldozer to shear the stumps just below ground level. A wood rake is then used to wind-row the debris. Bulldozers fitted with brush rakes push the wind-rows into piles which are then burnt. Other methods include the use of orchard flails to grind larger woody chunks into smaller more manageable pieces and the Merricrusher used by MacMillan Bloedel in the Pacific Northwest. Work is also continuing to develop stump grinders that will grind the debris and stump and reincorporate them into the soil. Herbicide and mechanical treatments continue as described previously with the exception that disks are more often used for mechanical cultivation for their ability to handle woody debris. Why go to such great measures to do site preparation? First, the weed bank is still there and needs to be handled. Second, stumps are hard on tractors, equipment, and the people needed for cultivation.

So what’s new? A program is now underway to genetically engineer hybrid poplar clones for resistance to some insects and herbicides. Many SRWC companies have joined the Tree Genetic Engineering Research Cooperative based at Oregon State University to work on developing and licensing these clones for operational use. Studies are on-going and some exciting results with glyphosate resistance have already been achieved. If successful, costs associated with hoeing and mechanical cultivation will be greatly reduced. New herbicides are being marketed and tested for compatibility with SRWC species. Many of these chemicals have lower active ingredient rates, less mobility in the soil and better weed control spectrums. In addition, labeled chemicals for poplars are being re-evaluated in different combinations to specifically target certain soil types and environmental conditions. Weed barriers currently being used in vegetable production are also being evaluated for use with hybrid poplar and other SRWC species. The film can be mechanically laid down to form rows with trees being planted directly through the plastic. The film will suppress most of the weeds around the tree and help conserve moisture during the drier parts of the growing season. Cost is a big concern with this technology, however, but it may have application to some situations. There is also concern that some of the plastic residue may find its way to the mill if the trees are processed in the field.

In summary, weed control in hybrid poplar and other SRWC species is essential for plantation success. The methods and intensity of weed control can vary by site, previous land use, tree spacing, clone, and equipment availability. This paper has examined some of the strategies which are successfully being employed to grow poplars in North America, however, any weed control strategy should be refined to the local conditions. Significant alterations in weed control can occur from the first to the second rotation due to post harvest field conditions. New technologies show promise for reducing weed control costs but applications may vary across regions.

File posted on March 17, 1998; Date Modified: February 21, 1999