Using Herbicides in Silviculture

Herbicides can be used to kill undesirable trees and other unwanted vegetation during both intermediate treatments and site preparation. They can be applied by:

1. Spraying or mistblowing it onto the foliage.
2. Injecting it into or spraying it onto the bark of target woody plants.
3. Spraying it over the stumps of cut trees and shrubs that might resprout.
4. Spreading the substance over or incorporating it into the soil.

Herbicides can be applied to individual stems, to spots or bands within a stand, or as broadcast applications. Factors influencing the choice include: (1) the scale of operations, (2) the cost of the herbicide, (3) the conditions of topography and obtrusive vegetation, (4) the purposes for the work, (5) the species involved, and (6) the amount a landowner will spend for the treatment (Nyland 1996).

Applications of herbicides for site preparation provide a number of advantages. Along with those advantages are concomitant disadvantages, in addition to precautions and environmental concerns that are less commonly encountered with other site preparation methods.

Individual Stem Treatments

Individual stem treatments substitute for tree cutting, girdling, or bulldozing. Commonly used compounds include picloram, tricopyr, and glyphosate. Techniques include injecting herbicide into frills or cuts that encircle the main stem, applying it to the bark as a basal spray, or spraying the top and sides of a stump. Special tools for stem injection include hypo-hatchets and tubular tree injectors. Herbicides for stem injection are often diluted in a water-based carrier. By contrast, herbicides for basal spraying are in an oil-based carrier because the active chemical must translocate into living tissues through the bark. Basal sprays usually prove effective for thin-bark species or small-diameter trees and shrubs. Along with spot applications, individual-stem treatments generally have greater efficacy than broadcast methods. However, individual-stem treatments normally cost more than broadcast applications if more than 400-500 stems per acre (990-1235/ha) need to be removed (Sage 1987).

Generally, foresters consider individual-stem treatments to be environmentally safe, because there is little chance for drift onto nontarget plants. Plus, the compounds quickly translocate into the stem after application and degrade within the treated tree shrub (Daniel and others 1979). Foresters may also hand-spread granular herbicides to treat individual trees and shrubs in small-scale operations. Soil-active chemicals work in two different ways. Some, such as granular hexazinone or imazapyr, enter the root systems and translocate into tissues of target plants. Other soil-active herbicides prevent germination of seeds, primarily of herbaceous species. Examples include atrazine, hexazinone, oxyfluorofen, simazine, and sulfometuron. However, tests with white pine in the Appalachians indicated that treating soil with granular herbicide closer than 3 or 4 feet from a 3 feet tall tree with a 0.9 in. basal diameter might kill it. In stands with larger saplings, workers should keep the herbicide spots even farther from the pines (Wendel and Kochenderfer 1988, Nyland 1996).

Spot and Band Application

Spot and band applications of herbicides are used when vegetation does not need to be controlled across an entire site, or where safety or a pest dictate a more selective approach. Often, backpack mistblowers are used for spot treatments. For band treatments, a sprayer is attached to a tree planting machine and used to treat a narrow band of vegetation within 2 to 3 feet of the planted dormant seedlings. Alternatively, herbicides can be applied during the growing season prior to planting (Nyland 1996).

Broadcast Application

Herbicides are broadcast to treat vegetation over a large area. Thay often are applied with helicopters or fixed-wing aircraft. A common use is to kill understory plants after a reproduction method cutting. Commonly used compounds include glyphosate, phenoxy, picloram, triclopyr, dicamba, and fosamine in water. These compounds can be used at relatively low volumes per unit of area. For example, effective control with glyphosate requires only 0.8-1.6 gal (3-6 l) of active ingredient in a total spray volume of as little as 5.3 gal/acre (20 1/ha). With aerial application, experienced crews can treat 1,000-1,800 acres (400-730 ha) per day by aircraft operations (McCormack 1991). Many conifer species survive low-dosage sprays that kill broadleaved competitors. In spruce-fir stands in the Northeast, triclopyr and glyphosate compounds kill broadleaved species such as maple, aspen, and birch without damaging conifers. Phenoxy herbicides leave more hardwood cover. Because most herbicides delivered from the air enter a plant through its foliage, foresters may actually postpone a treatment for up to 3 years to realize maximum benefits on high-quality sites, and 5 years on poorer ones (McCormack 1991). Granular herbicides absorbed through root systems may provide some added flexibility in the timing of an aerial treatment, because they must dissolve into the soil solution prior to uptake (Nyland 1996).

Ground application methods can be used for smaller stands or understory treatments prior to a reproduction cutting. These techniques may use backpack mistblowers, low-volume sprayers or granular herbicide spreaders mounted on 4-wheel-drive tractors. However, these ground applications have some disadvantages over aerial broadcast methods, including (McCormack 1991):

• Increased costs, especially in treating large areas.
• Rugged and irregular terrain makes uniform application nearly impossible.
• More herbicide and spray solution are needed.
• Tractors used to transport the sprayers or mistblowers may damage existing regeneration.
• Terrain, debris, standing trees, and other obstacles may impede access.
• Workers run a higher risk of exposure.

These ground applications work better for smaller areas with realtively sparse vegetation and less than 20 feet (6 m) tall (Nyland 1996).