Invasive Weeds in Forests

Invasive weeds have many opportunities to establish and spread in forests. Many of these species are exotic, but many native species also proliferate when given disturbed soils and protected environments. Landowners are urged to keep these pests under control, if only for the public good. Once such weeds are introduced, they are very costly to eradicate.

Many invasive species are woody plants that have been propagated and distributed commercially. English holly, English ivy, gorse, and Scotch broom have been planted as ornamentals and have escaped to cover large areas of pasture and cut-over forest lands. Himalayan and evergreen blackberries, brought by settlers into this region for their fruit, are popular for recreational picking and eating. Broom and gorse have lovely yellow flowers. All are highly invasive, have persistent seeds, and will occupy ground rapidly. Birds spread them by foraging on fruits and seeds. Deer and other wildlife carry seeds to provide new colonies. These species are equipped to survive many obstacles. Programs to eradicate them may appear successful in the short run but are likely to fail in the longer term. Control requires persistence and good recognition as well as understanding their means of survival and propagation. Establishing a conifer crop that can shade out these exotics is an important approach to managing this problem. For more detail, see "Section W. Control of Problem Weeds" in this handbook.

Some exotic plants are still sold in commercial nurseries. English holly and English ivy pose major long-term problems: they are very tolerant of shade, they flourish under dense second-growth stands, and they are difficult to kill with herbicides normally in a forester's tool kit (see Section "Control of Problem Weeds" in this handbook for more information on ivy control). The key to successful management is early recognition and eradication before plants get big enough to produce seed or reproduce vegetatively. Ivy and holly produce seeds when growing in shade; hence seedlings must be removed before plants reach seed-bearing size. Ivy, which must climb into upper crowns to produce seeds, will develop crowns capable of shading out second-growth Douglas-fir. Holly can create dense stands from scattered understory seedlings, eventually leading to shade so dense hemlock trees cannot survive. These attractive plants are serious pests. Periodic inventories of forest stands will determine where problems are likely and where attention should be directed next. For example, holly seedlings are often found along powerlines where birds tend to roost.

Many invasive plants are spread by machines, including logging equipment and various motor vehicles. One of the most important conduits for invasive plants is roads. Road graders, ditch cleaners, bulldozers, car tires, log trucks, and many other types of machinery that travel along roads pick up seeds and vegetative structures and move them to new areas. Maintaining weed-free road shoulders and backslopes is the only certain way to prevent spread. Some landowners require that all equipment brought in to do logging and road work be washed down before entering and leaving the property.

Weed control programs designed to maintain visibility along roads may help to reduce the spread of invasive species. They also provide unoccupied habitat where seeds, viable stem pieces, and whole plants have room to grow. Unless sources of contaminating plant material are kept under control, these species will spread through a road system and into clearcuts or otherwise disturbed areas. Use herbicide mixtures that favor the establishment of competitive cover such as low-flammability grasses while removing brush, and use native plants that are less likely to become weeds themselves. Low, dense vegetation beyond the road shoulders is a good way to minimize invaders.

Keep a road system relatively free from invasive plants by maintaining a section of road right-of-way as free as possible from invasive plants, long enough to prevent invasive plants from moving effectively. Control of vegetation on road shoulders with a broad-spectrum herbicide that inhibits germination or vegetative propagation can be effective if the general road system is relatively free from invasive plants. Patrol these roads two to three times each summer to recognize and control small invasions, and thus prevent their spread. Clean-up of small patches is far cheaper than clean-up of established exotic stands.

Many species, such as horseweed (marestail), spread very rapidly after fire or clear-cutting, especially if other weeds (i.e., cover that competes with the invasive species) are controlled with products with little soil residual in late spring. Horseweed germinates later than many weeds and also can develop herbicide resistance. Certain products (e.g., clopyralid and directed applications of aminopyralid), will control this weed, especially if applied in early summer. Aminopyralid is not selective over conifers.

Blackberries (Himalayan and evergreen species) are especially troublesome along rights-of-way where they spread across fences. Herbaceous species too numerous to mention are capable of colonizing new clearcuts and understories of thinned stands. Species such as Brachypodium (false brome), prickly lettuce, sweet cicely (a native or naturalized exotic), Roberts geranium, and many others will invade disturbed understory sites and exclude native herbs. Where wildlife species prefer native plants for forage, the invasive plants may degrade habitat by replacing native cover. One seldom knows the true cost of such invasions, yet the mixture of these invaders may be difficult to control economically until the site becomes occupied with desirable and dominant cover. Good site preparation and timely release often help minimize invasive species, but also may create openings for invasion if products with little residual activity are used.

The diversity of invasive plants almost guarantees failure to control all of them. Clearly, the best control is a dominant cover with a group of species that casts a dense shade. Douglas-fir and western red cedar are valuable species that cast dense shade from about ages 13 to 40 years in un-thinned stands. Western hemlock and grand fir may provide better shade than Douglas-fir and cedar, but tend to be lower in value. Regardless of the overstory species present, thinning will offer new opportunities for invasive plants as well as increase the growth of the conifer crops. In controlling invasive species, it will be essential to follow thinning with a weed control program designed to avoid damaging overstory trees and desirable forage species. This technology is in its infancy.

Identification of potential invasive species is essential, so that appropriate and common controls can most effectively prevent their establishment. Many herbicides and herbicide combinations are registered for use in young stands, and each will control a certain spectrum of weeds, depending on dosage and time of application. The nature of disturbance in the logging operation also will influence the composition of invaders. Site history (i.e., whether broom, blackberries, gorse, or other plants with persistent seeds were present before the current stand was harvested) will provide warning of a seed bank not yet emerged. One problem that will become worse is treatment tolerance: some exotic species may have a certain percentage of plants that tolerate a given treatment, and over time these may become resistant. It is especially important to remove such resistant individuals before they spread across the landscape. This, however, requires surveillance beyond most of our capabilities, and this problem will require extensive attention.

Recommendations for Directed Spot Spray, Tree Injection,
and Basal Bark Treatment

Hand applications of herbicides can be very effective in controlling individual plants or small areas of weeds. The information below lists applications for directed foliage spraying using common herbicide products. Following this is a species specific table with information on foliage sprays, basal applications and cut surface stem treatments. Spot foliage treatments can use larger volumes of spray solution than broadcast sprays. The table assumes volumes applied at 50-100 gallons per acre. Users should always read the label of the products they are using to make sure they don't exceed any per acre maximum listed and adjust the herbicide concentration accordingly. When spraying around conifers or other desirable plants, avoid foliage contact especially during active growth.