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 and can inflict serious harm on forest sites.
Many invasive species are plants that have been propagated and distributed commercially. English holly, English ivy, gorse, and Scotch broom, pampas grass, butterfly bush and false brome have been planted as ornamentals and have escaped to cover large areas of pasture and 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. Others, like knapweeds, starthistle, mare's tail, and exotic thistles, are widely distributed nationally and even globally. All are highly invasive, have persistent seeds, and will occupy ground rapidly. These species are equipped to survive many obstacles. 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 specific weed detail, see "Section W. Control of Problem Weeds" in this handbook.
Invasive plants can be 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 soil with seeds and vegetative structures and move them to new areas. Maintaining weed-free road shoulders 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.
The key to successful management is early recognition and eradication before plants get big enough to produce seed or reproduce vegetatively. The state department of agriculture in each state maintains lists of invasive and noxious weeds important locally, often online with pictures of major weeds and maps of their occurrence. Use these resources to keep road systems 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. Landowners may also have additional herbicide control options on roadsides since they are considered a separate use site from forest management sites. Herbicides useful for controlling plants on roadsides and non-crop sites can be found under the section "Non-Cropland and Right-of-Way" in this handbook.