Exclusion

Exclusion strategies decrease the likelihood of invasion by reducing the frequency of repeated weed introductions. The number and frequency of weeds arriving at a site seems to be the main component of successful invasion. Repeated introductions are frequently associated with terrestrial and water pathways or dispersal corridors, such as roads, railways, trails, rivers, irrigation ditches, and streams. Repeated human patterns of movement, such as cars and contaminated equipment traveling on roads, is the main dispersal factor for terrestrial pathways, and water is the agent that supports downstream weed movement. Periodic disturbance along roads and waterways creates openings in vegetative cover that encourage weed establishment.

Because human activity increases the number of weed introductions and disturbance, it is likely responsible for the quickest and most distant weed spread. Exclusion strategies, therefore, should focus on interrupting weed movement where humans are the dispersal agent. Preventable means by which weeds can be introduced from human action include roads and railways, cattle shipments, livestock movement from “weedy” areas to weed-free rangeland, and contaminated equipment and materials, such as forage and feed grains, seeds, soil, and gravel. Non-local angler and hunter activity may also be increasing accidental introductions. For example, recent studies have documented that an increase in the number of human visitors to an area leads to an increase in the number of invasive weeds.

Prevention strategies may include requesting weed-free material or closely monitoring sites for new weeds where at-risk material was applied. Another strategy is to hold livestock in easily accessible pastures for five to seven days prior to release into weed-free ecosystems. When these pastures are not available, livestock could be fed weed-free forage before being moved to weed-free areas. Clean-out sites could be designated for livestock shipping trailers so plants that establish can be destroyed before they set seed. Prevention strategies may also include posting signs that request anglers and hunters to clean their boots, dogs, horses, boats, and equipment prior to entering weed-free areas. Parking in designated areas and remaining on trails with all-terrain vehicles could make monitoring easier for landowners. The booklet entitled Invasive Plant Prevention Guidelines is a helpful reference for identifying site-specific prevention strategies. It is published by the Center for Invasive Plant Management and is available for purchase from the Extension Publications office at Montana State University.

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Last Modified: 08/02/2007