Plan Now for Noxious Weed Prevention

By Roger Sheley and Kim Goodwin

9/20/2000 BOZEMAN -- The most effective method in managing noxious weeds is to prevent their invasion into weed-free areas. In order to limit noxious weed invasion into weed-free areas, managers could contain small, nearby weed infestations by treating the infestations at the proper time with herbicides. Managers could also limit noxious weed invasion into weed-free areas by immediately revegetating with competitive grasses after any soil disturbance and by encouraging beneficial vegetation through the avoidance of improper grazing.

            A first step in preventing large-scale noxious weed infestations is quick eradication of individual weeds or small infestations through a landscape monitoring program that should be implemented two to three times during the growing season. A spring survey should be conducted to detect weeds early enough to allow cost-effective chemical control and / or removal through hand-pulling.

            Another landscape survey should be implemented during the summer and early fall. A summer survey will allow herbicide and / or mowing control of the plants that have matured to the flowering stage. An early fall survey will allow managers a final opportunity to control a small infestation by limiting seed dispersal through hand-clipping and burning (when safe to do so) of the seed-heads. In most cases, herbicide applications during this time can be effective if the weeds have not reached dormancy. However, applications will not affect seed dispersal into weed-free areas.

            Limiting seed dispersal of nearby infestations is central to preventing invasion into adjacent weed-free areas. Try not to hike or drive vehicles / machinery through weed infested areas during the seeding period. If this is unavoidable, be sure to thoroughly clean clothing and vehicles / machinery of seeds prior to entering weed-free areas. Also, during the seeding period, try not to graze livestock in weed infested areas. Seeds adhere to their coats and pass through their digestive tracts to infest rangelands that are weed-free. If grazing livestock in weed infested areas is unavoidable during the seeding period, place the livestock in a holding area for 14 days to allow passage of seeds before being moved to weed-free areas.

            Managing landscapes and rangelands to prevent noxious weed invasions should be based on the current vegetation type and condition. A recent Montana State University study documented that a diverse, well-established plant community may better resist weed invasion than those plant communities that are less diverse. In other words, landscapes that contain a diversity of healthy, vigorous vegetation and very little bare ground have the ability, in most cases, to deter noxious weed invasion. It is important to encourage beneficial vegetation growth by avoiding improper grazing of landscapes.

            When desirable vegetation is heavily grazed during the growing season, stress is applied and, subsequently, vigor against noxious weeds is reduced.

This encourages invasion leading to reduced forage production. Heavy grazing during the growing season also opens the shade canopy that was previously provided by the grasses that further deterred noxious weed establishment. To prevent improper grazing, implement season alteration of use, rotation of livestock to allow vegetation to recover before being re-grazed and promotion of litter accumulation on rangelands.

            With early fall, noxious weed infestations that have not been previously managed or contained this year have probably reached the seed dispersal stage. At this point, the only option available to contain infestations by limiting seed dispersal is to hand-clip and burn (when safe to do so) the seed-heads. If infestations are too large to clip and burn, application of herbicide on a non-dormant infestation can be helpful in affecting the infestation this year and can continue to affect the infestation next spring. However, applications will not affect dispersal of seed into weed-free areas. Re-seeding disturbed areas now with competitive grasses is another opportunity for prevention of noxious weed establishment next spring.

            For more information, contact your local county extension agent or weed district supervisor.

Send questions or comments to Carol Flaherty, MSU Communications Services, Bozeman, MT 59717 or email Roger Sheley and Carol at carolf@montana.edu.

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