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September a good time to apply herbicides for noxious weedsBy D. Bruce Bosley CSU Extension Agent, cropping systems Colorado September is a great month to apply herbicides for controlling perennial and biennial noxious or nuisance weeds. These are the plants that don't rely on seeds to replenish themselves next spring. Instead, perennials, store enough energy through the summer growing season to allow them to over winter and "spring" back next year when conditions are again favorable for plant growth. Perennial plants as a plant category are very common when you consider trees, shrubs, bluegrass lawns, and garden favorites like irises & roses and rhubarb. The two weedy perennial plants that are most troublesome are Canada thistle and field bindweed. Other troublesome perennial weeds include; dandelion, skeleton-leaf bursage (frequently called povertyweed), curly dock, and yellow nutsedge. Many native grasses are also perennial plants and have become weeds in no-till dryland rotational farming systems. Biennial weeds are plants that grow one year and produce seeds the second year. Sugar beets, carrots, onions are domesticated biennial crops. These crops are harvested in the fall of the first year when their stored energy (sugars and starches) are at their peak. Musk thistle and common salsify (goats beard) are examples of biennial weeds. The drop in nighttime temperatures triggers a switch in perennial and biennial plants to accelerate the storage of sugars into the plant roots and other storage structures. When herbicides are applied at this time these plant toxins are moved with the sugars into the same energy storage areas and destroy the cells that are most needed by the plants to survive through the winter. This is why fall applied herbicides are most effective in controlling perennial and biennial plants. Most plants are very susceptible to herbicides containing glyphosate as an active ingredient. Glyphosate containing herbicides are useful in controlling weeds under the following conditions: when the target weeds have no desired vegetation under or around them, when desirable vegetation has gone dormant, or when bare ground is the acceptable result. If none of these three conditions apply, a selective herbicide should be used instead. Nature abhors bare ground. Bare ground is an invitation to plant invasion. Normally weedy species are the first to arrive and the plant that dropped seeds before ground became bare will be the first again. Most broadleaf plants are susceptible to the phenoxy based herbicides: 2,4-D, Dicamba, and Tordon. These herbicides are useful on controlling broad leafed weeds when grasses are the desired ground cover. Tordon gives the most consistent long term control of perennial and biennial weeds but it also persists in the soil and leach away from the target area. Tordon shouldn't be used near trees, shrubs and other desired broad leafed plants. Dicamba containing herbicides are shorter lived but can persist in soils into the next season, especially when soils are dry. 2,4-D is fairly short lived and is usually broken down in soils within a matter of weeks. None of these phenoxy herbicides should be applied to seedling grasses. The key to controlling perennial weeds is persistence. Controlling field bindweed and Canada thistle take several years of concerted efforts to see much improvement. Combining competitive forage crops (alfalfa or grass forages) into the control effort will speed this up. Please contact me, Bruce Bosley about this article or other cropping systems or natural resource topics at 970-522-3200, extension 285 in Sterling or 970-542-3540 in Fort Morgan.
Date: 10/3/07
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