RELEASED WEEK OF FEBRUARY 17, 1997
University Extension Center
Courthouse Room 201
Palmyra MO 63461
(573) 769-2177
Contact: Area Agronomist, Jimmy Smith

Several factors have caused a few weed species to become more prevalent in Missouri. Trends in herbicide use have triggered species such as waterhemp to become a dominant pest across the Midwest. Biological factors such as seed size, dormancy mechanisms, and growth habit have played a vital role in the ecological succession of some weed species. Animals, wind, water, and farm equipment also contribute by spreading seeds. In addition to common waterhemp, weeds which are becoming more prevalent or at least established in Missouri are prickly sida, spurred anoda, hophornbeam copperleaf, common sunflower, Palmer amaranth, woolly cupgrass, and yellow nutsedge.

Prickly sida (Sida spinosa) is a member of the Malvaceae family which also includes important crop and weed species such as cotton and velvetleaf. It is widely distributed throughout the state and occurs as far north as Michigan and New England. It is most commonly a regular pest of agronomic crops in the Bootheel region of Missouri. Like prickly sida, spurred anoda (Anoda cristata) is a member of the Malvaceae family. It is not native to Missouri and its original distribution was reported only in the extreme southwest corner of the state. Presently, it occurs in several central Missouri counties along the Missouri River and the southeastern region of the state.

Four species of copperleaf including hophornbeam copperleaf (Acalypha ostryaefolia) are known to occur in Missouri. All are members of the Euphorbiaceae or spurge family. The appearance of these weeds may very well be related to the wide use of acetolactate synthase (ALS) inhibiting herbicides. Common sunflower (Helianthus annuus) is only one of nearly 20 sunflower species which occur in Missouri. Although not generally a species difficult to control, in recent years it has spread to several areas of the state, including the Bootheel, where it typically was not encountered. Palmer amaranth (Amaranthus palmeri) is another member of the troublesome pigweed family, Amaranthaceae. Unlike most amaranthus species, it is thought that Palmer amaranth was introduced from another state. Most of the present distribution reports find it occurring in the western and Bootheel regions of the state.

Woolly cupgrass (Eriochloa villosa) was imported from Asia. It has been a serious pest in areas of the northern corn belt, including Iowa, Minnesota, and Wisconsin. It was originally reported in Missouri's Audrain County in 1987. This past summer, it was discovered in Saline County. Yellow nutsedge (Cyperus esculentus) is one of nearly 200 sedge species occurring throughout Missouri. It has a vast range throughout much of both Americas, and doubtlessly occurs in every Missouri county. Although not a new weed, it is reported to occur in greater frequency.

Like other pest populations, weed species will continue to shift as long as the environment is manipulated. New classes of herbicides with new modes of action will continually influence weed species dynamics. Genetically engineered crop cultivars such as Roundup Ready soybeans will also impact weed populations due to the repeated use of only a few herbicide chemicals. This information was gleaned from Dr. Fred Fishel's report at the Agricultural Chemicals Short Course in Columbia on December 2-3, 1996. Dr. Fishel is the Coordinator of Pesticide Programs at the University of Missouri in Columbia. For more information about how to identify and control these potential weeds, please contact Jimmy Smith at (573) 769-2177.