A noxious, fast-spreading, exotic weed called tropical spiderwort has become very troublesome to farmers in the Southeast, particularly cotton farmers. But there may be a way for them to get ahead of the weed — by planting early.
Agricultural Research Service (ARS) agronomist Theodore Webster, in the agency's Crop Protection and Management Research Unit at Tifton, Ga., has found that tropical spiderwort (Commelina benghalensis L.) emerges later in the growing season than most weeds. That means planting cotton early would give the crop a head start before tropical spiderwort takes off.
Webster and his colleagues — Michael Burton of North Carolina State University; Stanley Culpepper, Tim Flanders and Tim Grey of the University of Georgia; and Barry Brecke of the University of Florida — monitor the weed's advances and work to understand its biology. According to Webster, the main reason tropical spiderwort has become a serious weed has to do with recent changes in cropping and production systems.
One such change is the widespread planting of so-called "Roundup Ready" crops that tolerate the herbicide. This helps growers to better manage weeds in cotton and increases the use of conservation-tillage. But tropical spiderwort has a natural tolerance to glyphosate, the active ingredient in Roundup.
According to Webster, tropical spiderwort also tolerates other common herbicides, and the ones that control it best increase production costs significantly.
A native of Africa and south Asia, tropical spiderwort was first observed in Florida in 1928. It is unique in the plant world because it produces both aerial and underground flowers, and both types form seeds. It gradually advanced into Georgia, but wasn't considered a troublesome weed there until 1999, according to Webster.
Annual control efforts now cost the Georgia cotton industry more than $1.2 million.
In Georgia, planting cotton in April and early May would help get a jump on tropical spiderwort. Typically, cotton is planted in late May and into June, the edge of the planting window for cotton in southern Georgia.
Read more about this research in the September issue of Agricultural Research magazine, available online at: http://www.ars.usda.gov/is/AR/archive/sep06/spider0906.htm 
ARS is the U.S. Department of Agriculture's chief scientific research agency.