Back before herbicide-resistant pigweed became enemy No. 1 for southern farmers, the exotic, invasive tropical spiderwort contended for the title. Tropical spiderwort is now trying to regain status as a major pest again for Georgia cotton farmers. Timely residuals can stop it.
"To control this weed, one must understand the importance of placing residual herbicides strategically throughout the growing season beginning at planting. The most important herbicide application may be the last one where an effective residual product must be applied where it contacts the soil, says Stanley Culpepper, University of Georgia Extension weed specialist.
The best way to apply that last application is with a "layby rig or hooded sprayer applying the herbicide where it needs to go, which is not over the top of the crop," he says.
He discusses the spiderwort issue in University of Georgia Cotton Team May newsletter, which provides good insight from other cotton team members on moving the cotton season forward after the region received a torrential downpour in late April.
Shading out spiderwort with cotton canopy doesn't work. It can grow in the dark.
When building a sound weed management program that includes spiderwort control, Culpeper provided some recommendations to consider:
- Residual activity from Warrant and Dual Magnum are critical to success; limited data suggests Outlook is also effective.
- Gramoxone and 2,4-D are very effective controlling emerged plants with timely applications.
- Dicamba is not overly effective but when mixed with glyphosate and applied sequentially, control of emerged plants is often acceptable, depending on the weeds size when the first application is made.
- Following sequential dicamba applications with a directed layby application, including an effective residual herbicide, will be important for long-term success with this system.
- Roundup + Staple remains effective on emerged plants if they are small.
- Liberty is not very effective.
- Layby materials such as diuron + MSMA, glyphosate + diuron, or any mixture with Aim can be quite effective. Include a residual herbicide with the layby application and follow all application requirements.
"Growers should contact their local county Extension agent for a season-long program approach for spiderwort, depending on their choice of technology being grown," he said.
According to USDA, tropical spiderwort, also called Benghal dayflower, is one of the world's worst weeds, econmically affecting more than two dozen agronomic crops. Originating in Asia and Africa, it was first detected in the U.S. in Florida in 1928. It has spread to Alabama, California, Georgia, Hawaii, Louisiana and Mississippi. According to Clemson University, it has been documented in the Virginia-Carolina region also. Naturally tolerant to glyphosate, it became a problem in cotton when growers shifted production practices to glyphosate-tolerant cotton technology.