The impending ban on methyl bromide has many U.S. vegetable growers scrambling to find new ways to control yield-robbing weeds, insects and plant diseases. But perhaps no problem poses a bigger economic threat to growers from Florida to California than nutsedge.
Purple nutsedge has been called the world’s worst weed. It can be found in more than 50 crops worldwide, thrives in all types of soils, and can survive the highest temperatures known in agriculture. Once established, purple nutsedge is very difficult to control due to its extensive network of underground tubers.
While not as aggressive as its cousin, yellow nutsedge can be found over a much wider range, due in part to its ability to withstand soil temperatures down to zero degrees. Purple nutsedge, on the other hand, does not tolerate soil temperatures much below freezing.
“The continuous tuber production of purple nutsedge makes it more aggressive and helps it spread rapidly throughout a field,” noted Steve Fennimore, University of California weed control specialist. “Nutsedge tubers have a lot more inherent energy than a seed, so when they come out of the ground, they’re ready to grow and rob the crop of vital soil moisture and nutrients.”
One reason why nutsedge is so hard to control is the depth at which some of the tubers germinate. According to Fennimore, about 80% of nutsedge tubers can be found in the top 6 inches of soil. He added that 95% can be found in the top 18 inches. “That leaves 5% that are very deep. It’s those deep tubers that can come up and cause you to lose control of a field if not controlled somehow.”
A Programmed Approach
Peter Dittmar, assistant professor of weed science at the University of Florida, said nutsedge control in vegetable crops requires a carefully planned and programmed approach. “We really don’t have a lot of options for controlling nutsedge in vegetables,” he said, “so we have to manage the problem with a combination of preplant herbicides, post-emergence sprays, fallow crop herbicide applications, cultivation, and cover crops. In a crop such as tomato, some growers use a preplant application of Reflex and Dual (both from Syngenta Crop Protection) followed by a postemergence spray of Sandea (Gowan) or Matrix (DuPont Crop Protection). Sandea or glyphosate works well to keep nutsedge in check during the fallow period. And cover crops such as sorghum sudangrass and sunn hemp can be planted in Florida to shade-out nutsedge on fallow ground.”
Further north, Stanley Culpepper at the University of Georgia stated that the loss of methyl bromide means increased complexity in managing nutsedge.
“In addition to a fumigant system using the proper mulch, nutsedge management in Georgia’s vegetable crops often requires preplant, postemergence and row middle herbicide applications,” he explained. “Sandea is among the most effective nutsedge management tools we have, since it’s one of the few herbicides registered for topical use in several vegetable crops, including tomato, cucumber, cantaloupe, and snap bean. Plus, it’s also labeled for row middles in many crops.”