Tropical spiderwort: Scientists looking to stop spread of noxious weed

Weed scientists in North Carolina are taking precautions to stop a weed so prolific that it forms a carpet in fields. Tropical spiderwort, a member of the dayflower family, so far has only been spotted in Wayne County, but North Carolina State University scientists are urging farmers to be on the lookout for this noxious weed that thrives in Roundup Ready systems.

In the past five years, tropical spiderwort has emerged as a major weed problem in cotton and peanuts in Georgia and Florida. “It's in four out of five counties in the Coastal Plain of Georgia,” says Mike Burton, a North Carolina State University weed ecologist. “In North Carolina, we didn't notice it until after a sequence of Roundup Ready soybeans and cotton.”

Tropical spiderwort is on the federal noxious weed list, and is therefore automatically on the noxious weed list in North Carolina. The North Carolina Department of Agriculture has funds to survey the extent of infestation in the state, Burton says, but not for eradication.

Above ground, the tropical spiderwort produces attractive flowers. But the noxious weed also produces flowers and seeds underground. “It's one of the most impressive flowering adaptations I've seen since the peanut,” Burton says. Pieces of stems cut by disking can also re-root to form new plants, if not buried too deeply. Left alone, it can sprawl out and fill the space between the crop rows.

In North Carolina and Georgia, researchers are conducting trials on pre- and post-emergence herbicides looking for possible control strategies. They have found strategies that rely on post-emergence herbicides alone are ineffective because of herbicide resistance and because the weed continues to germinate late into the summer.

In North Carolina, however, a methyl bromide trial at 340 pounds per acre gave complete control, Burton says. “Our colleagues in Georgia also tell us they haven't seen tropical spiderwort become a problem in fields that have been treated with methyl bromide as part of a crop rotation with vegetables.”

“Tropical spiderwort has emerged as a problem as producers have gone to more Roundup Ready systems,” Burton says. “Fields that have one Roundup Ready crop after another are the ones where we're finding spiderwort problems.”

Dayflower species, like tropical spiderwort, have flowers with a very short life — only a single morning — but each plant will produce several flowers per stem. The petals quickly decompose after blooming. They are monocot plants and have only one leaf when they emerge from the seed. Tropical spiderwort flowers have three petals: two blue or light purple petals and one smaller, white petal. Leaves of the tropical spiderwort are egg-shaped and about one and a half times longer than they are wide.

Tropical spiderwort has also been noted as a problem in agricultural production in Australia, where it grows through peanut canopies. Warm climates like those in the Southeast United States are nearly optimal for growth and reproduction of this troublesome weed.

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