The hardy, aquatic perennials with dark green and slightly heart-shaped leaves are growing so thick in some rice fields that farmers are worried about yields and the ability of rice plants to get needed nutrients.
"All of our aquatic weeds are becoming more of a problem," says Ron Strahan, an LSU AgCenter weed scientist.
Vermilion Parish rice farmer Errol Lounsberry has seen portions of his 1,200 acres of rice near Lake Arthur threatened by pickerelweed.
"It can be just as severe a problem as red rice," Lounsberry said. Red rice is another nuisance plant that can plague rice farmers.
Pickerelweed can grow to a height of 3 feet and overwhelm rice fields if left unchecked.
LSU AgCenter county agent Howard Cormier of Vermilion Parish and Strahan said pickerelweed is a particular problem because the plant spreads so quickly. It has a high seeding rate and also spreads by creating new plants from its rhizomes (creeping root structures).
"It competes with rice for space, nutrients and sunlight," Cormier said. "It's very aggressive."
Pickerelweed is not new to Louisiana. In the past, it has been controlled by applying Londax, a herbicide manufactured by DuPont. Londax is still effective in many cases, but in some fields Londax-resistant populations have developed because of repeated use of the herbicide.
Strahan said other farming practices also may have contributed to a thornier pickerelweed problem. For example, farmers who alternate rice with crawfish production may have let pickerelweed go untreated for two or three years in a row during their crawfish years, and now they're finding it difficult to control after planting rice once again, he said.
"Normally, if you'd plow up the land and let it dry, that would destroy most of the weed," Cormier said. But wet conditions hampered early work by farmers in their fields over this winter, allowing pickerelweed to gain ground.
New LSU AgCenter test plots in the far western corner of Vermilion Parish will examine at least eight herbicides, including an experimental chemical from Dow that may be two years from the commercial market.
Other options under study include Regiment, Regiment plus Aim, Grandstand, Grandstand plus Aim, Duet, and Basagran.
Strahan said any effective treatments will be replicated next month in additional tests. The goal is to have suggested alternative treatments to farmers by June, the LSU AgCenter expert said.
Most of the herbicides under study would burn the pickerelweed above the water line but not move through the weed into its rhizomes or roots. The goal of that approach is to do enough damage to the weed to allow rice plants to form a canopy over the top of the weeds, which will make harvesting easier.
Farmers like Lounsberry, meanwhile, are rooting for the LSU AgCenter to find a new weapon.
"The weed is particularly troublesome," Strahan said. "It produces a lot of seed, much of it viable, and in ponds where it's been left uncontrolled, it can get waist high if allowed to grow."