May 1, 2020
Tillage key to weed control for Southeast fall broccoli

Heavier rainfall and greater weed pressure in the eastern U.S. make weed control in broccoli tougher than in the arid states of California and Arizona where most of the country’s broccoli is grown. This makes weed control a major challenge to establishing an East Coast broccoli industry.

Fortunately, new research at Clemson University is showing tillage can effectively complement preemergence herbicides when growing broccoli in the fall.

Broccoli in South Carolina would typically transplant in September when temperatures are still warm and then have cooler temperatures for harvest. “Cooler temperatures mean better broccoli heads, but in that initial transplanting time, we get a lot of weeds,” said Matt Cutulle, Clemson University assistant professor of vegetable weed science.

Clemson University weed scientist Matt Cutulle. Photos: Clemson University

Cutulle spoke on weed control at the recent Ag Expo in Wilmington, North Carolina. Ag Expo is jointly presented by the North Carolina Vegetable Growers Association and the North Carolina Agribusiness Council.

Yellow nutsedge is a leading culprit. Preplant herbicides effectively control the weed for a while after transplanting but are eventually overwhelmed. “We don’t have a residual herbicide to control yellow nutsedge for the duration of the season,” Cutulle said.

Four herbicides were evaluated for their weed control in the study – with and without cultivation. Goaltender, Dual Magnum and Devrinol were the three products labeled for use on broccoli. Zidua was also tested although it’s not registered for the crop. “It has the same mode of action as Dual Magnum but binds to more sites and has a lower use rate so there’s less product in the environment,” Cutulle said.

All herbicides were soil-applied before transplanting. Herbicides alone effectively controlled weeds for a while. At seven days after transplanting all products provided more than 70% control with Dual Magnum exceeding 80%.

Herbicides alone also effectively controlled other important weeds. At 18 days after transplanting, all four products provided more than 85% control of carpet weed. Dual Magnum, Goaltender and Zidua provided more than 85% control of purslane, and Devrinol and Dual Magnum provided greater than 80% control of yellow foxtail.

Unfortunately, yellow nutsedge by then was out of control. “Initially, it all looked like good suppression of yellow nutsedge, but at 21 days after transplanting we’d lost effective control,” Cutulle said.

Researchers evaluated a single cultivation made three weeks after transplanting. “We can control that initial weed pressure with herbicides, but can cultivation control the later flush?” Cutulle asked. “We’re trying to get through to that first cold snap in the fall when yellow nutsedge goes away pretty quickly.”

Eight weeks after transplanting, the benefits of a follow up cultivation were clear. Both Dual Magnum and Goaltender followed by a cultivation provided 85% or more control eight weeks after planting. Zidua followed by a cultivation provided less control and Devrinol plus cultivation significantly less.

Goaltender, Dual Magnum, Devrinol and Zidua alone – without cultivation – each provided weed controls ranging from 10% to 27%.

Dual Magnum and Goaltender were both effective and the best choice for a field will probably depend on the weed species present.

Dual Magnum effectively controls most annual grasses and small-seeded broadleaf weeds. “Goaltender is not known for controlling grasses that well,” Cutulle said, but controls more broadleaf weeds with the exception of horseweed.

The Clemson study also found cultivation alone did surprising well. “Cultivation alone may provide adequate weed control if there’s very low weed pressure,” Cutulle said.

Researchers looked at two cultivars in the study – Emerald Crown and Lieutenant. Mean stem diameters were measured to assess the effects of the treatments on broccoli quality.

There was little statistical difference between the herbicides although the superior weed control of herbicides followed by cultivation did show an improvement. “Generally, we had higher mean stem diameters when cultivation was added,” Cutulle said.

Broccoli growers in the West can use Goaltender postemergence because the low humidity means thicker cuticles on the broccoli. “In the East Coast, there are no postemergence herbicides that provide selective control of broadleaves, with the exception of Stinger which typically only controls legume weeds,” Cutulle said.

Fortunately, cultivation may be a viable alternative.

— Dean Peterson, VGN correspondent


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