Organic practices complicate weed control

By Renee Stern
Contributing Editor

Weeds are the top pest problem for organic vegetable growers, who rely mainly on cultivation, hand-weeding and other herbicide alternatives.

“It all boils down to money,” says Richard Smith, a University of California Cooperative Extension farm adviser in Monterey County. “It’s an issue of how much money you have to throw at (weed control).”

Doing nothing isn’t an option, he says. “If you don’t deal with it, you’ll have a disaster on your hands.”

Bruce Rominger, co-owner of Rominger Brothers Farms in Winters, Calif., uses cultivation, crop rotations and “too much hoeing” for weed control on 400 acres of organic processing tomatoes, alfalfa and oat hay.

Joe Townsend, co-owner of The Farm in Atoka, Okla., grows strawberries and broccoli on plastic mulch with drip irrigation to keep down weeds, combined with tilling and hand-weeding. The 2-acre garden operation also has three hoophouses dedicated to onions, peppers and melons, along with lettuce, cucumbers and radishes for local school salad bars.

Acetic acid and other organic sprays aren’t cost-effective enough, he says.

Rominger says even the cost of hand-weeding doesn’t make organic herbicides economical in his operation. Plastic mulches are likewise out of reach.

Townsend participates in a state plasticulture program that lays down the material for growers. “If we had to do it ourselves, we would’ve just broken even with it,” he says.

For organic growers, plastic mulches sent to landfills after use raise philosophical conflicts, Rominger says.
The plastic, which must be replaced each year, is Townsend’s highest weed-control cost. To reduce erosion, between-row cover crops must be established as soon as possible.

While they control erosion and improve soil quality, cover crops also can encourage weed growth, he says.

“I like the hoophouses more and more all the time,” he says. “It’s a more controlled environment, and fewer weeds blow in.”

Preventing weeds is the key
Keeping weeds out in the first place is the best method, growers and researchers agree. Where possible, choose fields with fewer weeds, and time crops to avoid competing with dominant weeds, Smith says.

Preirrigate fields ahead of planting to germinate weeds, then till, flame or spray herbicides without endangering crops.

“Preirrigation is a powerful tool, but you need time to make it work,” Townsend says.

Fields with low weed densities may not need this step, says Tom Lanini, Extension weed ecologist at the University of California, Davis. For high weed densities, “it’s definitely needed.”

Knock down weeds before they set seed to reduce problems in subsequent years, says Chuck Webber, research agronomist at the U.S. Agriculture Department’s Agricultural Research Service in Lane, Okla.

“It all boils down to cultural practices,” Smith says. “None will help in an acute situation, but they’re more long-term solutions.”

“Nature abhors a vacuum,” Webber says. “If you till the soil and leave it empty, weeds will grow.”

He suggests mulching with either plastic or cover crops.

“Plastic is a tremendous resource for the organic grower,” Webber says. But application and disposal do raise costs.

Possible organic herbicides
Webber has compared watermelon transplants on plastic mulch using between-row cultivation with melon seeded directly into a winter cover crop of rye and sprayed with acetic acid around vining plants. The plastic-and-cultivation combination showed better results. Spraying weeds without hitting and damaging the watermelon vines was difficult, he says.

Available organic herbicides aren’t selective, requiring care when applying around crops, Lanini says. Still, they provide alternative weed control for growers who want to reduce tillage and fuel costs. Disturbing the soil through cultivation also brings up more weed seeds to germinate and create problems.

Much of Webber’s research has focused on corn gluten meal as a non-selective pre-emergent herbicide. Mustard meal also seems similarly effective, he says.

After initial applications of corn gluten meal, follow up with herbicide sprays on any surviving weeds. Most organic herbicides work best with smaller plants, so keep close watch on fields, Webber says.

These herbicides also tend to handle broadleaf weeds better than grasses, which may require more applications, he says.

Other weed-control options
Which methods growers choose depends on their crops, conditions and risk-tolerance. On wetter sites, cultivation may not be an option, so mulches or transplants may be better choices, Lanini says. Growers with drier, warmer conditions may pick solarization: burning out weeds under layers of clear plastic.

Tillage suits tomatoes, for example, but is less useful for viny, spreading crops such as melons or cucumbers, he says.

Subsurface drip irrigation prevents annual weeds from germinating in an even smaller portion of the field by reducing surface wetting, Lanini says. That’s ideal for transplants and low-till systems.

Growers looking to add animals to their system and with fields dominated by grassy weeds may find a fit with geese.

“They’re grass specialists,” Lanini says. “You can grow a lot of broadleaf crops and they won’t touch them.”

Drawbacks are food safety concerns and added management needs, he says.

Mechanical weeders that use precision guidance to take out only weeds in the plant row are hitting the market, and researchers are developing more advanced sensor systems.

No single weed control method is perfect, Smith says. “The emphasis needs to be on the technology that’s coming and on age-old practices.”