Organic weed control not easy

For that patch of dirt infested with field bindweed where you want to produce organic vegetables, the doctor recommends applying plenty of glyphosate and calling him in three years.

The doctor is Ph.D. Steve Fennimore, University of California Cooperative Extension weed specialist in Monterey County. He gets many calls like that from farmers who want to produce food organically.

Fennimore’s recommendation may be a bitter pill to swallow for those who want to leap on the organic bandwagon, but it is reality. The first rule in converting to organic is to select the right field for it. A field that is bindweed infested is not the right choice. Fennimore said his recommendation to the grower who called him was to farm it conventionally, controlling weeds with synthetic herbicides during the mandatory-three year transition to organic. Once the field is cleaned up, the odds are better for organic farming success.

Although some consider organic and conventional widely different farming methods, they share two common weed control goals; avoid at all cost expensive hand-weeding. It devours profit. And, it may be politically incorrect, but Fennimore said both systems demand that weeds be killed. A weed left a little green after a control method is a weed to return.

There are no silver bullets or easy solutions for organic farming weed control. However, in leading a discussion at the 62nd annual California Weed Science Society conference in Visalia, farmers, PCAs and research specialists said you can kill weeds in organic production, but it takes plenty of work. And, if you wait until tomorrow to get rid of small weeds, you will have a large weed problem quicker than you can say “call the hoe crew.”

Weed control in organics is perhaps the gnarliest element of organic farming. Conventional farming has long relied on synthetic herbicides to control weeds, and there are a growing number of registered organic herbicides that have emerged to meet commercial, organic farming.

The organic herbicides will work as well as synthetics; however, don’t expect them to come to the rescue of an out-of-control-weed infestation like synthetics often do, according to another UC weed specialist, Tom Lanini. In detailing what Fennimore calls “real world, pragmatic ideas,” controlling weeds largely without herbicides requires relearning weed control fundamentals:

• Field selection (none infested with field bindweed)

• Prevention

• Sanitation

• Control

“In weed management, we tend to think of control as the core of weed science. That is really a small part of it,” he said.

It starts with such things as using only composted manure.

The speakers all pointed out that cover crops are vital in organics to generate green manure for nutrients. Just as critical is not allowing cover crops to go to seed before disking.

Pre-irrigate to germinate shallow weed seed followed by cultivation to control the weeds that emerge. Cultivating too deep just brings up more weed seed.

“Good vegetable growers are masters at pre-irrigating and germinating shallow weed seeds to be cultivated out,” said Fennimore.

There are many methods of cultivating out weeds, but Lanini’s research has identified cultivators that move soil horizontally and not vertically as best at controlling shallow seeded weeds. Night cultivation actually enhances mechanical weed control, Lanini added.

Soil solarization also works. It is more adapted to California’s warmer climates than to coastal areas. Injecting steam also controls weeds. Tarping is another control method, as are the new generation of smart cultivators. In Fennimore’s trials, a yellow plastic has proven the most effective.

Fennimore has worked with a cultivator from England equipped with small coulters, a camera and hydraulic motors that swing the coulters around tiny celery plants. It costs $30,000 per seed line, holding back a stampede of buyers.

Dean Shiroyama, a PCA with Crop Production Services at Five Points, Calif., on the West Side of the San Joaquin Valley, said irrigating fields destined for organic product to germinate weeds followed by mechanical weed control works.

“You have to have a plan to make it work. It takes time and water, which may be in short supply on the West Side,” he added.

Sudangrass planted as a cover crop in late May and early June and incorporated in mid-August is a good green manure source and also controls weeds in Shiroyama’s area.

“Cover crops are more art than science,” said Fennimore. It takes experimentation to determine what works in specific areas. For example, Sudangrass would not work well on the coast since it requires warm temperatures. Mustard works better on the coast.

“The only rescue treatment with organics is a hoe,” emphasized Shiroyama.

Mulches are another good approach to organic weed control. “Anything that blocks light,” said Lanini. For controlling Johnsongrass and other grasses, weeder geese will work. Some San Joaquin Valley cotton growers used weeder geese up until the mid 1970s to weed emerged cotton. Cotton contains gossypol and geese will not eat cotton, but will eat grasses.

There are several organic herbicides. Most all work well at high concentrations in high water volumes. “With synthetic herbicides, we think of 20 to 30 gallons of volume per acre. With organic herbicides, it takes 60 to 70 gallons per acre to make them work,” Lanini said.

They work better in warm weather and on younger weed seedlings. Most, he added, work best with two applications within a 15-day interval. Use an organic adjuvant, he insisted. “There are many good ones on the market,” Lanini said.

Spot treatment of emerging weeds will keep you ahead of the game. Marking spots with GPS helps.

Ed Missarian of Bohosian Raisins, Fowler, Calif., grows both organic and conventional raisins. He has both Dried-on-the Vine (DOV) and open gable vineyards. High vigor, over the row vines shade out weeds.

He uses a wide array of weed control methods. A rotary hoe equipped with Bezzerides “really works. The faster you go, the better they work. You can travel down the row at 4 to 5 miles per hour,” he noted.

Weed Badger also works well, especially in a nettlesome weed situation. However, it is slower than the rotary hoe. A French plow also will clear out weeds, but again it is slow. He also uses some hand weeding with shovels.

He uses propane, flame weed control, but generally around anchors or on outside rows. He uses flame early.

He has experimented with organic herbicides. “They work well when weeds are small … 4 inches or less.”

Lambsquarter and mares tail are his toughest weed problems.

“You cannot stop thinking about weed control in organic production. You have to be in the vineyard constantly. Every time we are in the field, we are doing something to control weeds,” he added.

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TAGS: Management
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