Plant Mulches: Possible Methyl Bromide Alternative?

With only two full crop seasons remaining before the ban on the use and importation of methyl bromide in the United States, Government policy officials and researchers, industry, academia, and growers are leaving no stone unturned in the frantic search for alternatives. Vegetable growers and others are sounding the alarm that if no viable alternatives surface soon, they may go belly-up. So far, there is no commercially available alternative that does as good a job for growers as this soil fumigant.

The heat is on at USDA's Agricultural Research Service to exhaust all possibilities in finding some alternative that will ease growers' reliance on this chemical that for decades has been so effective in eliminating soilborne pests and diseases and ensuring high, quality yields.

Aref Abdul-Baki, a plant physiologist with ARS' Vegetable Laboratory in Beltsville, Maryland, has been growing vegetables with plant mulches for almost 10 years. He has worked with growers in Maryland, Virginia, and Pennsylvania who raise strawberries, squash, beans, corn, cantaloupes, peppers, and tomatoes. These growers have adopted the practice of plant mulches and have increased their yields and decreased production costs by using less fertilizer and less chemicals to control pests, disease, and weeds.

"I started with tomatoes," Abdul-Baki says, "And used hairy vetch instead of the traditional black polyethylene mulch. The idea is simple: plant a cover crop in the fall, mow it to about two to three inches in the spring, then immediately plant tomatoes without disturbing the soil."

Abdul-Baki followed a four-year crop rotation system in his test plots. The first year, he planted fresh-market tomatoes, followed by a cover crop of hairy vetch. The second year, he introduced a mixture of hairy vetch and crimson clover. Each year since then, he used a mixture of hairy vetch, crimson clover, and rye on the crops. He planted sweet corn the second year, followed by muskmelons the third, and snapbeans the fourth.

"Cover crops are planted in the fall after harvesting the vegetables and are cut in May the following year," he says. Granted, plant mulches add organic matter to the soil and increase its water-holding capacity, and they even add nitrogen, all of which help boost plants' productivity. But what about pests and weeds?

Pests

David Chitwood, head of the ARS Nematology Lab in Beltsville has compiled some data on nematode populations from Abdul-Baki's experimental plots.

"We found no increase in the numbers of nematodes in the test plots," Chitwood reports. "But we don't have a problem with root-knot nematodes in the area containing Abdul-Baki's plots."

Chitwood's lab monitored populations of five phytoparasitic nematodes: root-knot, lesion, stunt, spiral, and lance. Nematode numbers started low early in the growing season and remained below the levels that could cause economic losses through harvest.

Scientists were concerned about the numbers of nematodes in the soil because certain legumes, including hairy vetch, are somewhat susceptible hosts of nematodes. Therefore, under favorable conditions, numbers may increase, especially if cover-crop legumes are part of the crop rotation system.

"The cold winters here in the mid-Atlantic region could have played a part in this scenario," Chitwood says.

Weeds

John Teasdale, a plant physiologist in the ARS-Beltsville Weed Science Lab has collaborated with Abdul-Baki since 1991. "Some cover crops make such a heavy, matted mulch that it actually suppresses early-season weeds, eliminating the need for preplant herbicides," he says.

Generally, cover crops delay weed growth, but they don't completely control weeds, Teasdale reports. "Some other type of practice is usually necessary to control postemergence weeds. Plant mulches reduce the number of weeds by the sheer biomass of the cover crop residue. Using hairy vetch alone to grow vegetables doesn't provide enough plant material to adequately suppress weeds. That's why we use mixtures of vetch with crimson clover and rye."

Legumes like hairy vetch decompose quickly, adding more nitrogen to the soil. However, the rapid decomposition reduces the mulch biomass needed to help keep weed emergence at a minimum.

Several vegetable crops, including tomatoes, corn, beans, melons, and peppers have been grown with plant cover crops. "We've successfully slowed down the emergence of pigweed, lambsquarters, and foxtail—three weed species that plague these vegetable crops," Teasdale reports. "We found that small-seeded annual weeds tend to be the ones that are suppressed most by plant cover crops."

Matted plant mulches allow crops to become established and growers to identify what weeds emerge so that appropriate herbicides can be applied later in the growth cycle. Crops like peppers, which don't have a dense leaf canopy, don't fare as well as crops that produce enough cover to shade the emerging weeds.

"We feel that cover crops can be one piece of the puzzle for an integrated approach to the loss of methyl bromide," Teasdale says.

Future Plans

Some of the leading growers in Florida have said that unless a practical technology is developed soon which can effectively replace methyl bromide, commercial vegetable production may collapse in the subtropical area of southern Florida.

Would the idea of plant mulches work in a different environment such as the climate in Florida and perhaps be a potential alternative to methyl bromide for vegetable growers there? Well, Abdul-Baki may get a chance to see.

Waldemar Klassen, director of the University of Florida's Tropical Research and Education Center in Homestead, has invited Abdul-Baki to try his method on research fields in Florida.

"We've been looking at Abdul-Baki's research for some time," Klassen says. "We have several research projects on methyl bromide under way at Homestead. Also, we have a highly promising project on the legume sunn hemp that was inspired by ARS research at Auburn, Alabama, and another on perennial peanut sources. However, we haven't worked on plant mulches and are most anxious to collaborate with ARS to investigate the suppressive effects of mulches on soilborne pathogens that thrive in the unusual calcareous soils of the south Florida winter vegetable production area. Since south Florida soils are very different from those at Beltsville, Maryland, we need to work together to develop a practical technology in the short time we have left before Florida vegetable growers are denied the use of methyl bromide."

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Last Updated: October 6, 1998