The loud sucking sound echoing from the wallets of California bell pepper farmers is from the approximately $400 spent per acre annually to control weeds with herbicides, mechanical cultivation and hand hoeing.

Reducing weed control cost is the purpose of two ongoing research field trials underway since 2004 in central California. Leading the trials are University of California Cooperative Extension vegetable farm advisors Michelle Le Strange, Tulare County, and Richard Smith, Monterey County. 

Le Strange conducts research at the West Side Research and Extension Center in Five Points in Fresno County in the San Joaquin Valley (SJV). Smith’s studies are underway in commercial grower fields in Monterey County on the Central Coast. Trial results can vary due to the different geographical areas.

Le Strange discussed the latest study findings with pepper researchers from around the world during the International Pepper Conference in Las Cruces, N.M. this fall.

Weed challenges

In the SJV, summer weeds primarily impact bell pepper production. Among the most difficult to control, Le Strange says, include weeds in the nightshade family, especially hairy, black, cutleaf, and groundcherry nightshades.

In some years, the weed common purslane can grow “wall-to-wall” in bell pepper fields while the weed is absent in other years. Even with cultivation, purslane re-roots and grows back. Yellow and purple nutsedges are problematic perennial weeds.

Other SJV problem weeds in pepper fields include tumble, redroot, and prostrate pigweeds. Puncturevine is increasingly found on roadsides and in vegetable fields. While some weevils provide natural control, Le Strange says high numbers of puncturevine and weevils typically occur at different times limiting the effectiveness of the bugs. Puncturevine is a noxious weed in the field at harvest.

In Central Coast pepper fields, little mallow is an annual-biennial weed able to grow to 4 feet tall and interferes with harvest. No registered herbicide is effective on little mallow at layby. Other common weeds include groundsel, lambsquarters, shepherd’s purse and sowthistle.

Most Central Coast and SJV bell pepper farmers grow transplants which compete poorly against weeds for 40 to 60 days after transplanting.

“During the spring, rain and cooler temperatures delay transplant development and flushes of weeds typically occur until warmer temperatures take over,” Le Strange said.

Black plastic mulch is often laid on shaped beds as a method of weed management in peppers destined for the fresh market. Bell peppers in both regions are usually grown without plastic mulch for the processing market where the profit margin is smaller.

In 2008, California ranked first nationally in bell pepper production with 51 percent of the nation’s crop, according to the California Department of Food and Agriculture. About 400 hundredweight of bell peppers per acre were harvested on about 20,000 California acres. The crop value totaled about $290 million.

Herbicide trials

The herbicide trials have focused on developing a weed control strategy for peppers with pre-emergence herbicides applied at planting and/or at layby to provide weed control throughout the growing season.

Specific information on the Fresno County trials include: a Goal Tender preplant application March 15; transplanting the pepper variety Jupiter or Baron on April 15; two beds each at 40 inches wide by 70 feet long; 30 gallons of water /acre; layby herbicides applied from May 30 to mid June; sprinkler to furrow irrigation; Panoche clay loam soil, and harvest from Aug. 1-15.

In Monterey County, Smith applies Goal Tender as a preplant April 15; followed by transplanting on May 15; the Baron variety; one 40 inch wide by 25-foot long bed; 74 gallons of water; layby herbicides applied June 16; sprinkler to drip irrigation; Pacheco silt loam soil; and an Aug. 19 harvest.