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.

Herbicide solutions

Herbicides tested in the seven-year trials included: S-metolachlor (Dual Magnum); flumioxazin (Chateau); oxyfluorfen (Goal Tender); dimethenamid (Outlook); pendimethalin (Prowl H20); and napropamide (Devrinol) and Dacthal.

Among the findings, Le Strange reports that Dual Magnum and Goal Tender effectively control the nightshades. Goal Tender is very effective on puncturevine while other products including Dacthal, Devrinol, Dual Magnum, and Prowl H2O provide partial control. Dual Magnum is effective against yellow nutsedge, but not purple nutsedge. Most of the herbicides work against common purslane and the pigweeds.

For little mallow, no registered herbicides are currently registered for layby use. Goal Tender works against the weed but is not registered. Chateau works, but can cause crop phytotoxicity (stunting).

Dual Magnum was successfully utilized in commercial bell pepper production at planting from 2002 to 2007 due to a special 24c exemption permit gained by the California Pepper Commission.

The herbicide provided good weed control in the field trials. Le Strange and Smith worked with the manufacturer Syngenta in 2008 to gain an indemnified label for product use at layby. To gain label use, growers must visit Syngenta’s website http://farmassist.com.

“Dual Magnum at planting and layby provides very good weed control,” Le Strange said.

Chateau is not currently registered in bell peppers yet provided excellent weed control in the trials. Herbicide use can result in crop phytotoxicity. Le Strange is working with Valent to determine the best formulation (spray versus granule), timing, and application rate. Chateau is primarily used for weed control in orchard crops.

Goal Tender is registered for use in peppers grown with plastic mulch and is applied to shaped beds before planting. Le Strange says Goal Tender also works effectively without mulch. Leaf crinkling, necrotic spotting, and leaf death can occur if applied less than 30 days before transplanting.

Outlook currently lacks registration in peppers but provided excellent weed control in the trials. Work is underway with BASF to determine the best application method, timing, and application rate. Crop phytotoxicity with Outlook occurred once in the trials but was never replicated.

Prowl H2O is registered for planting and layby applications. The herbicide controls many broadleaf weeds including lambsquarters, goosefoot, and mustards. Prowl H2O is not effective against the nightshades or yellow nutsedge.

“We see very good weed results using Prowl H2O especially on grasses in the field,” Le Strange said.

The pepper herbicide research trials will continue in 2011.

“When we started the trials there were very few herbicides registered for use in peppers,” Le Strange said. “We have a few more now as a result of our research. This is good progress in the effort to control weeds and costs in the California pepper industry.”

[email protected]