Orchard growers consider new crops to suppress weeds

As some herbicides come under scrutiny, cover crops may give orchard growers another tool for managing the bare ground between rows. They can take advantage of that space, plus water and light, to grow cover crops that suppress weeds.

But the choice of cover crop varieties, planting dates and management program should be chosen to suit the purpose of preventing weeds from taking over that bare ground.

"Choose your cover crop species and management practices based on the specific functions that are important for your own orchard system," advised Steve Haring, a University of California, Davis, doctoral student in weed science. "Abundant cover crops limit bare ground and leave fewer resources available for unwanted weeds."

Growers came to see and discuss the latest research at the Nickels Soil Lab Annual Field Day in Arbuckle. There, Haring suggested: "Have a specific management goal for your cover crop and a plan to meet that goal."

Growers looking to use cover crops primarily to provide food for pollinators, prevent soil erosion, retain nutrients in the soil or improve soil structure and organic matter content may choose varieties and management practices to suit those purposes.

But if the main goal is to suppress weeds, the key is the rapid development of high biomass cover crops to quickly fill the space between the rows and crowd out the weeds.

"Select fast-growing, high biomass species like cereal rye," Haring advised. "Timely cover crop planting supports increased cover crop biomass."

Cover crops are one of the weed suppression tools that could gain in prominence as legal and health issues lead some growers to take a second look at glyphosate, the most widely used broad-spectrum herbicide in California orchards.

"Some growers and end users are concerned about glyphosate," said Brad Hanson, UC Cooperative Extension weed control specialist. "We can control weeds in orchards without glyphosate, but it's going to cost more. Glyphosate is pretty cheap."

Some orchard growers are switching to weed control programs based on the herbicide 2,4-D, according to Hanson. But that material also faces some controversy and potential regulatory pressure.

While cover crops can take up part of the slack in controlling weeds, high-biomass plants must be managed to fit with the overall orchard program.

"Consider how high-biomass cover crops may interact with irrigation, orchard sanitation or other orchard operations," Haring said.

For maximum weed suppression, the cover crop should be grown over a long season, from mid-October to mid-April. Haring found that waiting until after January sanitation did not give the cover crops enough time to grow.

Even though rapid development of biomass is a goal, it is not advised to use inputs to give cover crops a boost.

"If your goal is to suppress weeds, I wouldn't use a lot of inputs on my cover crop," Haring said. "If you fertilize your cover crop, you're going to get a flush of weeds."

The annual cost for cover crops is around $50 for 50 pounds of seeds and an additional $20 an acre to seed. However, some pollinator mixes are available at no charge.

"The (Project) Apis m. seed group has a 'Seeds for Bees' mix they send for free," Haring said.

(Bob Johnson is a reporter in Sacramento. He may be contacted at bjohn11135@gmail.com.)