Almond growers advised on how to attack orchard weeds

The topic for a talk at the Central Valley Almond Day in Fresno was "Managing the Top 10 Most Wanted Weeds in Almonds," but it could as well have been phrased as managing the least-wanted weeds.

Kurt Hembree, weed science advisor for the University of California Cooperative Extension in Fresno County, not only listed names of the top 10, he also gave some advice on how to keep weeds at bay.

The problematic weeds that made the list were hairy fleabane, horseweed, junglerice, sprangletop, yellow nutsedge, field bindweed, knotweed, evening-primrose, Russian thistle and prostrate spurge.

Some are particularly problematic because they are resistant to the herbicide glyphosate.

"There has been an increase in glyphosate-resistant weeds worldwide," Hembree said, adding that resistance often occurs when herbicides are limited to single modes of action.

"Resistance is a selection process," he explained, whereby resistant plants survive and sets seeds.

Hembree has long urged applications of products with multiple modes of action, but more recently he has also stepped up advice of tank-mixing at least two products with different modes in every spray application. Herbicides list a group number, Hembree said, and the idea is to combine products with differing numbers.

He said growers also need to time applications properly, making sure there is uniformity in coverage, the proper rate is used and to check that equipment is fine-tuned.

He gave these tips for optimum control with post-emergent herbicides:

• Hairy fleabane: Treat when there are fewer than eight leaves with a two-or-three way mix and good spray coverage.
• Horseweed: Treat when there are fewer than eight leaves with a two- or three- way mix and good spray coverage.
• Junglerice: Treat with glufosinate and paraquat before flower heads emerge or with a grass herbicide before they become drought stressed.
• Sprangletop: Treat with clethodim, glyphosate and glufosinate twice at a three-week interval before seed set.
• Yellow nutsedge: Treat with glyphosate when there are fewer than four leaves every 21 to 28 days or tank-mix glyphosate with rimsulfuron and apply twice at a two-week interval.
• Field bindweed: Treat with glyphosate when runners are 12 to 16 inches long at 10 percent flower in the fall.
• Knotweed: Treat at less than 21 days after emergence with appropriate tank-mix materials.
• Evening primrose: Treat with glyphosate and glufosinate less than 21 days after emergence.
• Russian thistle: Treat less than softball-sized plants with saflufenacil or similar contact material.
• Prostrate spurge: Treat less than 14 days after emergence. Seed set is about 21 days after emergence.

Hembree said it is important to avoid spray drift and that the age and health of trees should be taken into account when applying herbicides. He said environmental conditions need to be taken into account and spray nozzles should be selected to manage coverage and drift.

Hembree said more problems are being posed by another weed that appears to be spreading. It's commonly named alkaliweed and is native to the western United States and Mexico. It is a species of flowering plants in the morningglory family and grows in habitats with saline or alkaline soils.

It's a perennial herb producing an erect stem with many braches up to about 10 inches tall. The clump of stems are densely lined with silky hairs and studded with many small, oval leaves. Hembree said researchers are looking to learn more about the weed and how to combat it.

Hembree says high weed pressure can stunt tree growth, and weeds may harbor insect pests. Hembree said it is important to spray weeds when they are small.

He said it's important to identify target weeds. A weed photo gallery at the website ipm.ucanr.edu/PMG/weeds_all.html can help with that.

Applying both pre-emergent and post-emergent herbicides is recommended.

(Dennis Pollock is a reporter in Fresno. He may be contacted at agcompollock@yahoo.com.)