Bill McCloskey with weed File photo
Bill McCloskey, University of Arizona weed specialist, examines a little mallow weed in a pecan orchard.

Gaining the edge against weeds in Arizona pecan orchards

Good cover crops in pecan orchards include peas but not alfalfa

An effective weed management program in Southwest agriculture includes tillage, herbicides (preemergence and post-emergence), and cover crops, says University of Arizona weed specialist Bill McCloskey.

In pecans, the weed specialist says tillage on the orchard floor during the winter can provide a “clean season start,” and provide an excellent surface to apply preemergence herbicides to minimize early-season weeds. Tillage can also increase water infiltration in the soil.

“If you have a lot of ‘trash’ and organic debris on the soil surface (un-tilled) it can reduce the efficacy of applied preemergence herbicides,” said McCloskey during the 2017 Arizona Pecan Growers Association annual meeting.

Growing glyphosate resistance

Growers across the nation are well aware of the growing issue of glyphosate resistance to weeds, including Palmer amaranth especially in the Mid-South. Five years ago, McCloskey confirmed the first case of any herbicide resistance in Arizona - glyphosate to Palmer amaranth - in a Buckeye (West Phoenix) cotton field.

Since then, the weed specialist has confirmed glyphosate resistance to Palmer in Central and West Central Arizona (Eloy, Parker, and Marana); plus in southeastern Arizona’s Cochise County; the latter where McCloskey has also identified glyphosate resistance to hairy fleabane, including in pecan orchards.

More diversity

To reduce herbicide resistance, McCloskey continues to advocate growers’ use of multiple herbicidal modes of actions year after year; not repeatedly relying on lower-cost glyphosate.

“We need more herbicide diversity in our weed management programs to try to slowdown and reduce the risk of developing herbicide resistant weed populations.”

McCloskey says effective preemergence and post-emergence herbicides are solid performers in weed control. If growers use post-emergence herbicides to tackle weeds then six or seven applications can be needed per season.

Preemergence herbicide use, he explains, can reduce the number of annual post-emergence herbicides needed for weed control. Note that some of these herbicides can drift and cause tree damage, including pecan.

Herbicide options

McCloskey says the main culprit is glyphosate.

“The alternatives glufosinate (Rely, Lifeline) and paraquat (Gramoxone) only cause minor leaf speckling with drift with no lasting damage, and don’t translocate to the nuts, roots, or other plant parts. These can be used in conditions where glyphosate might cause some damage to the trees.” 

Cover crops

McCloskey was asked which cover crops are best in pecan orchards to help control weeds. He said peas and legumes can help young trees get started, plus help control dust.

“Peas are a good choice but not alfalfa,” the weed specialist said. “You only need a cover crop for a year or two in pecans as young trees mature. Mowed cover crops or resident weeds work well overall to help stop blowing dust.”

Keep in mind, he said, that the cover crop will be eventually inundated by the resident weed population and take over.

In organic pecans, McCloskey says some growers have successfully used some clover varieties which have helped weed suppression year after year by reseeding themselves.

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