The upcoming winter months offer pistachio growers the opportunity to reduce weed threats to next year’s crop by using soil-applied herbicides to prevent weeds from emerging in the spring.
In fact, the newer pre-emergent materials should be your first line of defense against the weeds, says Kurt Hembree, University of California Cooperative Extension weed management farm advisor for Fresno County.
“It’s really difficult to get ahead of weeds once they’ve emerged using only contact herbicides, like Roundup or Goal,” he says. “On large acreage, it can be a challenge to have enough equipment and labor to keep up. The newer pre-emergent herbicides should be the heavy lifters in your weed control program. Then, you can come back later with burn-down materials to mop up your fields and control any escapes.”
The new pre-emergent herbicides include Alion SG, Chateau, Matrix SG, Pindar GT, and Trellis. They cost more than most post-emergent materials, he notes. But, they also do more.
Depending on the product, the newer pre-emergent materials can control certain noxious weeds, including fleabane and marestail, which have developed resistance to glyphosate, the active ingredient in Roundup and similar products. These weeds aren’t easily controlled with the older pre-emergent herbicides, Hembree notes.
The spectrum of weeds controlled by these products, whether a newer or older type of herbicides, varies from one product to another. That’s why he stresses the need to match the herbicide to the specific weeds in an individual field. When the newer products are combined in appropriate tank mixes, with either other newer materials or older pre-emergent herbicides, they are effective against a broad spectrum of other difficult-to-control species, such as puncturevine, Palmer amaranth, nutsedge, and jungle rice.
The newer pre-emergent herbicides also offer long residual activity. “In many cases, we’ve seen them remain effective for 200 days or more after application,” Hembree says.
That’s not all. Drip irrigation systems keep soils wet throughout the summer, causing older pre-emergence herbicides to break down faster than in drier soils. The newer products tend to remain effective much longer when used with drip systems, he says.
Pre-emergent herbicides are designed to be applied on the soil surface before rainfall, which works the material down into the weed seed germination zone. As little as 1/8- to 1/4-inch of rain will activate the products.
“Because they adhere pretty tightly to soil particles, they stay where you put them. Hembree says. “You can apply them on up to 28 days ahead of rain and they’ll remain in place without volatilization. Generally, the closer to rainfall you apply them, the better the weed control you’ll achieve.”
While the newer pre-emergent herbicides can be effective when applied from early December through late March, Hembree recommends delaying until February and March, when rain is still likely, to extend control farther into summer.
“If you can get the product on then, generally, it will remain effective through the summer and even into harvest,” he says.
The latest pre-emergent materials, like Alion, Matrix, and Chateau, can also be used to help control jungle rice, Hembree notes. It’s become a problem around the perimeter of areas wetted by drip emitters and continues to spread throughout the San Joaquin Valley
He recommends following up a late-winter application of these pre-emergent products with an application of Prowl or Surflan in April or May in advance of predicted rain.
“This can extend effectiveness of Prowl or Surflan an extra few months to help control jungle rice,” he says.
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