New troubles with old weed problems call for different control approach

New troubles with old weed problems call for different control approach

The top challenges facing California’s tree nut growers in controlling weeds are not new weed species but new problems with long-standing weed threats. The biggest difficulty continues to be expansion of herbicide-resistant biotypes of existing weed species, says Brad Hanson of the University of California.


The top challenges facing California’s tree nut growers in controlling weeds aren’t new weed species but new problems with long-standing weed threats.

“The biggest difficulty continues to be expansion of herbicide-resistant biotypes of existing weed species,” says Brad Hanson, University of California Cooperative Extension weed specialist.

Much of that resistance involves glyphosate, the active ingredient in Roundup, and some other weed-killing products. Because it offers relatively inexpensive control of a wide range of weeds, glyphosate has long been the go-to choice of many growers for controlling broadleaf and grass weeds in their orchards.

However, heavy reliance on this one mode of action has selected for some species or populations that are able to survive glyphosate treatment.

Glyphosate resistance

State-wide the most wide-spread of these resistant weed types is probably hairy fleabane, Hanson notes. However, some growers in the Sacramento Valley and northern areas of the San Joaquin Valley, might disagree, he adds. They also rank ryegrass as among their greatest glyphosate-resistant weed problems.

Marestail or horseweed is another species found in California orchards that has developed resistance to glyphosate, Hanson points out.

So has junglerice. In the past few years, glyphosate-resistant stands have been confirmed in more and more areas of the Central Valley. That includes pockets from near Gridley in Butte County south as far as Kern County, Hanson reports.

The buildup of glyphosate resistance in junglerice has attracted special attention from weed scientists. Over the past decade or so, most resistance problems in orchards have been winter annuals. Junglerice is the first summer annual weed discovered to do so in California, he notes.

“That makes for a difficult control challenge,” Hanson says. “Junglerice begins germinating and emerging in May just as the residual activity of pre-emergent herbicides applied the previous December and January is beginning to run out.”


Weed scientists are also keeping their eye on two other annual weeds found in California orchards. One of them is annual bluegrass. Several cases of glyphosate-resistance in this weed have been confirmed, Hanson reports.

Another is three-spike goosegrass, a short-lived perennial. Some hot spots of suspected glyphosate-resistance of this weed have been found in the Merced and Atwater areas of Merced County as well as farther north in the Sacramento Valley.

“Those are the areas where I’ve had the most questions about possible resistance of three-spike goosegrass to glyphosate,” Hanson says. “But, it’s likely any resistance has spread to other areas where the weed is found.”

Three-spike goosegrass germinates from seed or starts re-growing from existing clumps in the spring before junglerice emerges,” Hanson notes.

“Three-spike goosegrass is particularly challenging,” he says. “Once it starts tillering, it’s difficult to control with glyphosate and other available post-emergence herbicides.”

The key to controlling this and other summer annual weeds in California’s tree nut orchards is a good pre-emergence herbicide program, Hanson says.

It can significantly reduce the weed populations that, otherwise, will require control later in the season. Although pre-emergence herbicides cost more to buy, their residual weed-controlling activity often can eliminate the need for one or more later spray applications. This, in turn, can offer some management flexibility while reducing operator and machine costs, he notes.

Primary drawback

The primary drawback to pre-emergence herbicides is the need to incorporate them into the top layer of soil where weed seed germinate, usually by rain or irrigation. For most commonly used pre-emergence materials, ¼- to ½-inch of water is recommended within a few weeks of application for best results.

“However,” Hanson adds, “even in last year’s drought conditions, many pre-emergence herbicides still worked surprisingly well, despite less than ideal conditions. This included several weeks on the soil surface with minimal incorporation.”

To get out in front of summer annual weed problems in tree nut orchards, he advises modifying the typical program used for controlling winter annual weeds, such as ryegrass, hairy fleabane, and cheeseweed.

As with conventional practices, Hanson advises starting with a December or January application of a pre-emergence product targeted against the winter annual weed spectrum.

However, if summer annual weeds are also a problem, growers may want to consider a sequential application of two pre-emergence herbicides – one applied early enough to control the winter annuals and one applied in late winter or early spring to target the summer grasses.

This approach may more effectively stretch weed control into the summer, compared to a one-shot tank mix of the two products in the winter, without increasing the total amount of herbicide used. The most aggressive and broadest spectrum material would be applied with the first treatment.

The second material, combined with a post-emergence partner, would control any escapes and more effectively target the summer weeds.

As Hanson explains, this approach doesn’t necessarily require an extra trip through the field, since many growers already put on a post-emergence treatment at this time to control any winter annuals that escaped the dormant season application. 

The idea is to come back in March, trying to catch one late rainfall to incorporate the pre-emergence material, rejuvenating control and getting a jump on junglerice and other summer annuals.

A contact or burn-down herbicide could then be applied in the summer to control later-emerging summer annuals as part of the normal pre-harvest preparations.

Glyphosate value

At the same time, Hanson is not suggesting growers eliminate glyphosate in their weed control program.

“Growers continue to find a lot of value in glyphosate,” he says. However, to broaden the weed control spectrum and reduce selection for weed populations resistant to glyphosate, I’m seeing a number of growers tank-mixing it with other products, such as Goal, Shark, Venue and Treevix, which provide different modes of chemical action for broadleaf weeds.”

Also, in planning this season’s weed control program, Hanson recommends checking product labels for the latest recommended rates and restrictions. Some important pre-emergence products, including Alion, have changed this year.

The October-December, 2014, issue of California Agriculture, published by UC Agriculture and Natural Resources, features several articles on herbicide-resistant weeds. (

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