Filaree, annual sowthistle, malva, horseweed and hairy fleabane are just a few of the usual annual visitors that will again be arriving at California tree nut orchards in the weeks and months ahead. And these are the type of visitors that insist on hanging around for a long time.

While late fall rain has been light or nonexistent in most areas and winter rains are only now arriving, as moisture builds in West Coast orchards, winter weeds will be quick to follow.

Researchers say winter weeds that become well established can rob nut trees of nutrition and vigor. If preemergent herbicides have not been applied by Dec. 1, that should happen as soon as possible. Be warned, the trunks and foliage of young trees need to be protected from contact with some postemergence herbicides. Be sure to check and follow individual label instructions.

Herbicide treatments

Postemergence herbicides are used on established weeds. Contact herbicides kill those parts of the plant that are sprayed, making good coverage and wetting essential. A single treatment can kill susceptible annual weeds, but re-treatment is necessary if perennial weeds regrow from roots or other underground structures or if new germination of annual weeds occurs after the initial application.

"Most growers are probably finished with the burn down of summer weeds and preemergent applications, but as the late season and winter rains begin to fall we can expect to see some of these winter weed varieties popping up," says Kurt Hembree, University of California Cooperative Extension weed management advisor in Fresno County.

He says once young winter weed varieties emerge ― if they emerge following fall preemergence applications ― it becomes prudent to identify them to know what tools to employ to avoid an outbreak before it starts. He also reminds growers to select and thoroughly clean the right spray nozzle and make spray pressure adjustments required for winter applications throughout the colder months leading up to early spring.

In addition to winter weed varieties mentioned above, broadleaf weeds including horseweed and grasses such as ryegrass and annual bluegrass are commonly found during the winter in California orchards.

Hembree also advises that if identifying young weed varieties is difficult they should consult their pest control advisor or refer to the University of California integrated pest management website and search 'weed gallery,' which provides photos of weed varieties for identification purposes.

Seasonal surveys

Researchers say a good weed management plan will also involve seasonal weed surveys. A late fall weed survey should be staged after the first rains of the fall when winter annuals have germinated and started to emerge. Dr. Brad Hanson, extension weed specialist and Vice Chair of Outreach and Extension at the University of California, Davis says monitoring weeds in fall accomplishes several tasks.

It helps to identify remaining summer species and perennial weeds that escaped the previous year's weed control program so that adjustments can be made to control these species in the next year.

Fall monitoring will also identify winter species that are emerging. He advises growers to keep records of their observations and map locations where weeds are problematic each year.

Hembree warns while most growers are familiar with what winter weed varieties are common for their orchard, he says some varieties have developed resistance to glyphosate and ALS-inhibiting herbicides and a good winter weed management plan should include the use of at least two modes of action to counter resistance problems.

Controlling weeds in an orchard is a year-round task and requires diligence and dedication to stay ahead of serious outbreaks. Winter weed control is where the weed season begins, so winter weed management is critical to starting the new year off on the right foot.

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