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Weed Pressure Building After Wet Spring


Spring rains and warm weather creating headaches.

Now that temperatures are reaching near normal seasonal levels on the heels of what many California growers are calling an exceptionally wet year, weed pressure is increasing in many orchards and the race is on to get the problem under control.

Mick Canevari, University of California professor emeritus in weed science and former University of California Cooperative Extension farm advisor in San Joaquin County, says not only has the wet spring added to weed pressure across much of the San Joaquin and Sacramento Valleys this year, weed varieties resistant to glyphosate are increasing across the Golden State.

“Herbicide resistant weeds are popping up here in California and that problem has been growing over the last decade or so. Most of our weed resistant issues are tied in with glyphosate. In California we are on the coat tails of what is happening in the Midwest and in Southern regions of the nation where glyphosate-resistant weed control is a major concern. But we are seeing an increase in resistant weeds and that is proving to be a troublesome problem,” Canevari told Western Farm Press.

Canevari says glyphosate resistant weeds in California are not at the same level of risk as they are in other parts of the country.

“I don’t want to say the glyphosate-resistant weed problem is out of control in California – it isn’t. We’re on top of it and have a number of tools to combat the problem. But it is a growing issue that has the potential of getting worse,” he added.

West Coast weed scientists warn that resistant weeds are not limited to row crops but are also a problem in nut orchards. California weed scientist Brad Hanson is among them.

“As most orchardists and pest control advisors are well aware, glyphosate-resistant weeds have been one of the biggest weed management challenges in California orchard crops for several years,” Hanson notes.

Winter annuals

Of particular concern have been hairy fleabane and horseweed, also commonly known as mare’s tail, which dominate the annual winter season. Winter annuals generally germinate and emerge during the cool season and reach a reproductive stage by spring or early summer. But in recent times glyphosate-resistant species have been reported in summer annual grasses. Summer resistant weeds then germinate and emerge as the season warms in late spring and summer and grow well into the summer season before reaching maturity.

Hanson notes these may include junglerice, threespike goosegrass, and several other glyphosate-questionable species such as feather fingergrass, sprangletop, and witchgrass.

These late emerging warm season resistant weed varieties are more problematic because fall and winter preemergence herbicide applications generally take place far too early to be effective on warm season varieties. Most orchard preemergence herbicide applications take place between late November through the month of February.

Hanson says this means if spring applications of foliar material like glyphosate fail because of resistant issues, weed problems will quickly multiply. He says it all comes down to application timing. Herbicide residuals are limited, meaning environmental conditions and other factors will determine how long a preemergence herbicide will work to control weeds.

The best approach may be to include preemergence herbicide applications along with your spring burndown schedule in the orchard, as late perhaps as early spring. Spreading out the preemergence herbicide application schedule could help maintain the herbicides effectiveness into the warm-weather season.

Hanson calls this a sequential approach to weed management that could serve to help control both winter and summer annual weed problems.

Canevari agrees that weed control in a fruit or nut orchard can quickly become complicated with the introduction of glyphosate resistant weed varieties. He warns that growers who opt to split their preemergence herbicide applications between the fall and spring seasons should be aware of their typical weed profile in their orchard and be prepared to make adjustments as environmental conditions – such as heavy spring rains – change.

While a sequential approach to combating annual winter and annual summer weed varieties can provide extended benefits for about the same input costs under some circumstances, only by a willingness to adjust the timing and the rate of herbicide applications can save you if conditions change and resistant weed emergence becomes obvious at any point in the winter and summer growing season.

For more news on tree nuts as reported by growers and farm advisors, subscribe to the Tree Nut Farm Press e-newsletter.

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