Effective Weed Management Requires Planning, Tenacity

(April 24, 2020) – You can’t have a conversation about weed management in the almond industry without talking about the work conducted by Brad Hanson. Hanson, a University of California (UC) Cooperative Extension weed specialist, has spent his career at UC Davis analyzing weeds and researching how to control them in California’s orchards and vineyards.

Last December, during a session devoted, in part, to weed management at The Almond Conference 2019, Hanson reminded growers that dealing with problematic weeds is a year-round issue and that simply spraying more frequently or in higher volumes is generally not the answer.

“My premise is, ‘We know we are going to manage weeds on every acre every year. Can we be smarter with the tools we use? Can we use them in a very specific manner to achieve weed management without just spraying again?’ It’s important to be thoughtful and prescriptive in what we’re doing, not just applying more,” Hanson said.

Spring is the time, Hanson advised, for growers to analyze their winter herbicide results and map where weeds are located in the orchard. He said there are a couple key questions growers should ask themselves.

“What worked this winter and where are the weak links,” Hanson said. “What do I need to do in the spring and are there more effective approaches to consider next year?

“Year to year, we see similar weeds. Mapping helps identify resistance, new invaders or other weed problems. It’s always best to identity a problem when it’s small: I’d rather spend $500 on one acre one time to get it in check rather than spending much more across an entire orchard for several years when it’s out of control.”

Orchard Survey Key to Understanding

To help growers optimize their weed management approach, the UC Integrated Pest Management (IPM) program provides a list of weeds that are commonly found in almonds in addition to guidelines on how to monitor for and combat pesky weeds. Those guidelines recommend the following:

• Survey and thoroughly monitor entire orchards, field margins, ditch banks and irrigation canals for weeds in late fall and again in late spring. Examine all areas that are susceptible to weed infestation, such as areas of high moisture, and document weed species, location in the field, the degree of control achieved with the current program and what herbicides were applied.
• Record observations on a survey form that includes a map so the infested sites can be revisited for weed control. Pay particular attention to perennial and other problematic weeds, noting their location.
  • Record weeds found in rows and middles separately. Weeds in tree rows must be managed, but annual weeds in row middles — i.e., cover crops — may have some benefit as an orchard floor cover or can be managed with mowing.

UC IPM’s guidelines recommend growers choose herbicides or other control techniques based on what kinds of weeds are present in the orchard. If growers are uncertain about what a weed is or how to address it, they should contact their PCA or local farm advisor.

Beware of Herbicide Resistance

Weed resistance to various herbicides – especially glyphosate, a.k.a. Roundup – is an ongoing issue for the almond industry and other crops. Roundup is widely used across row and specialty crops but faces increasing public scrutiny.

Hanson has studied glyphosate resistance for more than a decade, and unfortunately no one product has emerged to replace this product.

“It really is a cornerstone herbicide for all tree crops,” he said. “At this time, we don’t have a drop-in replacement that works quite as well, particularly on large weeds or perennial species.”

Growers can observe patterns of herbicide resistance, which are identified by patches of dense weeds with less dense populations radiating out from the central patch and weeds that have escaped control scattered throughout the field.

To combat resistance and reduce the spread of weed seeds, UC IPM recommends growers:

• rotate herbicides that have different modes of action and Weed Science Society of America (WSSA) group numbers,
• monitor for weed survival after an herbicide application,
• include non-chemical weed control methods, such as mowing, especially for middles,
• clean equipment after it’s been through in weed-contaminated orchards to prevent the spread of weed seeds,
• control weeds suspected of herbicide resistance before they can produce seed, (If weeds escape treatment, use shovels, hoes and other hand tools to cut the plants below the soil surface to prevent flowering.), and
• use a pre-emergent herbicide before weeds appear. When the weeds emerge in spring and fall, consider splitting applications to meet the multiple emergence windows.

Hanson recognizes that determining how to effectively manage weeds is just one out of many decisions that growers must make to responsibly and efficiently grow their crop. Still, he is encouraged by the industry’s commitment to “smarter weed control” at a “lower cost and pesticide load on the environment.”

“The industry should be aiming to use the least amount of herbicides while maintaining the best weed control possible,” he said. “It’s about making smart decisions, not just using a recipe. It’s part of being good stewards to our system. It’s often easy to keep doing what works, but I want to help growers move the needle while avoiding over-treating their crop.”