While some management practices are one-time events, weed control for tree nut orchards is a year-round challenge for growers, one that gets particularly intense as harvest nears.
Brad Hanson makes weed management his fulltime job as part of the University of California, Davis Plant Sciences Department where he uses field-based research to reduce the impact of weeds on cropping system productivity. His integrated-approach-to-weed-management toolbox includes chemical, cultural, biological, and physical manipulations of the cropping system.
While a common weed control program consists of winter pre-emergence herbicides with desiccation treatments before harvest, some summer-germinating species or herbicide-resistant biotypes have become troublesome with growers who are now reporting difficulties in controlling summer-germinating grass weeds that show resistance to herbicide programs.
“We’ve been looking at a sequential pre-emergence approach to control both winter- and summer-germinating orchard weeds,” the scientist says. “Field experiments have been conducted in tree nut orchards to evaluate the efficacy of common winter herbicide programs and a sequential herbicide program for control of key summer grass weed species.”
The field trials of sequential application strategy all involved the addition of pendimethalin during both seasons, a move that enhanced summer grass weed control by up to 31%. Applying at least part of the pendimethalin in the spring improved control of junglerice by up to 49%.
“Tailoring sequential herbicide programs to address specific weed challenges can be a viable strategy for improving orchard weed control — without increasing herbicide use in some situations,” the cooperative extension specialist says.
“The thought process was — rather than apply a lot of herbicide in the winter trying to get control eight months later, what if we applied some of that herbicide closer to the time when those summer weeds would appear — and it worked pretty well.
Achieving control without over-treating
“It’s an approach that some growers have already started using to maintain weed control and decrease herbicide load in the environment, using a tool in the most appropriate time to achieve the goal without over-treating,” he says. “It’s one of my basic mantras — a smarter use of tools rather than an increased use of tools.”
Over the course of an almond orchard’s life, the competition between weeds and trees can create some inefficiencies. If the weeds use fertilizer and water that the trees would otherwise be using, weeds will interfere with nut production, especially so at harvest time.
“The physical impediment of having live or dead weed debris on the orchard floor can interfere with nut recovery as well as the operation of irrigation equipment. The physical presence of weeds will become problematic,” Hanson says.
The year 2020 doesn’t seem to have brought anything strange in the way of weeds.
“I haven’t heard of anything out of the ordinary,” he says. “There are certain weeds that are particularly problematic in certain areas, but usually, year- over-year, weed problems are related to weather. It depends on how rainfall occurred during the winter because that impacts how the herbicides either do or don’t perform. Winter rainfall patterns really dictate how many panicked phone calls I get from growers.”
Asked if the new procedure would help win the crop-versus-weed war, Hanson chuckled before acknowledging, “I don’t think you’ll ever defeat weeds. I think mostly you’re just trying to hold your own in the battle, getting weeds to evolve so we can control them with different management practices, managing them to minimize their impact on our production systems.”
The chuckle turned into a laugh when he acknowledged, “I’m not worried about whether or not I’m working my way out of a job. I suspect there will be plenty of weeds for me to conduct research on for the remainder of my career.”
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