Effectively reducing weed populations in orchards requires persistence and investment to control weeds before they set seed. Poor weed control can result in an abundance of seed set and heavy weed pressure for several years into the future.

Data and Scouting

An effective weed management program starts with data. This could be actual weed counts or a mental assessment after walking or driving through the orchard multiple times during the season. Weed management is not one and done; it should be fine-tuned based on what is or isn’t working and any new weed problems coming into the orchard, according to Brad Hanson, UCCE weed science extension specialist.

Ryan Hill, UCCE agronomy and weed science advisor for Shasta, Tehama and Glenn counties, said, “This is why it’s important to know what is in the orchard and to map and scout weed populations, especially if there’s been a repeat use of the same treatment in a field.”

Repeated use is the No. 1 risk factor for resistance. “If you’ve been doing that, you need to start, not just to scout your fields, but I would encourage people to map their fields and record where different weed populations are,” Hill said.

Field scouting is also an opportunity to identify and manage problems such as new species or suspected herbicide resistance while the problem is small and localized. This could save a great deal of effort and expense later, Hanson said.

Post-season is a good time to ask what happened, what worked and what didn’t work. Why didn’t it work? What should I do differently next year?

“It’s an iterative process to develop and then fine tune a program and modify it as needed as the situations change, as the orchard gets older, as different weeds come in, as we make changes in our production system that affect our weed control,” Hanson said.

Split Approach for Preemergents

Hanson has suggested a split approach for preemergent applications: One in the fall and one in the spring. “That’s one of the approaches that I’ve been recommending the last few years. What I think happens relatively frequently with a couple of our key preemergent herbicides is some growers delay their application, thinking, ‘Well, if I wait until January, then that’ll carry me through harvest,’” Hanson said, but actually growers would be better off if they started earlier (November or early December) because that gets the preemergence herbicide into the soil in time to catch the first flush of weeds.

Importantly, that earlier timing also reduces the risk of a major rain event, like recent storms in northern California, keeping the spray rigs out of the field and missing the ideal PRE application window altogether, Hanson continued.

That delayed application is unlikely to totally get you through harvest anyway, Hanson said. “If you put it on in February instead of December, you’ll still see some weeds breaking through in the spring as the herbicides degrade in the soil, and those processes happen quickly as the soil warms up in the spring, and as you start putting the water on.

“I would rather see those really highly effective preemergent herbicides used early to get the early emerging weeds. That could be ryegrass if you’re in the north, it could be fleabane just about anywhere in the state,” Hanson said, adding this will more likely result in a longer duration of weed control and more effective weed control by catching the early emergers.

“No matter what you use, you’re going to start to have some breakthroughs in the springtime,” Hanson said. “I think using those effective PRE materials early gives more bang for the buck and leaves fewer breakthrough weeds in the spring to clean up.”

Often growers use high rates in the winter in an effort to get six or eight months of weed control.

“In my mind, that’s not the most effective way to use those herbicides,” Hanson said. “If you’ve got challenges with summer emerging weeds, tailor a weed management program to address those weeds specifically.”

It makes more sense to use the right herbicide, at the right rate, at the right time, and apply another shot if more herbicide is needed for a different weed problem at a different time of the year, he said.

Incorporation

Preemergents must be incorporated. Orchards with solid-set sprinklers or microsprinklers can use irrigation to incorporate, but with drip irrigation, it requires rain.

“A solid-set sprinkler system is going to be pretty much just as effective as rain. I haven’t done this test, but my sense is with the micro sprinklers most orchards have sufficient coverage in the tree row, so I think we can do a good job incorporating herbicides with our microsprinklers, too,” Hanson said.

Drip irrigation is a different story. “We’re really not getting much activation or incorporation with a drip system. It’s a very good system for micromanaging water and fertilizers, but it’s not a very helpful system for incorporating herbicides,” Hanson said.

Orchard Debris

Orchard debris can be problematic when applying a preemergent. “Blowing or sweeping those berms is really, really important in walnuts, in grapes, in pears, any of those plants that have really durable long, lasting leaf material,” Hanson said.

Hill agreed the orchard floor needs to be clear of debris before a preemergent application is made. “Many preemergents will bind tightly with organic material. If you can’t get the herbicide down to the soil, it’s not going to be effective,” he said.

“There are some herbicides that handle thatch and leaves well, and water will wash it down to the soil, but that’s not something you want to rely on. You need to get those strips clean before you spray, especially in walnut orchards,” Hill said.

Hanson’s colleague, John Roncoroni, did a project in vineyards several years ago. Roncoroni did a split experiment where he blew some of the berms and others he didn’t.

“My recollection was he got about a 30% improvement in weed control with just about any preemergent herbicide by putting it on the soil rather than on the leaf material,” Hanson said.

“No question there’s a cost to sweeping or blowing the berms, but it also improves the efficacy of the herbicide, which is also very costly,” he added. “Most of the nut growers are doing some kind of winter sanitation, so they’re going to be sweeping and blowing the orchards anyway.”

Early Preemergent Applications

Hanson prefers earlier applications. “We had some old herbicides that were more volatile or more prone to degradation in sunlight, but those aren’t our primary herbicides anymore. That’s why I tend to err on the slightly earlier side because it’s easier to manage the debris, it’s easier to get the preemergent on and incorporated before the weeds germinate,” he said, and growers with microsprinklers or solid-set sprinklers aren’t dependent on rain to incorporate the preemergent.

“If you have the luxury of putting a half inch of water on with your irrigation system, then I would definitely start on the early side with my preemergent programs,” Hanson said, especially with weeds like fleabane and ryegrass that start germinating early and are widely resistant to glyphosate.

These weeds are the ones that growers have the most trouble with, Hanson said. “It’s because a postharvest irrigation or an early rain gets enough moisture in the soil for those weeds to germinate and become established before we get our preemergence on.”

It’s easier to keep things clean than it is to get them clean again, he said.

Managing the Middles

There are some important things to consider with the middles of the orchard, Hill said. Scout weeds that are growing in the middles. That’s going to be a source of weed pressure for your spray strips, he added.

“I’ve seen orchards that are solid stand of junglerice. Junglerice has glyphosate resistance issues, and it also has very shallow roots,” Hill said.

Annual bluegrass also has very shallow roots, and this can be an indicator of soil compaction. It also doesn’t allow water to infiltrate as far into the soil because the paths that are available are so shallow, Hill explained.

“Active management of those middles to promote species that have deeper tap roots can be a benefit,” Hill said, adding cover cropping is the way to do that. “It is a lot cheaper to use resident vegetation, but the composition of the weed population matters.

“If you’re seeing standing water in your orchard, maybe your resident vegetation isn’t doing what you hope it could and you’re missing an opportunity,” he said.

Resistance

It’s important to rotate the preemergents to prevent resistance from building. “Rotating chemistries is just a good idea generally for resistance management,” Hanson said.

“There is resistance to several preemergent herbicide chemistries in the U.S. and in the world. We don’t have any resistance to any of our major preemergents in orchards in California yet, but it’s not that it’s impossible,” Hanson said.

Resistance management before there’s resistances is important. Switching chemistries every one out of three years or one year out of four years is a good resistance management plan, Hanson said.

Hill agreed rotating chemistries helps reduce the risk of resistance. “Glyphosate resistance is getting very common, but resistance to group 2 herbicides is the most prevalent worldwide. Folks should rotate herbicides carefully if they are using any of these products.

“It can also be tempting to overuse the strongest preemergents available and tempt resistance, but it is easier to rotate other chemistries in if you use Hanson’s split application method,” he said.

“None of these weed management decisions are ‘set it and forget it.’ It comes down to making smart decisions,” Hanson said.

Think about what the problems are, and what herbicides will work on those problems. “If you can mix it up every year, every other year, every third year, those are all risk mitigation approaches. It’s better to mitigate the risk than to try to resolve it after you have the resistance issues,” Hanson said.

Resistance to our key preemergent herbicides would be a big problem and it’s just as important to do resistant management for preemergents as post-emergent herbicides, Hanson said.

“A lot of these decisions are really trying to make sure we get the most bang for our buck, getting the most weed control we want for the longest period of time, reducing the risk of injury to the crop, reducing the risk of harm to the environment,” Hanson said.

“You’re buying the stuff to kill weeds, so kill as many weeds with it as you can,” Hanson said. “ Use your weed management tools well.”