Weeding Out the Competition: Integrated Weed Management in California Citrus

For California’s citrus growers, weed control isn’t a game; it’s a competition. In a sense, it comes down to this: It’s either weeds or trees.

Weed control comprises 10-15% of production costs in citrus, according to a 2012 study by Analiza H.M. Ramirez and Megh Singh at the Citrus Research and Education Center in Lake Alfred, FL.¹

That study focused on Florida citrus, but the precepts are applicable in California.

“Weed species compete with citrus trees in many ways and with varying intensities, so management of this problem is an early and common challenge for California citrus growers,” says Sonia Rios, area subtropical horticulture farm advisor with University of California Cooperative Extension (UCCE) in Riverside County.

Weeds pose multiple challenges for California citrus growers:

• They compete with trees for water and nutrients.
• They serve as alternative hosts for insects, notably thrips.
• They are conduits for disease.
• They interfere with measures to fight potentially lethal freeze events.

To fight weeds and mitigate resistance, growers should apply glyphosate on a rotational basis with several other pre- and post-emergent herbicides that have different modes of action. Growers must be proactive with their integrated weed management (IWM) programs to best protect their trees.

“Our weed management program has to be just about perfect for our frost-protection activities to work,” says Joel Nelsen, president of the Exeter-based California Citrus Mutual.

The Threats

Weeds pose one of the most persistent threats to healthy citrus groves, growers and researchers agree. Horseweed is among the most vexing problems in orchards because it is aggressive in competing with trees for moisture.

Rios says that if left unchecked, weed-related problems can manifest themselves in multiple ways, including:

• Weeds disrupt cultural operations such as pruning and harvest.
• Weeds impact tree growth and tree yields by altering the spray pattern of low-volume irrigation systems.
• Weeds can intercept soil-applied crop protection products.

Weeds pose an even greater threat in younger groves, Rios says. Younger trees can be stunted because they can’t compete as aggressively for water.

Ben Faber, soils/water/subtropical crops advisor with UCCE in Ventura, points out additional problems weeds can cause in the grove if they are unmanaged and uncontrolled:

• Weeds reduce grove temperatures during freeze events.
• Wading through wet weeds to get to trees makes it hard for workers to stay dry.
• Weeds increase the incidence of invertebrate pests and unwanted wildlife such as voles and gophers, as these animals are harder to find in the weeds.
• Weeds growing around tree trunks create wet conditions that encourage the growth of pathogens that infect the trunk and roots, increasing the incidence of trunk rot.

Faber explains that weeds can reduce grove temperatures, which isn’t so bad during hot summer months but can be deadly during frost season and freeze events. Ground cover can drop temperatures three to five degrees, which can be significant.

In addition to the danger of snakes hiding in wet weeds, “wildlife such as gophers, rabbits and voles hide in weed vegetation, which makes them harder to control,” Faber notes. “They can attack the roots and bark of trees and kill them.”

Numerous other types of California wildlife, including coyotes, wild pigs and birds, present additional threats by spreading weed seed, Rios adds.

“Birds and other mammals can spread weed seeds when they defecate,” she says. “Weed seeds are very resilient and can survive in the digestive tract. Also, weed seeds stick to fur and feathers, which can help weed seeds spread.”

Weeds also serve as reservoirs for plant pathogens that can affect crops, Rios says, noting that weeds can serve as “the initial inoculum” for epidemics in crops and can intensify existing problems by providing multiple hosts for reproduction.

Diagnosing a problem early is advisable, but it’s also common sense, advises Faber. The goal is simple: Reduce weed populations to a level that will lessen the impact of competition.

Stopping a Problem Before It’s a Problem

Getting ahead of the weed problem before it starts is the best way to combat the issue. Employing four strategies – prioritizing, scouting, herbicide application and cultivation practices – helps growers tackle the problem head-on.

In general, an effective weed-fighting arsenal contains numerous weapons and expertise on how to use them. Growers must keep in mind that a balance of tools is probably as important as the tools themselves when it comes to preventing the emergence of herbicide-resistant weeds.

“They pose a danger for increasing in the future unless we use rotation methods,” Faber says.

Prioritizing

Growers prioritize weeds by their competitiveness. Citrus growers’ main aggressive weed priorities include:

• Fleabane
• Horseweed
• Johnsongrass
• Vetch
• Dallisgrass

Less-competitive weeds include:

• Puncturevine
• Spiny cocklebur
• Stinging nettle
• Bull thistle
• Bristly oxtongue

Less-competitive weeds may require more active management because they can deter labor operations.

Scouting

In addition to prioritizing weeds, growers should have a robust scouting program.

Among the most effective early strategies available is “prevention by scouting – taking care of visible weeds when they are young, before they can reproduce,” Rios says. She also advises growers to keep an eye out for weed shifts when scouting.

Weed management is about managing the weed seed bank, says Brad Hanson, Extension weed specialist at UC-Davis. For weeds that are already present, growers should control them before they set seed. “It’s easier to control a new weed problem in a smaller area than after it is spread over the whole field or region; that’s where the scouting comes in,” Hanson adds.

Scouting, identification and early management combine to create a good integrated approach to prevent weeds from going to seed and are crucial in avoiding weed proliferation. “If I go out and see a weed I’ve never seen before and it doesn’t have seed yet, I should pull it out or chop it down before that single plant has a chance to produce 1,000 or 100,000 seeds,” Hanson says. “Those seeds will produces the weeds that then have to be controlled for the next several years.”

Herbicide Application

By prioritizing and scouting for weeds, growers determine their weed pressures, which can help them determine the best herbicides for their weed-management programs. This integrated approach makes it possible to attack even the smallest populations of weeds before they become a problem.

When it comes to herbicides, Faber says there are some labels for irrigation-applied herbicides, but most growers prefer to control weeds by broadcasting or spraying three or four times per year.²

Managing herbicides is important for keeping the weed population under control and avoiding herbicide-resistant weeds. Nelson and Hanson recommend the following:

• Know what each herbicide does.
• Rotate modes of action.
• Use both pre-emergent and post-emergent herbicides.
• Include at least three different chemistries and mixtures in rotations.
• Tank mix different modes of action.

When it comes to pre-emergent and post-emergent options, growers employing a comprehensive IWM program have an array of tools at their disposal. Among the former are simazine, indaziflam and rimsulfuron. Post-emergent tools include glyphosate, glufosinate and saflufenacil.

Bayer offers Alion^® herbicide for weed control in citrus orchards. Alion offers pre-emergence control of a broad range of grass and broadleaf weeds, including ALS-, ACC-ase-, triazine- and glyphosate-resistant species. The chemistry in Alion features a cellulose-biosynthesis inhibitor as its active ingredient.

Cultivation Practices

The last piece of the weed-prevention puzzle is cultivation. Helpful cultivation practices include:

• Tillage: Approach tillage with caution. Hanson points out that because shallow roots in established orchards could be damaged, tillage is discouraged, although some growers do use it. Nelsen adds that another downside to tillage is that it encourages weeds to grow by bringing weed seed to the surface.
• Mulch: Mulch can serve a dual purpose – to prevent weeds and maintain soil moisture. Some mulch, however, can be embedded with weed seeds, so it’s important to know the source of the mulch.
• Cover crops: Some growers employ cover crops as weed fighters. Cover crops (clover varieties are popular for this purpose) can also improve water infiltration rates, help control erosion and mitigate dust. On the other hand, this approach also has risks. Cover crops can increase the risk of frost damage, and some attract snails, slugs and gophers. Because watering cover crops adds expense, extended periods of drought have lessened the use of cover crops in recent years.

Having a sound IWM approach can help citrus growers fight the weed problem successfully, says UC-Davis’ Hanson.

“There’s nothing particularly novel about IWM; it’s a mindset more than an approach,” he says. “You’re thinking, ‘What weed problems am I trying to manage?’ A part of that is identification and quantification of problems, really trying to understand what weed problems need to be addressed. Another part of IWM is the tools at our disposal, and those run a full spectrum, from cultural practices such as canopy management, water placement and timing of irrigation to mechanical controls such as mowing and tillage, to herbicides. Any of those approaches can be a part of an IWM as long as growers really consider the specifics of a weed problem, what tools they have at their disposal, and how they can best be economically and environmentally sustainable.”

Sources

¹Ramirez, Analiza H.M., and Megh Singh. “Weed Management Recommendations for Florida Citrus.” Southwest Florida Research & Education Center, University of Florida, 24 July 2012, swfrec.ifas.ufl.edu/docs/pdf/squeezer/2012/july/ramirez_singh.pdf.

²“Citrus Agricultural Pest Management.” UC IPM Online, University of California Agriculture and Natural Resources, ipm.ucanr.edu/PMG/r107700411.html.

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