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How to Manage Pests

UC Pest Management Guidelines


Pistachio

Integrated Weed Management

(Reviewed 2/07, updated 2/09)

In this Guideline: More about weeds in pistachio:

Weeds compete with pistachio trees for water and nutrients. The competition for these resources is of greater concern when trees are young because weeds can delay growth and production. Weeds can also harbor pests and pathogens, interfere with irrigation uniformity and distribution, and reduce harvest efficiency. Integrated weed management involves the use of multiple strategies to manage weed populations in a manner that is economically and environmentally sound. Such strategies include cultural, mechanical, chemical, and biological methods.

Integrated weed management strategies vary from orchard to orchard. Location in the state, climatic conditions, soils, irrigation practices, topography, and grower preferences influence pistachio floor management decisions and the tools used. Weeds are commonly controlled either chemically or mechanically in a 4- to 6-foot-wide strip in the tree row. The area between the tree rows may be chemically treated, mechanically mowed, or tilled. Mulches, subsurface irrigation, and flamers can also be used to control weeds in orchards. Often several weed management options are used in an orchard depending on the types of weeds present, age of the trees, soil conditions, and grower preference.

Irrigation method, amount of water applied, and pattern of rainfall affect weed growth as well as the frequency and timing of cultivation and selection of herbicides and their residual properties. For example, soils that receive frequent, low-volume, drip or micro-sprinkler irrigation increase the degradation of herbicides in the soil. Herbicides are degraded faster in warm, moist soils as compared to cold, dry soils.

Although there are not many herbicides registered in California for use in pistachios, they are an important component of an integrated weed management program. Herbicide selection is an important process and is determined by the species of weeds present, stage of weed development, weed density, herbicide toxicity, herbicide persistence, soil type, soil moisture, irrigation method, environmental conditions, labor and equipment availability, and economics. Referring to the weed susceptibility charts and herbicide tables shown in this text can help in the process of herbicide selection and effectiveness.

Herbicides are either applied to the soil surface before weeds germinate and emerge (preemergent) or are applied directly to the foliage of small, actively growing weeds (postemergent).

Preemergent herbicides prevent weed germination and emergence; they do not control established plants. The effect of preemergent herbicides can last up to a year or more, depending on the solubility of the material, adsorption to the soil, weed species, and dosage applied. Soil type and irrigation methods are very important to the effectiveness of these herbicides. They tend to be more stable when used on heavier soil types than on lighter ones. These herbicides can also be lost as a result of leaching and runoff. Herbicide leaching is more extensive on sandy soils and runoff occurs more on clay soils. As a general rule, preemergent herbicides require rainfall or irrigation following treatment to activate them. Subsequent irrigations are less important on the movement of the herbicide. In most instances, combinations or sequential applications of herbicides will be needed to provide effective, economical control.

Postemergent herbicides are applied directly to weed foliage and they control weeds either by contact or through translocation. A contact herbicide, such as paraquat, kills young weeds by direct contact of the foliage. Therefore, it is essential to have good spray coverage and wetting for this type of herbicide to be effective. Translocated herbicides, like glyphosate, move within the plant to kill it. Complete coverage of weeds with translocated herbicides is not necessary. Furthermore, postemergent herbicides can be selective or nonselective in their control. Selective herbicides, like sethoxydim and fluazifop-p-butyl, control only grassy weed species. Nonselective herbicides, like glyphosate and paraquat, control a broad spectrum of both grasses and broadleaf weeds. Regardless of the type of postemergent herbicides used, the best time to treat weeds is when they are in the seedling stage and actively growing.

PREVENTION

A good weed management strategy in pistachio orchards begins with prevention. Keep irrigation canals, ditch banks, and irrigation systems free of weeds and weed seeds. Install filters in canals and irrigation systems to prevent weed seeds from entering the orchard. Prevent leaks in the irrigation system and the accumulation of water in furrows or low-lying areas, which encourage weed growth. Control weeds along field margins before they produce seed that can be dispersed readily into the orchard. Clean the undercarriage and tires of tractors and other equipment before entering new fields because weed seeds and reproductive parts of weeds can be transported along with them. This is especially important in preventing perennial weeds from entering a previously uninfected field.

MONITORING

Detecting new weeds and weeds that escaped previous control efforts is essential in preventing weed establishment or identifying shifts in weed populations. Regular monitoring or scouting is a very important component of an integrated plan. For weed monitoring to be useful it is important to correctly identify the weed species present in and around the orchard. Try to identify and control weeds when they are in the seedling stage. Most weeds are poor competitors for water and nutrients when they are small, but some can become very aggressive as they become large. Furthermore, it is easier to control annual weeds with chemical or mechanical tools when they are small and have not become established. Perennial weeds are more vulnerable to control at the early bud stage or during fall when the plants begin to go dormant. Herbicides applied at these stages can be translocated to the roots or rhizomes to better kill the weed. For assistance in identifying weeds in different stages of growth, consult the color photos in the online version of this guideline that are linked to the weeds listed in Common and Scientific Names of Weeds.

Monitor the orchard in a thorough and systematic manner. Include the entire orchard as well as field margins, ditch banks, and irrigation canals in your survey. Monitor at least four times a year (spring, summer, winter, and fall). Examine all areas that are susceptible to weed infestation, like areas of high moisture. Items of interest include weed species, location in the field, degree of control achieved with current program, and herbicides and other options used (including timing, rates, and dates treated). Record observations so the infested sites can be revisited for weed control. Maintain monitoring information for the life of the orchard. Over several years this information will help in determining changes in the weed species that are present. Comparing this information with the past and current weed management methods can help in evaluating the success of the techniques used and in deciding future strategies.

WEED MANAGEMENT BEFORE PLANTING

Whenever possible, avoid fields known to be infested with perennial weeds such as johnsongrass, field bindweed, bermudagrass, and nutsedge. If perennial weeds infest a potential orchard site, control them before the final land preparation for planting because they can cause problems and increase management costs in the future. There are no preemergent herbicides that can be used before planting pistachio trees. It is important to note that young pistachio trees are very sensitive to soil residuals of certain preemergent herbicides such as diuron (Karmex), simazine (Princep), and bromoxynil (Buctril); carefully follow all label plantback restrictions in orchard sites where preemergent herbicides have been used. The only chemical option of controlling weed seeds in the orchard site is soil fumigation, which can be expensive.

Identify and control weeds that are growing on the orchard site either chemically with postemergent materials or mechanically before planting. It is important to control annual weeds before they produce seeds. Perennial weeds can be mechanically controlled by repeated discings in summer or chemically treated with a postemergent herbicide in the early fall while the perennial weeds are still flowering. Re-treat with a postemergent herbicide the following spring to kill regrowth. Follow the re-treatment by discing the orchard site 2-3 weeks later to expose weed roots to drying.

Other herbicides, such as paraquat, can also be used to control weeds before planting. It may be necessary to irrigate the field before treatment to encourage weed emergence and growth. This can also significantly reduce the amount of weed seed remaining in the soil and increase the degree of control of perennial weeds.

WEED MANAGEMENT IN NEWLY PLANTED ORCHARDS

Weed control is especially important during the first few years of orchard establishment. Competition from weeds during this period can result in reduced tree vigor and productivity. Weedy orchards may require several more years to become economically productive than weed-free orchards. Regardless of the method to control weeds, be careful not to injure the young trees with herbicides or to mechanically damage the trunk or roots.

Cultivation. Weeds may be managed without the use of herbicides for the first few years after planting with cross discing or shallow cultivation in the tree row (less than 2" deep) and discing or mowing between tree rows along with some hand weeding in the area at the base of the trees. Cultivation is best done when weeds are in the seedling stage and easy to dislodge from the soil. Hand weeding is usually required up next to the trees to remove weeds missed during cultivation. Hand-held weed eaters can be used to kill small weeds around the trees, but take care not to injure the bark of young trees. Damage to either the bark or the roots can allow soil pathogens in, causing further damage to the trees.

Herbicides. Some weeds are best controlled in the early years of an orchard before the trees come into production because certain herbicides are registered only for use during the nonbearing period (normally four years). For example, the preemergent herbicide thiazopyr (Visor) can be used during the nonbearing years to help control nutsedge; however, once the orchard is in production, there are no preemergent herbicides to control nutsedge in pistachio.

Preemergent herbicides. If using preemergent herbicides to control weeds in a newly planted orchard, apply them to the soil only after the trees have completely settled in to reduce the likelihood of tree damage. The risk of damage is greater if the trees settle after treatment because the herbicide has a greater chance of coming into direct contact with tree roots.

Preemergent herbicides that may only be applied during the nonbearing years in pistachios are isoxaben (Gallery), pendimethalin (Prowl), and thiazopyr (Visor). Preemergent herbicides that are registered for both the nonbearing and bearing years in pistachio include napropamide (Devrinol), oryzalin (Surflan), and oxyfluorfen (Goal).

Preemergent herbicides can be applied to a small area immediately around the tree or in a 4- to 6-foot strip down the center of the tree rows. The soil surface should be free of leaves and other debris because some herbicides may adhere to the trash on the soil surface and not make adequate contact with the soil surface. Adequate rainfall is also required to activate the herbicides following application. Some herbicides, like napropamide, must be activated with rainfall or irrigation water within 4 to 5 days of treatment, while others, like oxyfluorfen and oryzalin can remain stable on the soil surface for 21 days or more.

Postemergent herbicides. Postemergent herbicides can also be used following planting once weeds have emerged. Herbicides registered for the nonbearing years only include clethodim (Select Max), diquat (Reglone), fluazifop-p (Fusilade), and sethoxydim (Poast). Postemergent herbicides registered for both the nonbearing and the bearing years include glyphosate (Roundup PowerMax, etc), paraquat (Gramoxone Inteon), and 2,4-D amine (Orchard Master, etc). Postemergent herbicides usually require the addition of an adjuvant (either a nonionic surfactant or a nonphytotoxic oil) to be effective.

Regardless of the postemergent herbicide used, protect the foliage and bark of young trees from direct spray or spray drift in order to avoid tree injury. Young pistachio trees are very susceptible to damage from herbicides. Placing plastic or paper wraps around the tree trunks is helpful in preventing herbicide contact with young trees.

WEED MANAGEMENT IN ESTABLISHED OR BEARING ORCHARDS

It takes about 6 years in most situations for pistachio trees to come into full production so they can be mechanically harvested. Trees less than 6 years old are generally harvested by knocking the nuts onto tarps on the ground. Generally, weeds are controlled between the tree rows by discing or mowing and herbicide strip applications directed down the tree row.

Cultivation. Cultivation can be used to manage annual and biennial weeds both between and within tree rows. Large weeds, perennials, or weeds with hearty roots and/or crowns (like cheeseweed and hairy fleabane) will not be controlled mechanically and may require postemergent herbicide treatments.

Generally, weeds growing between the rows of trees, in the alleys or middles, are disced 3 to 5 times a year. The size of the weeds is usually not a concern as long as the disc blades cut deep enough to destroy the weeds and seed has not yet been produced. Weeds within the tree row can be managed with a second pass of the cultivator. However, cross discing must be carefully done to avoid damaging the trees and their roots. Injury to trees can lead to invasion by crown-rotting organisms. Leave a 1- to 2-foot strip next to the trees to prevent injury. Weeds in this undisturbed area can be removed by hand or spot treated with postemergent herbicides where appropriate (see section below). In-row mulching cultivators also can be used as long as the trees are not damaged. Shallow (less than 2 inches deep) mulching will destroy most annual and seedling biennial weeds.

Some problems that can develop with repeated discing are soil compaction, dust, reduced water infiltration, and soil erosion in hilly terrains and sloping lands. Discing may also bring some buried weed seeds to the surface or spread rhizomes, tubers, or stolons throughout the orchard. Therefore, some growers maintain a planted cover crop or resident vegetation that they mow. Where resident vegetation is maintained, a flail mower is used as needed to maintain the plants in a low-growing state. Mowing too close to the soil surface creates dust and should be avoided. If self-reseeding of a cover crop is desired, a final mowing should not be made until the plants have set seed. For more information on cover crops, consult UC/ ANR Publication 21471, Covercrops for California Agriculture.

Herbicides. Established trees are generally more tolerant of many herbicides than newly planted ones. There are only three preemergent herbicides that are registered for use in a bearing pistachio orchard (napropamide-Devrinol, oryzalin-Surflan, and oxyfluorfen-Goal). When using a preemergent herbicide or combination of herbicides, apply a treatment in fall following harvest, in late winter before bloom, or split into two applications (fall and spring). Add a postemergent herbicide if weeds have emerged at the time of application. For the greatest safety, direct the spray to the base of the trees to avoid contact with young wood and foliage.

Apply preemergent herbicides as close to rainfall as possible to improve efficacy, and remove leaves or other debris that may be covering the tree row, preventing the herbicide from contacting the soil. Select the appropriate postemergent herbicide that best controls the weeds present in the field. Occasionally, a tank-mix of one or more herbicides may be required to control all the weeds. In situations where weeds are spotty, the amount of herbicide needed can be reduced by making spot applications or using a visual weed-seeking sprayer. With postemergent herbicides, it is important that weeds are small, not stressed for moisture, or too large for control. Some weeds, like spotted spurge, set seed soon after emergence, so they must be sprayed often when they are small to provide adequate control.

Herbicides and irrigation. In established pistachio orchards, chemical weed control has to be adjusted to the irrigation method used. In California, pistachios are irrigated by several methods, such as low-volume drip, micro-sprinklers, misters, solid-set sprinkler, furrow, or basin flood. Low-volume irrigation is common in California pistachio orchards because of better uniformity in irrigation application and efficiency than other methods. However, under certain conditions, low-volume irrigation water applied too frequently can increase the chance of leaching and herbicide degradation and the areas around the emitters often have vigorously growing weeds. It is important to monitor these areas closely monitored and spot treat, when necessary with postemergent sprays.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Pistachio
UC ANR Publication 3461
Weeds
K. J. Hembree, UC Cooperative Extension, Fresno County
A. Shrestha, UC IPM Program, Kearney Agricultural Center, Parlier

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