Integrated Weed Management

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 tree growth and productivity. 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, and chemical 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 chemically in a 4- to 6-foot-wide strip within the tree row. The area between the tree rows is mechanically mowed, tilled, or sometimes chemically treated. Fabric or plant-based mulches, subsurface irrigation, and flamers can also be used to help manage 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 rate of herbicide degradation in the soil. Herbicides are degraded faster in warm, moist soils as compared to cold, dry soils.

Herbicides registered in California for use in pistachios are an important component of an integrated weed management program. Herbicide selection is an important process and should be based on 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, size of farming operation, and economics. Referring to the WEED SUSCEPTIBILITY CHARTS and HERBICIDE TABLE shown in this text can help in the process of herbicide selection and effectiveness.

Before using herbicides, remove leaves and debris from the treatment area. Use drift-reducing spray nozzles, other techniques (i.e. low spray pressure, slower travel speed, etc.), or a combination of them, and apply herbicides only during favorable environmental conditions to enhance coverage and reduce drift.

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

Preemergence herbicides inhibit weed germination and emergence; they usually do not control established plants. The effect of preemergence herbicides can last up to a year or more, depending on the solubility of the material, adsorption to the soil, soil temperature and moisture, weed species, and dosage applied. Soil type and irrigation methods greatly influence the effectiveness of these herbicides. In some situations, these herbicides can 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, preemergence herbicides require rainfall or irrigation following treatment to activate them. After the activation and incorporation, subsequent irrigations have less of an impact on the movement of the herbicide. In most instances, combinations or sequential applications of herbicides will be needed to provide effective, economical control.

Postemergence herbicides are applied directly to weed foliage and they control weeds either by contact or through translocation. A contact herbicide, such as glufosinate, kills young weeds by directly damaging treated foliage. Therefore, it is essential to have good spray coverage and wetting for this type of herbicide to be effective. Effective control with contact herbicides is reduced as weeds become larger and denser because coverage is often reduced. Translocated herbicides, like glyphosate, move within the plant and damage tissues that were not directly sprayed. Complete coverage of weeds with translocated herbicides is not necessary. Furthermore, postemergence herbicides can be selective or nonselective in their control. Selective herbicides, like sethoxydim and fluazifop, 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 postemergence herbicides used, the best time to treat weeds is when they are in the seedling stage and actively growing.

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 uninfested field.

Detecting new weeds and weeds that escaped previous control efforts is essential in preventing weed establishment or identifying changes or 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 before they surpass 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 weeds in fall, late spring, and following any control effort. 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, noting locations of problematic weeds, so 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.

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. Pendimethalin (Prowl) is the only preemergence herbicide 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 preemergence herbicides such as diuron (Karmex), simazine (Princep), and bromacil (Hyvar); carefully follow all label plantback restrictions in orchard sites where preemergence herbicides have previously been used. The only other chemical option of controlling weed seeds in the orchard site during this time is soil fumigation, which can be expensive.

Identify and control weeds that are growing on the orchard site either chemically with postemergence 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 postemergence herbicide in the early fall while the perennial weeds are still flowering. Re-treat with a postemergence herbicide the following spring to kill regrowth. Follow the re-treatment by discing the orchard site 2 to 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 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. In addition, managing weeds that host invertebrate pests helps reduce their numbers as well. For instance, London rocket and spotted spurge are preferred hosts for chinch bugs. Clovers, Russian thistle, and birdsfoot trefoil are a few of the weed hosts of lygus and stinkbugs. 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. Furthermore, grassy weeds, particularly foxtails, create a good habitat for vertebrate pests like field mice, which can feed on young tree trunks.

Cultivation

Weeds may be managed without the use of herbicides for the first few years after planting with cross discing or shallow (less than 2 inches deep) cultivation in the tree row; discing or mowing between tree rows; and hand weeding in the area at the base of the trees. Cultivation is best done when weeds are seedlings and easy to dislodge from the soil. Hand weeding is usually required close 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

Tree age should always be considered when selecting specific herbicides to use, because it can affect whether or not tree injury could occur. Certain herbicides are registered only for use during the nonbearing period (normally the first five to six years after planting), some can be used only after trees have been planted for a given number of years, and still others can be used throughout most of the life of the trees.

Preemergence herbicides

Apply preemergence herbicides in a newly planted orchard only after the soil around the trees has completely settled in to reduce the likelihood of tree damage through direct contact with tree roots.

The only preemergence herbicide that can be used exclusively during the nonbearing years in pistachio is pendimethalin (Prowl). There are several preemergence herbicides registered for both the nonbearing and bearing years in pistachio including flumioxazin (Chateau), indaziflam (Alion), isoxaben (Trellis), oryzalin (Surflan), oxyfluorfen (Goal), and others. Refer to the HERBICIDE TREATMENT TABLE for herbicides registered, tree age restrictions, and general label recommendations.

Preemergence herbicides are most commonly applied in a 4- to 6-foot-wide strip down the center of the tree rows. For best results herbicides should be applied to a clean soil surface. A mechanical leaf blower can be used to remove leaves and other debris from the tree rows to improve herbicide contact with the soil surface. Adequate rainfall or irrigation is also required to activate the herbicides following application. Most herbicides require at least 0.25 to 0.5 inch of rainfall or irrigation water within 21 to 28 days of treatment to be adequately activated and incorporated.

Postemergence herbicides

Postemergence herbicides can also be used following planting once weeds have emerged. Herbicides registered for the nonbearing years only include clethodim (Select Max), diquat (Reglone), and fluazifop (Fusilade). Postemergence herbicides registered for both the nonbearing and the bearing years include carfentrazone (Shark), glufosinate (Rely 280), glyphosate (Roundup PowerMax, etc.), oxyfluorfen (Goal), and others. Refer to the HERBICIDE TREATMENT TABLE for herbicides registered, tree age restrictions, and general label recommendations. Postemergence herbicides usually require the addition of an adjuvant (either a nonionic surfactant or a nonphytotoxic oil) to be effective. Ammonium sulfate is often added to the spray water first, before adding herbicide(s), to condition the water and help improve herbicide uptake by weeds, particularly where water high in calcium, sodium, magnesium, and iron is used.

Regardless of the postemergence 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. However, using "protective" wraps will not compensate for poor spraying techniques.

It takes about six years in most situations for pistachio trees to begin full production and mechanical harvest. Trees less than six years old are generally harvested by knocking the nuts onto tarps on the ground.

Weeds are generally 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 or crowns or both (like cheeseweed and hairy fleabane) will not be controlled mechanically and may require postemergence herbicide treatments.

Generally, weeds growing between the rows of trees, in the alleys or middles, are disced 3 to 5 times per 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, perpendicular to the first pass. 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 postemergence 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 reproductive structures like rhizomes, tubers, or stolons throughout the orchard. Therefore, some growers maintain a planted cover crop or resident vegetation (existing weeds) 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.
• For frost protection and to reduce plant bug and false chinch bug numbers, ground cover should be mowed before pistachio bud break. Refrain from spraying herbicides during bloom to reduce potential injury to developing fruit.
• 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 herbicides than newly planted ones. Preemergence herbicides registered for use in bearing pistachio orchards include flumioxazin (Chateau), indaziflam (Alion), penoxsulam+oxyfluorfen (Pindar GT), rimsulfuron (Matrix), and others. Refer to the HERBICIDE TREATMENT TABLE for herbicides registered, tree age restrictions, and general label recommendations. When using a preemergence herbicide or combination of herbicides, apply a treatment in fall following harvest, in late winter before bloom, or split into two applications (fall and late winter). Add a postemergence 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. Use spraying methods that reduce likelihood of spray drift (e.g., drift-reducing spray nozzles) where possible and apply herbicides only when environmental conditions favor less drift.

Apply preemergence herbicides before and as close to a rainfall event (0.25–0.50 inch) as possible to improve efficacy. Avoid applying herbicide if a large (2 or more inches) single precipitation event is expected before herbicide is incorporated into the soil. Before application, mechanically remove leaves or other debris that may be covering the tree row, preventing the herbicide from contacting the soil. Select the appropriate postemergence herbicide that best controls the weeds known to be 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 postemergence herbicides, it is important that weeds are small, not stressed for moisture, or too large to adequately cover. Some weeds, like spotted spurge, set seed soon after emergence and should be sprayed when small to provide adequate control and prevent new seed formation. Two or more applications may be needed.

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 causing the areas around the emitters to have vigorously growing weeds. It is important to monitor these areas closely and spot treat, when necessary, with postemergence sprays.