Integrated Weed Management

Weed management is an integral part of an overall orchard management system. A good weed management program should start before trees are planted. The more difficult-to-control weeds (particularly perennial species) are easier to manage before trees are planted. Weed control in orchards enhances the establishment of newly planted trees and can improve the growth and yield of established trees.

Competition is most severe during the first four years. Weeds can reduce tree growth and yield by competing for water, nutrients, and sunlight. They also interfere with irrigation uniformity and distribution and, once the trees reach bearing age, can reduce harvest efficiency by making it more difficult to recover nuts from the orchard floor. Maintain a weed-free strip at least 30 inches from the trunk on each side of the tree to prevent weeds from competing with the developing tree.

Plants growing on the orchard floor influence the presence of other pests such as vertebrates, insects, mites, nematodes, and diseases.

• Weeds growing around the trunk compete directly with young tree growth, and provide a good habitat for field mice or voles.
• Gophers are most often found in nontilled orchards and are common where broadleaf weeds, such as field bindweed and perennial clovers, predominate.
• Crown rot in trees can also be a problem when weeds are allowed to remain around the trunks. Weeds must be controlled around the trunks, preferably without disks or other mechanical control that may cut roots or hit the trunks or main roots and cause wounding. These wounds are often an entry point for crown or root pathogens, such as crown gall.

Weeds are usually controlled either chemically or mechanically in a 4- to 10-foot-wide strip (depending on crop and age) in the tree row. The area between the tree rows may be sprayed, mowed, or tilled. Mulches, subsurface irrigation, and flamers can also be used to control weeds in orchards. Growers have many weed management tools available to achieve this objective, but the method in which these tools are utilized varies from year to year and orchard to orchard.

Soil Type Considerations

Soil characteristics play an important role in weed management. Soil texture and organic matter influence the composition of weed species present, the number and timing of cultivations required, and the residual activity of herbicides.

• On light textured soils, annual species such as puncturevine, crabgrass, horseweed, and Panicum spp., and perennial weeds such as johnsongrass, nutsedge, and bermudagrass are more prevalent.
• On heavier-textured soils, perennial weeds such as curly dock, field bindweed, and dallisgrass are commonly found.
• Higher rates of preemergence herbicides may be needed in clay or clay loam soils to achieve the same level of weed control as a lower rate in light, sandy soils. Many herbicide labels recommend that lower rates of the product be used on soils considered high in sand or low in organic matter.
• Soil texture affects water-holding capacity, which influences irrigation frequency and amounts. Soil moisture and wet-dry cycles can influence weed germination and establishment as well as persistence of residual herbicides.

Irrigation System Considerations

Weed growth is affected by the method of irrigation, amount of water applied, amount and timing of rainfall received, frequency and timing of cultivation, the herbicides used and their residual soil activity.

• During dry winters or in orchards with limited irrigation capacity during certain times of the year, effective weed control can increase soil water that is available to the trees.
• Frequent wetting also promotes herbicide degradation in the soil and, thus, degradation is generally faster under drip emitters, or micro-sprinklers, than under furrow irrigation.

Areas around sprinklers and emitters may require additional weed control measures, such as a postemergence herbicide applications or removal by hand. However, in the dry area between the sprinklers, weeds are less of a problem than in orchards with other types of irrigation. The first irrigation following an herbicide application is the most critical in determining the depth the herbicide is moved into the soil; subsequent irrigation is less important to the movement of the herbicide.

Preemergence herbicides can be incorporated using tillage, rainfall, or sprinkler, but not drip or furrow, irrigation. Flood irrigation will provide uniform incorporation of herbicides ONLY when the water distribution is uniform. Even when distribution is uniform, more water is usually applied by flood irrigation than is needed for herbicide incorporation; in sandy soils, excess irrigation water may move the herbicide deeper in the soil than is desired for optimum weed control. In furrow and basin flood systems with berms, preemergence herbicides dissipate more slowly on the berms because the soil surface in this area remains drier.

Choosing an Herbicide

Herbicides are traditionally discussed as belonging to two groups: those that are active against germinating weed seeds and very small seedlings (preemergence herbicides) and those active on emerged, growing plants (postemergence herbicides). Some herbicides have both pre- and postemergence activity. Herbicides vary in their ability to control different weed species. Before using any herbicide, identify the weed species to be controlled, then check the Susceptibility of Weeds to Herbicide Control tables and carefully follow the product labels for specific weed control activity. In most situations, combinations or sequential applications of herbicides will be required to provide effective and economical year-round control of a broad spectrum of weeds.

Monitoring

Detecting new weeds and weeds that escaped previous control efforts is essential for 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 that it be done at the right time and to correctly identify the weed species present in and around the orchard. Annual weeds generally fall into those that germinate in the fall and early winter (November&–January) and those that germinate in the spring and summer (March&–August). Try to identify and control weeds when they are in the seedling stage. 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.

Many herbicides are effective only against certain weed species. Regular monitoring will help to properly choose and time treatments. Follow-up monitoring allows you to assess if treatments were successful. Weeds often grow in patches, so it may not be necessary to apply postemergence herbicides or use mechanical control in the whole orchard. A spot treatment may save time and money while still achieving good weed control. Use the susceptibility charts in this guideline to determine alternative herbicides to control the weeds that escaped previous herbicide treatments.

Weed species that are not controlled by herbicides listed as effective against these weeds should be a warning to the manager: if there is no obvious pattern that could be attributed to sprayer malfunction or a misapplication, then herbicide resistance could be developing. See Herbicide Resistance: Definition and Management Strategies, UC ANR Leaflet 8012 and Preventing and Managing Glyphosate-Resistant Weeds in Orchards and Vineyards, UC ANR Publication 8501, for suggestions on how to adjust management to avoid development of herbicide resistance.

How to Monitor

• Survey your orchard for weeds in late fall and again in late spring
• 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.
• Examine all areas that are susceptible to weed infestation, like areas of high moisture. Important information includes 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 on a survey form that includes a map so the infested sites can be revisited for weed control. Pay particular attention to perennial weeds and other problem weeds and note their location on the map.
• Record weeds found in rows and middles separately. Weeds in tree rows must be managed, but annual weeds in row middles may have some benefit as an orchard floor cover.

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 actions including timing, rates and dates of herbicide applications and cultivations can help in evaluating the success of the techniques used and in deciding future strategies.

Late Fall Weed Survey

Survey your orchard after the first rains of the fall when winter annuals have germinated and started to emerge. Monitoring weeds in fall accomplishes several tasks. It will identify remaining summer species and perennial weeds that escaped the previous year's weed control program so that adjustments can be made to control these species in the next year. Fall monitoring will also identify winter species that are emerging. Keep records of your observations and use the map to show areas of problem weeds example weed survey form (PDF).

Late Spring Weed Survey

Survey your orchard in late spring or early summer, after summer annuals have started emerging. By surveying weeds at this time, you can identify any species that escape control from earlier management and know what perennial weeds are present. If herbicides were used, monitoring can help identify any need for changing to another herbicide. Pay particular attention to perennial weeds and check for their regrowth a few weeks after a mechanical or chemical control operation. Keep records of your observations and use the map to show areas of problem weeds example weed survey form (PDF).

Weed Management Before Planting

• Grade the orchard site to ensure even drainage. Low spots within the orchard promote perennial weed growth that is difficult to control and causes continuing problems.
• Maintain proper drainage, which keeps puddles from forming within the tree row. Puddles accelerate the dissipation of herbicides; this leads to weed growth that competes with the young trees.
• Avoid fields known to be infested with perennial weeds such as johnsongrass, field bindweed, bermudagrass, and nutsedge whenever possible.
• Control annual and perennial weeds before planting an orchard to reduce competition during orchard establishment.
• Control annual weeds before they produce seeds and established stands of perennial weeds before trees are planted. This will also reduce potential injury to young trees from herbicides that would otherwise be used after planting the trees.

Nonchemical Controls

Cultivation, followed by irrigation to germinate new weeds, and then cultivation again to kill weed seedlings is an especially effective weed control method that can be used before planting trees. Several cycles of germination and unsuccessful establishment reduces the number of weed seeds in the upper layers of the soil, thus reducing weed numbers. At least two cycles of cultivation, then irrigation, followed by a shallow cultivation are needed to achieve a marked reduction in weed seedlings. This method is not effective for established perennial weeds.

Cultivation when the soil is very dry is an effective method to control perennial grasses such as bermudagrass and johnsongrass. Cultivation cuts the rhizomes into small pieces so they can dry out. Rework the soil frequently with a spring tooth harrow to pull new rhizomes to the surface to dry out. If the site is irrigated, or rain occurs before total control of the perennial plant is achieved, the rhizome pieces may begin to grow, which can greatly reduce the effectiveness of this practice. Tillage in moist or wet soil can increase the number of perennial weeds because each piece of cut rhizome can root and develop into a new plant.

Field bindweed growth can be reduced for up to two years by deep plowing or using a reclamation blade (a large V-shaped blade) to cut the roots at a depth of 16 to 18 inches in dry soil. Nutsedge infestations can be reduced by deep plowing with large moldboard plows that bury the nutlets to a depth of at least 12 inches, but, subsequent deep plowing may bring viable seeds and nutlets back to the surface. Seedlings of annual and many perennial weeds can be controlled with repeated, timely cultivation.

Chemical Control

Annual weeds can be controlled with preemergence herbicides before planting an orchard. Any established annual weeds should first be controlled with postemergence herbicides. Preemergence herbicides should be used in conjunction with a rotation crop, making sure the residual period of the herbicide is not so long that it will interfere with planting the trees. Most annual weeds can be controlled in a strip down the proposed tree row by using a preemergence herbicide (e.g., Treflan) and incorporating it into the soil. However, many growers prefer to use preemergence herbicides only after the trees have been planted and soil has settled around the plants to avoid possible exposure to herbicides that may be in the backfill soil.

When planting trees, be careful not to mix preemergence herbicide-treated soil into the planting hole or severe injury can result. When planting the trees, place untreated soil (from the untreated middles) directly around the roots and then cover it with a surface layer of treated soil. Many growers use glyphosate before planting and then follow planting with an application of a preemergence herbicide after the soil has settled. Follow all label plantback restrictions in orchard sites where preemergence herbicides have been used.

Follow all label plantback restrictions in orchard sites where preemergence herbicides have been used.

Postemergence herbicides generally have a little or no soil residual activity and typically are safer to use before planting trees. A common practice to control perennial weeds such as dallisgrass, bermudagrass, and johnsongrass is to apply glyphosate (Roundup) in summer when the weeds are growing vigorously and then follow with cultivation 2 weeks later. If the soil and plant material can be dried after treatment, increased control is achieved. Field bindweed can be suppressed, but usually is not eradicated with this method.

Soil Solarization

Soil solarization is a nonpesticidalmethod of controlling soil-borne pests by placing clear plastic sheets on moist soil during periods of long day length. Soil solarization can be used in the area planned for tree rows to significantly reduce weed numbers and species. The plastic sheets trap the sun's radiant energy inthe soil, heating the upper levels to temperatures (108–131°F at a depth of about 3 inches) that kill many disease-causing organisms (pathogens), nematodes, and weed seeds and seedlings.

In areas where summer fog is not a concern, solarization should be done when day length is as long as possible (from mid-June to mid-August), or at the latest by the beginning of August to have sufficient time (4 to 6 weeks) to complete the process. In areas where summer fog is prevalent, solarization should be done during the warm fall months when there are fewer foggy days. The soil in the area designated for solarization must be moist and the treated area should be at least 6 feet wide. Use 1.5 to 2 mil thick clear plastic that is impregnated with a UV-inhibiting component to ensure that it will not break down before solarization is completed. Black plastic suppresses weed-seed germination but will not heat the soil to the same degree as clear plastic.

Effective soil solarization begins with preparing a smooth seed bed so the plastic can be placed as close as possible to the soil surface.

• Disc to break up clods and then smooth the soil (e.g. with a roller).
• Remove any material that will puncture or raise the plastic sheets such as rocks and weeds.
• Irrigate before or after applying the plastic because wet soil conducts heat better than dry soil.
• If irrigating before applying the plastic: cover the soil with plastic as soon as feasible after irrigating. After irrigation, allow the soil to dry somewhat to avoid compaction by heavy equipment.
• To irrigate after laying the plastic: install the drip system or microsprinkler line (with only the spaghetti tubing) before planting or use furrow irrigation under the plastic. (If the entire site is irrigated, weed growth will occur in the untarped centers and will be difficult to control without disturbing the plastic.) The plastic should be buried on all sides to create a seal on the soil and help prevent the plastic from being blown away by wind. Implements are available that assist in laying down the plastic and automate this otherwise labor-intensive process.
• Remove plastic before planting.
• Cultivate solarized soil less than 3 inches deep to avoid bringing viable weed seeds to the surface where they can germinate and establish.

The effect of solarization diminishes at greater depths, and it does not control perennial species as well as annuals. Seeds and seedlings of bermudagrass, johnsongrass, and field bindweed are controlled, but established plants are more difficult to control. Yellow nutsedge is partially controlled, while purple nutsedge is not significantly affected. Solarization leaves no toxic residues and can be used on a small or large scale. Soil solarization may also improve soil structure and increases the availability of nitrogen (N) and other essential plant nutrients.

For additional information see Soil Solarization, U.C. Publications, 21377.

Weed Management in Tree Row

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 orchards with effective weed management. Regardless of the method to control weeds, care must be taken not to injure the young trees with herbicides or to mechanically damage the trunk or roots. As the orchard becomes established, competition from weeds is lessened as shade from the tree canopy, especially in densely-planted orchards or in orchards with large-stature trees, reduces weed growth.

Weeds growing directly around the bases of trees can be controlled using a number of methods. A selective preemergence herbicide can be applied in a strip down the tree row or around the tree as soon as the soil has settled following planting. Do not let the spray contact tree leaves or the bark of trees less than 3 years old. Cardboard cartons are often used to protect trees at this stage. Trees are most sensitive to herbicides when they are young.In walnut orchards planted on Paradox rootstocks, the use of herbicides instead of mechanical control reduces the danger of wounding young trees with a disk or hoe and creating an entry for crown gall bacteria.

The area around young trees may also be hand hoed until the trees are 3 to 4 years old, at which point a swing mower or disc can be used to control the weeds between the tree rows. Other in-row mechanical tools available include discs, weed knives, cultivators, and rotary tillers. It is best to hoe when the weeds are a few inches tall; hoeing becomes difficult when weeds are allowed to get larger. Another alternative is the use of synthetic mulches made of polyethylene, polypropylene, or polyester around the base of trees to discourage weed growth; the weeds between the tree rows can be mowed or disced. However use these mulches with caution, since they may harbor voles that may feed on tree trunks.

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. Cartons, sleeves, or wraps can help to protect trees from string trimmers. 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 during the nonbearing period (normally four–six years) before the trees are at full production. Certain herbicides are registered for use only during this nonbearing period.

Preemergence herbicides
If using preemergence herbicides to control weeds in a newly planted orchard, apply them to the soil only after the soil has completely settled around the trees 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. Refer to the HERBICIDE TREATMENT TABLE for herbicides registered, tree age restrictions, and general label recommendations.

Postemergence herbicides
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 trees are very susceptible to damage from herbicides. Placing plastic or cardboard wraps, cartons, or sleeves around the tree trunks is helpful in preventing herbicide contact with young trees.

Established Orchards

Depending on the species and variety, it usually takes about 3 to 7 years in most situations for nut trees to come into production. Once the orchard is established, the area around the base of the tree should continue to be kept weed-free. By removing weeds from around the base of the tree, weed competition and the potential for rodent damage are reduced. In conventional orchards weeds are generally controlled between the tree rows by discing or mowing (see middles management) and in the tree row with an herbicide strip or with cultivation.

Herbicides

Herbicides are traditionally discussed as belonging to two groups: those that are active against germinating weed seeds (preemergence herbicides) and those active on emerged plants (postemergence herbicides). Some herbicides have both pre- and postemergence activity. In most orchards, herbicides are only used on a 4- to 10-feet-wide strip (depending on crop) centered on the tree row.

Preemergence herbicides
Preemergence herbicides are active in the soil against germinating weed seedlings. These herbicides should be applied to bare soil and then moved into the soil with rain or irrigation, where they can affect germinating weed seeds. If herbicides remain on the soil surface without being activated by rain or irrigation, some will degrade rapidly from sunlight and the resulting weed control will be reduced. Large weed seeds, such as wild oat, may germinate in the soil below the herbicide zone and not be controlled by the treatment.

For best results, most preemergence herbicides should be applied to the soil just prior to an irrigation or rainfall (0.25-0.5 inches) to be moved into the soil where the weed seeds germinate. Do not make an application if a large amount of precipitation is expected in a short period, as runoff or leaching of the herbicide may occur. Preemergence herbicides can provide control for several months or up to a year, depending on the soil type, solubility of the material, adsorption of the material to soil, the weed species present, and the dosage applied. Leaching from the soil is more extensive on sandy than on clay soils. Leaves or other debris covering the tree row can prevent the herbicide from contacting the soil; performance can often be increased by blowing or sweeping the rows right before application of preemergence herbicides.

Proper incorporation is important for the effectiveness of preemergence herbicides. This may be achieved mechanically (power incorporation or discing) or through irrigation. Rainfall may also be used for herbicide incorporation; however, weeds may germinate before a consistent rainfall pattern is established. As a result, postemergence herbicides are often used in combination with preemergence herbicides to control weeds that have germinated and emerged before the preemergence herbicide is properly incorporated.

Postemergence herbicides
Postemergence herbicides are applied to control weeds that have germinated and emerged. They can be combined with preemergence herbicides early in the season, alone as a broadcast treatment, or as spot treatments during the growing season. The trunks and foliage of young trees need to be protected from contact with some postemergence herbicides. Be sure to check and follow individual label instructions. Select the appropriate postemergence herbicide that best controls the weeds present. A tank mix of one or more herbicides may be required to control all the weeds.

Apply postemergence herbicides when weeds are small and not under moisture stress. If the weed population is sparse or patchy, the amount of herbicide needed can be reduced by making spot applications or by using a visual weed-seeking sprayer. Some weeds, like spotted spurge, set seed soon after emergence, so they must be treated frequently to provide adequate control if a postemergence-only strategy is used.

Postemergence herbicides are used on established weeds. Contact herbicides, such as paraquat, kill those parts of the plant that are actually sprayed, making good coverage and wetting essential. A single treatment can kill susceptible annual weeds, but re-treatment is necessary if perennial weeds regrow from roots or other underground structures or if new germination of annual weeds occurs after the initial application. Translocated herbicides, such as glyphosate, move into the plant and are moved to other above- and below-ground portions of the plant and kill them. (glyphosate, however, does not translocate into mature nutsedge tubers.) Complete coverage with translocated herbicides is not as essential as with contact herbicides but better coverage will often result in better weed control efficacy with both types of herbicides.

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. Many factors affect the performance of postemergence herbicides including: dust, spray volume, and hard water. For more information on the effective use of postemergence herbicides, see Glyphosate Stewardship: Maintaining the Effectiveness of a Widely Used Herbicide, ANR Publication 8492.

Application equipment must be accurately calibrated to apply the proper amount of herbicide to the soil and young growing weeds. For safe application and to minimize drift, spray equipment should be equipped with a short boom that has nozzles designed to minimize the amount of very small spray particles generated. Nozzle technology has advanced significantly in recent years and many manufactures have developed nozzles, or attachments to decrease the proportion of very small droplets in the spray pattern.

Herbicides and irrigation
In established orchards, chemical weed control must be adjusted to the irrigation method used. In California, nut trees 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 orchards because it provides better uniformity in irrigation application and efficiency when compared to other methods. However, low-volume irrigation water applied too frequently can, under certain circumstances, increase the chance of leaching and herbicide degradation, often leaving the areas around the emitters with vigorously growing weeds. It is important to monitor these areas closely and spot treat, when necessary, with postemergence herbicides. Prolonged moist conditions during winter in furrow bottoms or around low-volume emitters during irrigation favor the breakdown and leaching of herbicides.

Cultivation

Cultivation can be used to manage annual and biennial weeds both between and within tree rows. Large weeds, perennial species, or weeds with hardy roots or crowns (like cheeseweed) may not be completely controlled mechanically and require postemergence herbicide treatments. Mechanical methods of weed control include hoeing or using weed knives in the row and cultivating between rows. Mechanical cultivators, such as a Weed Badger, will be effective if used on loose soil that does not contain large rocks. These practices need to be done frequently when weeds are small to reduce competition and seed production. If weeds are allowed to mature the plants often become a fire hazard, but more importantly, can produce enough seeds to ensure many years of weeds.

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 postemergence herbicides where appropriate (see section above). 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.

Middles Management

Weed management in orchards is often separated into two categories: weed control in the tree row and weed control in the middles. Weed control in the middle is often combined with cover cropping and known as "middles management."

Some growers prefer to maintain a planted cover crop or resident vegetation because of problems that can develop with repeated discing including:

• soil compaction
• dust
• reduced water infiltration
• 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.

If resident vegetation does not grow uniformly enough to compete well with newly-invading weeds, consider planting a cover crop in the area between the tree rows. An annual cover crop, such as sub-clovers that reseed themselves, will compete against weeds; however, it may also require occasional reseeding to maintain a uniform cover crop stand. 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.

Cover Crops

Two primary reasons for planting a cover crop are: to enhance soil quality by adding organic matter and to increase soil nitrogen with legume cover crops. Other benefits cover crops can provide include:

• improved orchard access during the rainy season
• enhanced water infiltration
• suppression of winter weed species (and summer species if cover crop remains or regrows through late spring)
• reduction of:
  • soil compaction and crusting
  • irrigation and rain runoff
  • off-site movement of pesticides and nitrogen
  • erosion on slopes
  • dust (reduced dust minimizes spider mite infestations)

Plan for the additional water needs of the cover crop so that it does not compete with trees for available water, or, in the case of dryland orchards, disc under the cover crop in spring to maximize the amount of water available to the tree.

Newly established cover crops may be seriously damaged by fall and winter orchard traffic during operations such as pruning, brush removal, chipping and shredding, and spraying. In orchards where these operations are planned, cover crops may be seeded in alternate middles and these operations carried out in the nonseeded middles. Or, plant cover crops in years when these operations are not planned in the orchard.

Although cover crops will be most competitive if mowing is avoided, mowing once before bloom is recommended, especially in almonds, to reduce frost hazard and eliminate pollination competition during flowering. The cover crop will regrow and flower later in the season. However, cover crops that are not mowed or are mowed infrequently in order to reseed also provide excellent cover for gophers. Gopher populations can frequently build up in cover-cropped orchards, and during harvest they have nothing to feed on except tree roots. Thus, it is imperative that gopher control is maintained, regardless of the middles management system employed, but particularly if cover crops are utilized.

An alternative to mowing is to let a cover crop grow until it is nearly mature and then roll it with a ring-roller to press the vegetation down. This accelerates the senescence process but allows for some seeds to mature. In addition, the intact mulch shades the soil and may prevent some weed seeds from germinating. This mulch usually degrades by harvest, but if it hasn't degraded at least 3 weeks before harvest, a close mowing with a flail mower will chop the cover crop into pieces that will degrade or not interfere with harvest operations.

A once common practice was ‘chemical mowing' in which the vegetation middles, consisting of either resident plants or a planted cover crop were treated with low rates of postemergence herbicide to stunt the plants rather than killing them. This practice is no longer recommended due the potential selection for herbicide-resistant weeds.

For more information on choosing a cover crop, how and when to plant, and suggestions of cover crop mixes, consult UC ANR Publication 21471, Covercrops for California Agriculture or Cover Crops for Walnut Orchards, ANR Publication No. 21627.

Herbicide Resistance

Herbicide resistance is the inherited ability of weeds to survive and grow at herbicide dosages many times greater than what is usually needed for control of that species. The potential risk for the development of herbicide resistance is greatest when the same herbicide is used repeatedly, as is often done in orchards. To prevent the development of herbicide resistance, use a variety of weed-control strategies, including cultural practices and alternating herbicides with different modes of action. Failure to do this can result in the rapid loss of herbicides as an effective pest management tool, although cultivation remains an option.

If resistant populations are observed, avoid moving resistant weeds from one field to another by cleaning equipment before moving out of a field with known herbicide-resistant weeds. Consider scheduling fields with known resistance problems as the last ones for field operations. Some populations of annual bluegrass (Poa annua), horseweed (Conyza canadensis), annual or Italian ryegrass (Lolium multiflorum), junglerice (Echinochloa colona), and hairy fleabane (Conyza bonariensis) have developed resistance to glyphosate in California.

Detection

The first step in preventing herbicide resistance is early detection. Be on the lookout when monitoring for patterns that indicate resistance including:

• patches of dense weeds, with less dense populations radiating out from the central patch.
• weeds that have escaped control scattered in no particular pattern throughout the field.

Prevention and Management

One of the most important control strategies in managing resistant populations of these weeds is to not let the plants produce seed. To help prevent the development of resistance to herbicides in orchards:

• Rotate herbicides that have different modes of action and WSSA Group numbers.
• Monitor for weed survival after an herbicide application.
• Include nonchemical weed-control methods such as cultivation or hand weeding.
• Clean equipment after working in weed-contaminated orchards to prevent the spread of weed seeds.
• Control weeds suspected of herbicide resistance before they can produce seed.
• If weeds escape treatment, use shovels, hoes, and other hand tools to cut the plants below the soil surface to prevent flowering.
• Use a preemergence herbicide before weeds emerge. Where the weeds emerge in fall and spring, consider splitting applications to meet the multiple emergence windows.

For more information on herbicide resistance, see Selection Pressure, Shifting Populations, and Herbicide Resistance and Tolerance, UC ANR Publication 8493 and Preventing and Managing Glyphosate-Resistant Weeds in Orchards and Vineyards, UC ANR Publication 8501.

If horseweed and hairy fleabane are already growing in the orchard, either treat them with a postemergence herbicide or use mechanical cultivation before they get larger than 18 to 21 leaves. In established orchards, use 2,4-D, glufosinate (Rely 280), or saflufenacil (Treevix) to control these weeds early. Glyphosate can also work well for the non-resistant horseweeds if rates are 1 to 2 lb a.i./acre but even on the susceptible populations, control is best when plants are small. Mixing 2,4-D or glufosinate with glyphosate will improve control if resistance is suspected. Similarly, glyphosate-resistant ryegrass can be controlled with alternative herbicides such as paraquat (Gramoxone) or glufosinate if used at the appropriate stage. Closely monitor the weeds following treatment to assess the treatment's effectiveness.