Integrated Weed Management

Integrated weed management uses multiple strategies to manage weed numbers in an economically and environmentally sound manner. Strategies usually include various combinations of cultural, mechanical, chemical, and biological methods.

Weeds compete with trees for water and nutrients, primarily during the grove's early years of growth. Weeds are usually a minor concern in healthy, mature groves that have thick mulch and a dense canopy that shades the ground. Weed competition may be a problem in nurseries and in older groves where trees have been heavily pruned or are unhealthy and have a sparse canopy, allowing more light to reach the soil surface. A dense canopy that shades the soil reduces the amount of sunlight that weeds need to grow to be competitive with the trees. Competition is strongest from perennial and summer annual weeds. If tall or dense weeds are allowed to excessively compete with young trees, the trees may start producing fruit much later than those with minimal competition. Weeds within groves, along orchard borders, and on roadsides may host or harbor pests that attack fruits or trees. Weeds on field margins can also serve as a reservoir for future weed problems, because seeds can enter the orchard with the wind or on equipment. Additionally, weedy vegetation along field margins can spread wildland fires into groves.

Effective management of orchard floor vegetation can provide benefits, such as reducing soil erosion, improving water infiltration, and limiting movement of potential contaminants off-site into surface or ground water. For example, waterlogged soil around young trees can lead to root asphyxiation. This problem can be reduced if weeds or cover crops are allowed to grow, reducing soil moisture in those areas. Proper vegetation management can also provide alternative hosts and shelter for parasites and predators of invertebrate pests and can reduce dust that favors mite outbreaks. Maintaining cover crops on slopes and in open areas between trees, and having vegetative filter strips along borders and roads, can be especially beneficial to limit undesirable surface water movement.

Integrated weed management strategies can vary from grove to grove. Practices are influenced by location in the state, climatic conditions, soil texture and profile, irrigation practices, topography, cost, and grower preferences. A good weed management program utilizes a combination of strategies, including prevention and monitoring.

Preventing the introduction or establishment of weeds is the most effective method of managing weeds. Choose an appropriate growing site and prepare it properly before planting. Avocados are frequently grown on rocky slopes with limited mechanical access and care must be taken to minimize operations that compact or disturb soil around the shallow-rooted trees. New weed infestations from blown-in or carried-in weed seeds or propagules can be minimized, if not prevented, using herbicides, cultivating, and mowing during site preparation, when trees are small or widely spaced, and along roadsides and borders.

After planting, maintain a thick layer of organic mulch beneath the canopy to just beyond edge of the tree canopy (i.e. the drip line). However, to avoid crown disease problems, keep mulch thin near the tree trunks. Mulching and hand-weeding provide the best and safest weed control around young trees in most situations. As groves become established, exclude weeds primarily by providing trees with appropriate cultural care and good growing conditions. Healthy and productive trees develop a dense canopy that shades the soil and produce a natural mulch through leaf drop.

MONITORING

Regular monitoring of weed species and numbers will help to select effective weed control methods and helps to determine when to take further action. Detect, identify, and keep a record of new weed species and weeds that escaped previous management actions. This will help to prevent new weed species from establishing or prevent shifts in weed populations. For monitoring to be effective, correctly identify the weed species present in and around the grove, especially 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 and difficult to manage as they become large. It is easier to control annual weeds with tillage or herbicides when they are small and not well-established. Perennial weeds are more vulnerable to control at the early bud stage and during fall just before the plants begin to go dormant; systemic herbicides applied at these times will be translocated to the roots or rhizomes to better kill the weed.

Most management actions differ in effectiveness depending on the species of weeds. Regular monitoring will determine if your control actions are working. Because weeds often grow in patches, locating small infestations and managing these may save time and money while still achieving good control. Therefore, it is important to know which weed species are present, their growth stage, their abundance, and their location in the orchard. A handheld GPS (global positioning system) unit is useful for marking and recording patches of troublesome weeds for spot management and subsequent monitoring.

Monitor and Manage Weeds Throughout the Year

Monitor for weeds at least twice each year, midwinter and late spring. Additional monitoring in midsummer and late fall is desirable, especially before planting a new grove and during the first several years of tree growth, because this will help to reveal the full spectrum of weed species that are present. Record the results of your monitoring on a weed survey form.

Fall

Fall rains stimulate weed germination and growth. If fall rains occur early (in September and early October) during typically warm months, weeds develop sooner, grow larger, and require earlier or more vigorous control actions. If rains begin later after temperatures have cooled, winter annuals grow more slowly.

Future and new groves. After the first fall rains, look for and record the species of perennial seedlings or sprouts, winter annual seedlings, and mature summer annuals.

Established groves. Inspect the thickness and effectiveness of mulch and assess whether additional control efforts are warranted near trunks. Check row middles primarily for perennial seedlings and sprouts. If young trees are present, monitor if weeds are present near the trunk.

Winter

Cold, dry winters delay weed emergence. A warm, rainy winter means that winter annuals will grow large and will be capable of producing many seeds, perhaps requiring earlier and more vigorous control action than during drier winters. Examine untreated ground in February to observe the full range of winter annual species present and the vigor of their development.

Spring

Monitor in late spring, after summer annuals have germinated, when winter annuals are flowering, and perennial weeds are evident. Spring monitoring a year before planting indicates what species are present at a time when virtually all control methods can be used and most actions are more effective and easier to use. After the grove is planted, spring monitoring around trunks tells you which species have not been controlled by actions such as cover cropping, mulching, and preemergence herbicides.

Summer

Monitor in midsummer for perennial weeds, especially before planting. Monitor for perennial weeds at least 2 weeks before cultivation and for a few weeks after a management procedure to determine if retreatment is needed.

Begin weed management before you plant the grove. Do a weed survey at least twice a year as described above to learn what species are present before you clear, cultivate, or grade the site. Record your results on a weed survey form (PDF).

Choose an appropriate growing site and prepare it properly before planting. Provide proper drainage during site preparation, such as by ripping through any impervious soil layers or hardpan near the surface, installing drains, and grading soil to eliminate areas of standing water. Consider planting trees on a berm or mound of raised soil. Before planting on raised soil, prepare subsurface layers to encourage good rooting and avoid lodging.

Coordinate preplanting weed management with other site preparation activities such as amending soil to improve pH and installing the irrigation system. Be aware that weed problems can change or can be created anytime when soil is disturbed. It is especially important to eliminate perennial species before planting or moving soil around the site; otherwise perennial weeds may be spread throughout the field.

Control Perennial Weeds

Controlling aggressive perennial weeds should be a focus of your vegetation management program. Where feasible, use of repeated cultivations of dry soil during the summer before planting exposes vegetative propagules (like rhizomes and stolons) of bermudagrass, dallisgrass, and johnsongrass to drying, killing them. Additionally, treating perennial weeds with a translocating herbicide 1 to 2 weeks before cultivation can help kill underground structures, preventing the spread of live weeds by cultivation. Monitor a few weeks after herbicide application and retreat any regrowth. Cultivation does not eliminate the reservoir of seeds, which will continue to germinate for a period of years.

1. Control emerging weed seedlings with cover cropping, cultivating, flaming, mulching, and herbicides to prevent seedlings from forming perennial structures.
2. In early fall, treat perennial weeds with a systemic postemergence herbicide when the plants are flowering.
3. Repeat cultivation 10 to 20 days after herbicide application to expose the root systems to further drying.
4. Monitor the next spring and spot-treat any regrowth.

If field bindweed is present, the above program will reduce the infestation, but because this weed has a deep perennial root system, it is likely to persist and require ongoing controls until a mature tree canopy heavily shades the ground. Even in mature groves, perennial weeds such as field bindweed and wild cucumbers can be common along borders and roadsides. In places where they receive light, these vinelike weeds often grow up into the canopy of trees.

Control Annual Weeds and Perennial Weed Seedlings

Management strategies including cover cropping, cultivating, flaming, hand-weeding, mowing, mulching, spraying herbicides, and cultural practices that promote a dense tree canopy will provide good control of annual weeds.

Where adequate water is available and irrigation is feasible, the numbers of annual weeds and perennial weed seedlings can be greatly reduced by practicing a cultivation-irrigation cycle:

1. Cultivate.
2. Irrigate to germinate seeds near the surface.
3. Cultivate shallowly to destroy seedlings.
4. Repeat at least once to be effective

It is possible to apply preemergence herbicides before planting to the entire grove or to strips 4 to 6 feet wide where trees will be planted. If weeds have emerged, use methods such as cultivation or contact herbicides before planting trees. After trees are planted, a preemergence herbicide can be applied additionally. Be aware that some preemergence herbicides should not be applied before planting or near young trees because they can injure avocado trees. Be sure to follow the instructions of the current herbicide label.

Continue to focus on controlling perennial weeds and any weeds near trunks after trees are planted. Apply and maintain a thick layer of organic mulch or a weed barrier fabric as discussed in the section on mulch below. Hand-weed or spot-treat any weeds growing in or near edges of mulch. Be careful not to injure trees when using herbicides or tools such as flamers, hoes, machetes, mowers, and weed trimmers. Management strategies include strip weed control, basal control, and total control.

Strip Weed Control

Strip weed control maintains a 2 to 6 foot wideweed-free area in the tree row. By allowing vegetation in middles, strip control improves soil structure, reduces erosion, and requires less effort than trying to control weeds over the entire grove floor. However, perennial weeds can establish quickly in the tree rows because there is no competition. Check regularly for perennial weeds and spot-treat small infestations as needed.

Basal Weed Control

Basal control eliminates weeds in a 4- to 8-square-foot area, or within a several foot diameter around each tree trunk. Allowing vegetative cover in middles and in rows (some distance back from trunks) minimizes the area where control actions are taken.

Total Weed Control

Attempting to keep the entire grove floor free of vegetation generally requires extensive herbicide use and is unrealistic and potentially detrimental. If attempting total weed control, herbicides should be used in combination with other methods, especially using mulching near trunks to economically manage weeds.

Soil compaction, erosion, reduced water infiltration, contaminant movement off-site, and injury to trees can be serious problems with total vegetation control. Because no single chemical can control all species of weeds, combinations of herbicides or sequential treatments usually must be made. Weeds must be more carefully monitored to determine which herbicides to apply and when to apply them. Perennial weeds such as bermudagrass and nutsedges may become established quickly in the absence of competition. Repeated applications may lead to the buildup of species tolerant of, or resistant to, that herbicide. It is very difficult to completely avoid injuring avocado trees with herbicide if groves are frequently sprayed.

As healthy trees develop a wide dense canopy, they naturally produce their own leaf mulch. As a result, canopy and mulch increasingly shade other vegetation. Borders and roadsides become the primary locations where weed control is needed. Weeds can also become a problem around trees that are extensively pruned or unhealthy (sparsely foliated), especially if soil is disturbed and not thickly covered with mulch.

The chosen control methods depend on the extent of canopy shading and mulching, irrigation frequency and method, need for erosion and runoff control, soil type and rockiness, spectrum of weeds present, terrain, tree spacing, and considerations such as economics and grower preferences. Combine several methods and take action at the appropriate time to obtain good control.

Sanitation

Avoid introducing weeds and eliminate conditions that favor weed development. Clean equipment after working infested ground to prevent the spread of weed seeds and perennial structures. When working several sites, work the most infested sites last or thoroughly clean equipment between sites.

Irrigation

Avocado groves are irrigated primarily with sprinklers, micro-sprinklers, or drip. The method and frequency of irrigation strongly affect weed growth. If sunlight reaches the soil, the frequency of weed management activities increases when water is applied over a larger surface area and when water is applied more frequently. Weed management is needed less often when trees are on berms because the soil surface within the tree row remains drier.

Mulch

Mulch is a layer of material covering the soil. Mulch controls weeds by excluding sunlight, and to some extent by providing a physical barrier to weed growth. Applying mulch is expensive, because it requires significant labor, especially if it is difficult for mechanical mulch spreaders or haulers to reach the site. However, the improved tree health and increased fruit yield from mulching provide substantial benefits.

In comparison with organic material, weed fabrics (water-permeable polypropylene or polyester mulches) have certain advantages and are occasionally used around young trees. Weed fabrics suppress perennials that can grow through organic mulch. They are relatively efficient to apply and are longer-lasting than organic mulch. However, weed fabrics are usually more expensive than wood chips or other natural materials. Unless covered with material, such as a coarse organic mulch, fabrics breakdown within a few years because of exposure to ultraviolet (UV) light.

A layer of coarse organic material, about 4 to 6 inches thick, provides the best weed control. Bark, greenwaste (residential yard trimmings), straw, and wood chips make good mulch. Apply organic mulch over a several foot wide area around newly planted avocado trees, taking care to keep mulch thin near trunks or about 6 inches away from the trunk. Reapply mulch annually during the first several years of tree growth. If later in the trees' life the natural leaf mulch is insufficient, consider applying additional organic mulch at least once every several years to maintain a thick enough layer to suppress weeds, especially if leaf mulch has been blown or washed away.

Avoid removing or disturbing dropped avocado leaves, because they act as a natural mulch. Take steps to retain leaves under trees. Keep tree skirts low. In addition to weed control, organic mulch provides many benefits, such as reducing Phytophthora root rot, conserving soil moisture, and gradually improving soil quality.

A planted cover crop or resident vegetation that produces a large amount of biomass can provide a living mulch. Where terrain and tree spacing permit, specially designed mowers can be used to blow cut vegetation from the middles into the tree rows to provide weed-suppressing mulch. Be sure to remove any vegetation near trunks before blowing mulch into tree rows. Cereal cover crops, such as forage oats, work particularly well for this "mow and throw" technique because cereal crop biomass does not degrade as quickly as broadleaf vegetation, such as vetch. Plant the cover crop in about October and mow in late March (exact timing for best results depends on the location and weather).

Cover crops

Cover crops are useful in young groves and among older trees that have been extensively pruned. Vegetation along grove borders and roadsides can also be managed as a cover crop. A cover crop consists of the resident vegetation (the least expensive choice), one or more seeded annual species (such as commercially available self-seeding mixes), or a blend of resident and seeded vegetation. The best cover crop for your grove depends on the age of the grove, the irrigation system, location, soil conditions, and weather. For example, with sprinkler irrigation you have more cover crop options than with drip or micro-sprinkler systems.

Proper cover crop management minimizes the need for additional water, improves water infiltration, and reduces erosion and off-site movement of potential water contaminants. Cover crops provide habitat for beneficial insects and reduce dustiness that favors mite outbreaks. Competition from desirable cover crop species helps keep weeds from building up.

Mulching within the tree drip line and good cover crop species selection can reduce the need to manage the cover crop. Depending on the situation, cover crop management methods include herbicide application, cultivation, hand-weeding, mowing, utilization of a ring roller to injure the stems, and withholding irrigation. Keep cover crops away from tree trunks to minimize competition with the trees and to reduce the habitat for pests such as pocket gophers, snails, and voles. In comparison with bare soil, cover crops may increase frost hazard, especially in low-lying areas.

For information on choosing a cover crop, see Covercrops for California Agriculture, or Cover Cropping in Vineyards: A Grower’s Handbook, UC ANR Publication 3338. Detailed information on a wide range of cover crops is also in the Cover Crops Database.

Hand-weeding

Cutting weeds with a portable string trimmer or mechanical weed whip, hoeing, using a machete, and hand-pulling large weeds are commonly used in avocado. Hand-weeding controls weeds near trunks, including scattered weeds that sprout in mulch and tall weeds or problem species growing near the edge of mulch. Avoid injuring trees by using careful technique, trunk guards, and reapplying mulch to maintain a thick layer of organic material within the drip line.

Cultivation

At level sites, cultivate before planting, along borders and possibly in the middles when trees are young. Spring is an ideal time, because cultivation can be combined with turning under a cover crop and weeds are small. Preferably, weeds should be less than 4 inches tall. Cultivation soon after emergence is especially important for perennials such as nutsedge, dallisgrass, and Johnsongrass. Once these weeds have developed more than a few leaves, they produce underground tubers or rhizomes from which they can regrow after cultivation. However, cultivation combined with an application of translocated herbicides can provide effective control of some established perennial weeds.

Rugged terrain, rocky soil, the presence of fixed irrigation systems, and the spreading shallow root system of avocado trees make cultivation not feasible or undesirable in many situations. Cultivating too closely to the tree row can cut feeder roots, reducing water and nutrients uptake. Cultivation may also spread the propagules of root pathogens and vegetative structures of perennial weeds. Excessive cultivation increases dust, erosion, and soil moisture loss, can damage soil structure, and may lead to reduced water infiltration. Once trees are about 6 to 8 feet tall, discontinue cultivation to avoid damage to root systems.

Mowing

Mow or flail before planting where it is feasible; along grove borders and edges, and in the middles between young trees. In comparison with cultivated or bare soil, mowing reduces erosion and allows the roots of cover crops or resident vegetation to maintain good water penetration. During dry weather, dusty conditions are reduced. Mowing equipment is less expensive and easier to operate than cultivators. Equipment is lighter, so soil compaction is less. Special mowers can be used in the tree row to mow weeds around trunks or to blow cuttings from middles into the tree row to provide mulch. When mowed, healthy cover crops are more resistant to invasion of new weeds than is resident vegetation. Be aware that repeated mowing promotes a shift to species that tolerate mowing, such as weeds with prostrate growth habits and grasses.

Flaming

Weeds can be controlled with specially designed flamers, most using propane. Other equipment, without an open flame, includes hot water or steam applicators and infrared devices. Flamers can be handheld or mounted on a handcart or tractor. Mechanized flamers have multiple burners, while small devices usually have a single flame source.

Fire is a serious hazard when flaming weeds. Only an experienced operator with demonstrated skill and good judgment should be allowed to flame weeds. Wet conditions during the rainy season or after a thorough irrigation are often good times to flame. Work in early morning or late evening when winds are lower and any open flame is more visible. Move the flamer slowly though the grove or briefly touch the basal stem area with the tip of a flame. Do not flame weeds to the point where they char and burn; only brief contact with high temperatures is needed to disrupt cells. Proper flaming should not create smoldering vegetation or air pollution other than fuel burning emissions.

Use good judgment to identify hazardous situations where flaming should not be conducted due to the risk of fire. Do not use flame weeders in dry areas or during the dry season when conditions are conducive to causing a fire. Be especially cautious around mulch and leaf litter. Keep fire suppression equipment, such as a fire extinguisher, shovel, and water handy in case of accidents. Keep flamers away from unguarded trunks to avoid crop injury. Consider flaming only during the rainy season and during or soon after precipitation when surfaces are moist and humidity is high.

The advantages of flaming include broad-spectrum control of broadleaf weeds, relatively low cost (depending on fuel and labor cost), and lack of chemical residue. Winter weeds, germinating after wet weather and after the end of the wildland fire season, are a good target for flaming. Disadvantages of flaming include a lack of residual control, poor control of some grasses and perennial weeds, critical timing requirement to ensure adequate weed control, hazards associated with handling pressurized flammable gas, the risk of damaging irrigation tubing, and the potential for fire associated particularly with use on steep hillsides.

Flame weeds while they are less than two inches tall. Broadleaved seedlings are most sensitive to flaming, while grasses and perennial weeds are less susceptible. Repeated flaming can lead to weed population shifts where perennial species and grasses dominate the orchard floor vegetation, unless other control techniques are used.

Determine the correct working pace or travel speed by checking weeds after flaming a test area. Weeds are being killed if gently pressing their leaves between your thumb and index finger creates a water-soaked appearance, indicating that cell membranes have ruptured. Plants may wilt, change color, or appear unaffected soon after flaming. Even if no change in the weeds is evident immediately, proper flaming causes plants toyellow and die within several days.

Herbicides

When using herbicides, choose materials and rates according to the weed species you need to control, your soil type, your irrigation method, and their risk of damaging trees. Where herbicides are relied upon, combinations of materials or sequential treatments with different materials are often needed, since no single herbicide controls all weed species.

Preemergence herbicides

Preemergence herbicides kill susceptible plants during and shortly after germination; most are not effective on emerged weeds. To be effective, most preemergence herbicides must be incorporated into the upper one or two inches of soil by rain, light irrigation of about 0.5 inch, or by cultivation. Some preemergence herbicides must be moved into the soil immediately to avoid a loss in effectiveness, others may remain on the surface for a short time before incorporation, and still other preemergence herbicides lose effectiveness if the soil is cultivated after application. Follow specific label directions regarding incorporation method and timing ahead of rainfall or irrigation.

Residual weed control from preemergence herbicides typically lasts for two to five months after application, but sometimes for more than a year. Persistence is affected by application rate, soil conditions, amount of rainfall or frequency of irrigation, and whether soil is disturbed or not. For example, herbicide activity dissipates more quickly during prolonged wet weather and in areas that remain wet, as around low-volume drip emitters.

Postemergence herbicides

Foliar-applied or postemergence herbicides are sprayed on the leaves and stems of weeds that have emerged. They are generally classified as either contact or systemic herbicides. Contact herbicides kill only the plant parts that are sprayed and thorough spray coverage is critical, especially for large weeds or dense infestations. Contact herbicides usually are most effective on seedlings and young weeds.

Translocated or systemic herbicides, such as glyphosate, are transported from the treated foliage to other parts of the plant, including roots and rhizomes, via the plant's vascular system. They are more effective on actively growing plants and are the most effective materials for controlling perennials weeds. However, in comparison with contact herbicides, translocated herbicides pose a greater risk of phytotoxicity to avocado.

A combination of preemergence and nonselective postemergence herbicides are often applied if some weeds have already emerged.

Phytotoxicity

Many herbicides can injure avocado. Nonselective, translocated (systemic) herbicides and certain preemergence herbicides can cause phytotoxicity by contacting the tree's shallow surface roots. Avoid applying most herbicides if they may contact avocado roots. Be sure not to spray postemergence herbicides onto cracked bark, green wood, leaves, or shoots. Do not use a material around trees or before planting unless phytotoxicity can confidently be avoided. Use herbicides primarily before planting, in areas away from trees, and to control perennial weeds.

Herbicide-resistant weeds

Tolerance and resistance to herbicides prevents some herbicides from controlling certain weeds. Tolerant plant species have a natural lack of susceptibility to certain herbicides. Crop tolerance to herbicides can be desirable because it allows the use of selective herbicides, such as those controlling grasses, but do not damage avocado or other broadleaves.

Resistance is present when a pest population is no longer controlled by pesticides that previously provided control. After repeated exposure of a weed population to the same herbicide or herbicides with the same mode of action, weed populations may be dominated by plant biotypes resistant to that class of herbicides. For example, in California, where glyphosate has been repeatedly applied to weed populations, rigid ryegrass (Lolium rigidum), Italian ryegrass (Lolium multiflorum), junglerice (Echinochloa colona), annual bluegrass (Poa annua), horseweed (Conyza canadensis), and hairy fleabane (Conyza bonariensis) have developed resistance to glyphosate (the active ingredient in Roundup).

Minimize development of herbicide resistance by integrating multiple other weed management methods such as cultivation, mulching, and hand-weeding. If herbicides are used, avoid repeatedly applying a single herbicide or herbicides with the same mode of action by either rotating or tank-mixing herbicide classes, or both. Scout orchards and field margins and note weed escapes or species shifts over time. Avoid spreading weed seed and propagules from infested areas by controlling runoff water and cleaning equipment before moving to another site.

Consult SUSCEPTIBILITY OF WEEDS TO HERBICIDE CONTROL for guidelines on proper herbicide selection. Suspect herbicide resistance when a weed is not controlled by an appropriate herbicide that was properly applied and there is no obvious pattern that could be attributed to application errors (such as skips) or sprayer malfunction. For more suggestions on how to adjust management to avoid development of herbicide resistance, see Herbicide Resistance: Definition and Management Strategies (PDF).

Repeated use of low rates of an herbicide can cause a shift in the weed population; careful monitoring is essential. Perennial or annual weeds not controlled by these lower-than-label rates may quickly take over because of the reduced competition; spot treat these according to label rates before they become dominant and extremely difficult to manage. Lower-than-label rates applications may select for herbicide-resistant plants. Use alternate strategies within a season or between seasons.