Agricultural pest management
Integrated Weed Management
(Reviewed 1/07, updated 1/07)
Integrated weed management uses multiple strategies to manage weed populations in an economically and environmentally sound manner. Strategies include cultural, mechanical, chemical, and biological methods.
Weeds compete with trees for water and nutrients, primarily during the grove's early years of growth. Weed competition is also a problem in nurseries and in older groves where trees have been heavily pruned or are unhealthy and have a sparse canopy. Competition is strongest from perennial and summer annual weeds. If tall or dense weeds are allowed to grow around the young tree trunks, the trees may start producing fruit much later than the trees without any weeds around them. Weeds within groves and along borders and roadsides may host or harbor pests that can move to attack fruit or trees. These weeds can also serve as a reservoir because seeds can be blown in with the wind or can move into the grove with implements. Wildland fires can spread from unmanaged vegetation into groves. Weeds usually are a minor concern within healthy mature groves that have thick mulch and a dense canopy that shades the ground.
Grove vegetation must also be managed to provide benefits, such as reducing soil erosion, improving water infiltration, and limiting movement of potential contaminants off-site into surface water. Where soil becomes waterlogged around young trees, trees can be damaged by root asphyxiation. Allowing weeds or cover crops to grow and transpire will reduce soil moisture and the resulting damage. Vegetation provides alternative hosts and shelter for parasites and predators of invertebrate pests and reduces dust that can favor mite outbreaks. Cover crops on slopes and in open areas between trees, and vegetative filter strips along borders and roads, can be especially beneficial.
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 includes prevention, monitoring, and a combination of management strategies.
Good weed management begins with prevention. Prevention is the most effective method of managing weeds. Choose an appropriate growing site and prepare it well before planting. Applying herbicides, cultivating, and mowing are important primarily during site preparation before planting, when trees are small or widely spaced, and along roadsides and borders. Chemical and mechanical methods are limited after planting. Avocados are frequently grown on rocky slopes with limited mechanical access. Trees can be injured by herbicides and any activity that compacts or disturbs soil around their shallow roots.
After planting, maintain a thick layer of organic mulch beneath the canopy to just beyond the drip line. Keep mulch about 6 inches back from the trunk or keep mulch thin there. Mulching and spot hand-weeding provide the best 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. Keep trees healthy so they develop a dense canopy that shades the soil and produce their own mulch through natural leaf drop.
Regular monitoring helps you select effective control methods and tells you when to take action. Detect, identify, and keep a record of new weeds and those weeds that escape management actions to help you prevent new weeds from establishing or to 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 as they become large. It is easier to control annual weeds when they are small and have not become established. Perennial weeds are more vulnerable to control at the early bud stage and during fall when 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. Weeds often grow in patches and, locating and spot managing weeds may save time and money while achieving good control. Therefore, it is important to know what weed species are present, their growth stage, and their abundance and location in the orchard. A handheld GPS (global positioning system) unit is useful for marking patches of troublesome weeds for spot management and subsequent monitoring.
When and How to Monitor
Fall monitoring. 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.
At sites to be planted and at least around young trees, after the first fall rains look for and record the species of perennial seedlings or sprouts, winter annual seedlings, and mature summer annuals, which are easier to identify than seedlings. In planted groves, inspect the thickness and effectiveness of mulch and assess whether additional controls are warranted near trunks. Check row middles primarily for perennial seedlings and sprouts.
Winter monitoring. Cold, dry winters delay weed emergence. A warm, rainy winter means that winter annuals will grow large and will be capable of producing many seed, perhaps requiring earlier and more vigorous control action than during drier winters. Examine bare or untreated ground in February to observe the full range of winter annual species present and the vigor of their development. If weeds are abundant near trunks, consider actions during winter or early spring such as flaming, hand-weeding, or postemergence herbicide application.
Spring monitoring. 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 applying preemergent herbicides.
Summer monitoring. Monitor in midsummer for perennials, especially before planting. Monitor for perennials at least 2 weeks before any cultivation. Treating perennials with a translocated herbicide 1 to 2 weeks before cultivation can kill underground structures so the weeds will not be spread by cultivation. Monitor again a few weeks later and retreat any perennial regrowth.
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 .
Choose an appropriate growing site and prepare it well 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 that soil is disturbed. It is especially important to eliminate perennial species before planting or moving soil around the site.
Control perennial weeds. Aggressive perennial control should be the focus of your program. Where feasible, repeated cultivations of dry soil during the summer before planting exposes rhizomes and stolons to drying, killing vegetative propagules of perennials such as bermudagrass, dallisgrass, and Johnsongrass. Cultivation does not eliminate the reservoir of seeds, which will continue to germinate for a period of years. Control emerging seedlings with cover cropping, cultivating, flaming, mulching, and applying herbicides to prevent seedlings from forming perennial structures. In early fall, treat perennials with a systemic postemergent herbicide when the plants flower. Repeat any cultivation 10 to 20 days after herbicide application to expose the root systems to further drying. Monitor the next spring and spot-treat any regrowth.
If field bindweed is present, the above program will reduce infestations, but because of its deep perennial root system field bindweed will likely persist and require ongoing controls until a mature tree canopy heavily shades the ground. Even in mature groves, perennials such as field bindweed and wild cucumbers can be common along borders and roadsides. Where they receive light, these vinelike weeds often grow up into trees.
Control annuals and seedlings. Various management strategies including cover cropping, cultivating, flaming, hand-weeding, mowing, mulching, spraying herbicides, and (in established groves) cultural practices that promote a dense tree canopy will provide good control of annuals. Where irrigation is feasible, the population of annual weeds and perennial seedlings can be greatly reduced by cultivating, irrigating to germinate seeds near the surface, then cultivating shallowly to destroy seedlings. Repeat this irrigate-wait-then-cultivate cycle at least once.
Before planting you can apply preemergent herbicides to the entire grove or to strips 4 to 6 feet wide where trees will be planted. Alternatively, control emerged weeds using methods such as cultivation or contact herbicides, plant trees, then apply preemergent herbicide after planting. Be aware that some preemergent herbicides should not be applied before planting or near young trees because they can injure avocado. Be sure to follow label restrictions.
Focus on controlling perennials and any weeds near trunks after trees are planted. Apply and maintain a thick layer of organic mulch, or perhaps a weed barrier fabric, as discussed in the section on mulch below. Hand-weed or spot treat any weeds that grow in or near edges of mulch. Be careful not to injure trees with herbicide or damage roots or trunks when using 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 wide area weed-free 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, perennials can establish quickly in the tree rows because there is no competition. Check regularly for perennials and spot-treat them.
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. However, herbicides should be used in combination with other methods, especially with mulching near trunks.
Soil compaction, erosion, reduced water infiltration, contaminant movement off-site, and injury to trees can be serious problems with total control. Because no single chemical controls 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. Perennials 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 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, other vegetation is increasingly shaded by the canopy and the mulch. Borders and roadsides become the primary locations for weed control. 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 choice of control methods depends 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 keep from spreading 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 tree row surface 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 mechanical access is limited. 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. Weed fabrics are relatively efficient to apply and are longer-lasting than organic mulch. However, weed fabrics are relatively expensive. 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. Reapply mulch annually during the first several years of tree growth. Consider applying additional organic mulch at least once every several years throughout trees' life, especially if leaf mulch has been blown or washed away. Keep any applied mulch thin near trunks or about 6 inches away from the trunk.
Avoid removing or disturbing dropped avocado leaves, which naturally provide mulch. Take steps to retain leaves under trees. Keep tree skirts low. Install leaf barriers such as a moat of other organic material encircling the drip line. 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 mulch. Where terrain and tree spacing permit their use, specially designed mowers blow vegetation from middles into the tree rows, providing mulch that suppresses weed growth. Be sure to control any vegetation near trunks before you blow on the mulch. Cereal cover crops such as forage oats work particularly well for this "mow and throw" technique because the biomass does not degrade as quickly as does that of broadleaf vegetation, such as vetch. Plant the cover crop in about October and mow in late March (the 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 species selection reduce the need to manage the cover crop. Depending on the situation, cover crop management methods include applying herbicides, cultivating, hand-weeding, mowing, rolling a ring roller over it to injure the stems, and withholding irrigation. Keep cover crops away from tree trunks to minimize competition with trees and to reduce 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, UC ANR Publication 21471. Detailed information on a wide range of cover crops is also in the Cover Crops Database at http://www.sarep.ucdavis.edu.
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, such as in the spring to turn under a cover crop. Cultivate weeds when they are small, preferably under 4 inches tall. Cultivation soon after emergence is especially important for perennials such as dallisgrass and Johnsongrass; once these weeds have developed more than a few leaves, they produce underground rhizomes from which they will regrow after cultivation.
Rugged terrain, rocky soil, the presence of a fixed irrigation system, and avocado's spreading shallow root system make cultivation infeasible or undesirable in many situations. Cultivating too closely cuts trees' feeder roots, reducing water and nutrients uptake. Cultivation may spread pathogen propagules and increase the risk of pathogens infecting roots. Excessive cultivation increases dust, erosion, and soil moisture loss. Cultivation damages soil structure, reducing water infiltration. Cultivation can spread perennial weeds. However, cultivation combined with an application of translocated herbicides controls some established perennials. Once trees are about 6 to 8 feet tall, discontinue cultivation to avoid damaging root systems.
Mowing. Mow or flail where feasible before planting, along grove borders and edges, and perhaps 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 invasions of new weeds than is resident vegetation. Be aware that repeated mowing promotes a shift to species that tolerate mowing, especially grasses.
Flaming. Weeds can be controlled with specially designed flamers, most using propane. Other equipment includes hot water or steam applicators and infrared devices containing heating elements without an open flame. Flamers can be handheld or mounted on a hand cart or tractor. Mechanized flamers have multiple burners, while small devices 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.
Take care to avoid starting a fire. Use good judgment to identify hazardous situations where flaming should not be conducted. Do not use flames in dry areas or during the dry season where fire is a hazard. Keep fire suppression equipment handy, such as a fire extinguisher, shovel, and water, in case of accidents. Keep flamers away from unguarded trunks. Be especially cautious around dry vegetation, mulch, and leaf litter. 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. A good target for flaming is winter weeds germinating after wet weather, after the end of the wildland fire season. Disadvantages of flaming include a lack of residual control, poor control of some grasses and perennials, critical timing requirement to ensure adequate control, hazards associated with handling pressurized flammable gas, and the potential for fire.
Flame weeds while they are less than or about 2 inches tall. Broadleaved seedlings are most sensitive to flaming. Perennials and grasses are less susceptible and repeated flaming leads to perennials and grasses dominating the vegetation unless other controls 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 to yellow and die within several days.
Herbicides. Postemergent herbicides are applied after weed emergence. Preemergent herbicides are applied before weeds emerge, such as winter application to minimize perennial seedlings and summer annuals. A preemergent and nonselective postemergent herbicide are often applied in combination if some weeds have already emerged. 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.
Phytotoxicity. Many herbicides can injure avocado. Nonselective, translocated (systemic) herbicides and certain preemergents can cause phytotoxicity by contacting tree's shallow surface roots. Avoid applying most herbicides if they may contact avocado roots. Be sure not to spray postemergent 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 perennials.
Herbicide-resistant weeds. Tolerance and resistance prevent 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 use of selective herbicides, such as those that control grasses but do not damage avocado or other broadleaves.
Resistance occurs 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, rigid ryegrass (Lolium rigidum) resistant to glyphosate (Roundup) has developed in California where glyphosate has been repeatedly applied.
Minimize resistance development by using cultivation, mulch, and hand-weeding. Avoid repeatedly applying a single herbicide, or herbicides with the same site of action. Scout growing areas and note weed escapees or species shifts over time. Avoid spreading weed seeds and propagules from infested areas, for example, 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, UC ANR Publication 8012.
Postemergent herbicides. Foliar-applied or postemergent herbicides are sprayed on the leaves of weeds that have emerged. They are classified as contact herbicides if they kill only the plant parts that are sprayed. Contact herbicides are most effective on seedlings and young weeds.
Translocated or systemic herbicides such as glyphosate are transported via the plant's vascular system from contacted foliage to other parts of the plant, including roots and rhizomes. They are more effective on actively growing weeds and are the most effective materials for controlling perennials. However, in comparison with contact herbicides, translocated herbicides pose a greater risk of phytotoxicity to avocado.
Preemergent herbicides. Preemergent herbicides kill susceptible plants as they germinate; most are effective only against germinating seeds. To be effective, the chemical must be moved into about the upper 2 inches of soil by rain, light irrigation of about 0.5 inch, or by cultivation. Some preemergent herbicides must be moved into the soil immediately, while others may remain on the surface for a short time before incorporation. Certain materials lose effectiveness if soil is cultivated after application. Follow label directions regarding incorporation.
Efficacy typically persists for 2 to 5 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. For example, herbicide activity dissipates more quickly during prolonged wet weather and in areas that remain wet, such as around low-volume drip emitters.
Lower-than-label rates may be effective if the weed spectrum in the grove is particularly sensitive to herbicides, in areas with annual rainfall of less than 11 inches, and where trees are planted on berms, because there is less leaching and breakdown from moisture. Consider avoiding preemergent herbicides for 1 or 2 years before planting or replanting to minimize the possibility that residues will injure new trees.
Chemical mowing. Using a low rate of herbicide retards the growth of weeds without killing them, thus maintaining a ground cover. The application timing and rates for chemical mowing (perhaps two or three times per season) depend on the vegetation present, its stage of growth, and growing conditions. Young winter annuals can be controlled with very low rates applied in about January or February. Summer annuals and older winter annuals are harder to control. Plants are harder to control in spring or summer. When stressed by periods of drought weeds require even higher herbicide rates.
Repeated use of low rates of an herbicide can cause a shift in the weed population; careful monitoring is essential. Perennials or annuals not controlled by chemical mowing may quickly take over because of the reduced competition; spot treat these before they become dominant and extremely difficult to manage. Chemical mowing may select for herbicide-resistant plants. Use alternate strategies within a season or between seasons.
UC IPM Pest Management Guidelines: Avocado
B. A. Faber, UC Cooperative Extension, Santa Barbara/Ventura counties
Acknowledgment for contributions to Weeds:W. T. Lanini, Weed Science/Plant Sciences, UC Davis
A. Shrestha, UC Statewide IPM Program, Kearney Agricultural Center, Parlier
P. A. Phillips, UC IPM Program, UC Cooperative Extension, Ventura County