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How to Manage Pests

UC Pest Management Guidelines


Olive

Integrated Weed Management

(Reviewed 1/08, updated 1/09)

In this Guideline: More about weeds in olive:

Weed control in olive orchards enhances the development of newly planted trees and improves the growth and yield of established trees. Growers have many weed management tools available to achieve these objectives; however, the best strategy for employing these tools will vary from year to year and from orchard to orchard, according to local conditions.

Weed management is part of an overall orchard management system; plants on the orchard floor can influence other pests such as insects, mites, nematodes, and diseases. A weed management program should start before trees are planted because the more difficult-to-control weeds (particularly perennials) are easier to manage before planting. Weeds reduce tree growth and yields by competing for water, nutrients, and sunlight. Competition is most severe during the first 5 years of the tree's life or where root growth is limited. Weeds around the tree trunk not only compete directly with tree growth, but provide a good habitat for field mice or voles, which can girdle and kill young trees. Gophers are most prevalent in nontilled orchards and are common where broadleaf weeds, such as field bindweed and perennial clovers, predominate. They feed on the roots and weaken or kill young trees. Dry weed growth is a fire hazard. For optimum yields and tree health, control weed growth, especially within 3 feet of a young tree's trunk.

After about the fourth year, the effect of competition from weeds is somewhat lessened as trees become established and shading from the orchard canopy reduces weed growth. In older orchards, however, weeds result in colder orchard conditions, increased frost hazards, and the potential for olive knot. Weeds also increase humidity, making trees more susceptible to infection by the peacock spot fungus. In addition, weed growth can interfere with cultural practices and harvest. For example, weeds can disrupt the application pattern of water from sprinklers and low-volume spray emitters. Olive trees are shallow rooted and frequent cultivation near trees can injure tree trunks and promote suckering. Tree trunk injuries can result in crown gall or olive knot infections.

Orchard floor management decisions and the management methods used are significantly influenced by location in the state, climatic conditions, soils, irrigation practices, topography, and grower preferences. Weeds are commonly controlled either chemically or mechanically in a 2- to 5-foot-wide strip in the tree row. The area between tree rows may also be chemically treated or mechanically mowed or tilled. Alternatively, mulches, subsurface irrigation, and flamers can be used. Often several weed management techniques are combined.

Soil characteristics are important to weed management. Soil texture and/or organic matter influence which weed species are present, the number and timing of cultivations required, and the activity and residual effects of herbicides. Annual species such as puncturevine, crabgrass, sandbur, and Panicum spp. or perennials like johnsongrass, nutsedge, and bermudagrass are more prevalent on light-textured soil, while perennials such as curly dock, field bindweed, and dallisgrass are more common on heavier-textured soils. Less preemergent herbicide is required for weed control on sandy, light soils, but residual control may be shorter than on clay or clay loam soils. Use low rates of herbicide on sandy soils or those low in organic matter. Clay soils are slower to dry for effective cultivation than sandy loam soils; thus, more frequent cultivation is practiced on lighter soils than heavy soils.

The irrigation method, amount of water applied, and pattern of rainfall affects the frequency and timing of cultivation as well as the selection of chemicals and their residual activities. Frequent wetting of the soil promotes more rapid herbicide degradation in the soil. Herbicide degradation is generally faster in moist, warm soils than in dry, cold soils. Degradation is also more rapid under drip emitters or microsprinklers than under furrow or sprinkler irrigation. The first irrigation following an herbicide application is the most critical in terms of how far the preemergent herbicide is moved into the soil; subsequent irrigation is less important to the movement of the herbicide. The optimum amount of water for herbicide activity is from 0.5 to 1 inch. Greater amounts of water (3–6 inches) could move the herbicide far enough into the soil, especially in sandy areas, so that it is absorbed by the tree's roots.

When properly used, herbicides registered for use in olives can control most weed species. In many orchards, combinations and/or sequential applications of herbicides are required to provide effective, economical control. Before using any herbicide, identify the weed species to be controlled, then read and follow product label directions carefully.

Herbicides are traditionally discussed as two groups: those that are active against germinating weed seeds (preemergent herbicides) and those that are active on growing plants (postemergent herbicides). Some herbicides have both pre- and postemergent activity. Herbicides vary in their ability to control different weed species. Check the SUSCEPTIBILITY OF WEEDS TO HERBICIDE CONTROL tables and consult product labels for specific weed control activity. Most herbicides can be combined for controlling a broader spectrum of weeds.

Preemergent herbicides. Preemergent herbicides are applied to bare soil and are leached into the soil with rain or irrigation where they are active against germinating weed seeds. If herbicides remain on the soil surface without incorporation, some will degrade rapidly from sunlight. Weeds that emerge while the herbicide is on the surface, before it is activated by rain or irrigation, will not be controlled. Also, large weed seeds, such as wild oat, may germinate in the soil below the herbicide zone and still be able to emerge. Herbicides such as napropamide (Devrinol), and oryzalin (Surflan) are safe to use as a directed spray in newly planted olive orchards. Additional products are available for use in established orchards (see Herbicide Treatment Table).

Postemergent herbicides. Postemergent herbicides are applied to control weeds already growing in the orchard. They may be contact herbicides, such as paraquat and oxyfluorfen, or a translocated, systemic herbicide such as glyphosate, sethoxydim, and others. Contact herbicides are most effective on young weeds, whereas translocated herbicides are effective on both young and older weeds. No herbicide is effective on old, dusty mature weeds.

Postemergent herbicides can be combined with preemergent herbicides or applied as spot treatments during the growing season. In newly planted orchards, selective postemergent herbicides are available for the control of most annual and perennial grasses, but not broadleaf weeds. Young trees need to be protected from contact by some postemergent sprays. Be sure to check and follow individual label instructions.

In most orchards, herbicides are used only on a narrow strip of soil centered on the orchard row; thus, the area treated with herbicides in these orchards is 20 to 30% of the total orchard area.

Application equipment must be accurately calibrated to apply the proper amount of herbicide to the soil and young growing weeds. To minimize drift, spray equipment should be equipped with a short boom that has low pressure (LP), flat fan nozzles. Off center (OC) nozzles are often used on the end of the boom to apply chemicals in the orchard row. Some herbicides require special use precautions as indicated in the table below. Always read and follow the entire product label before using any pesticide.

For treatment of small areas, especially for perennial weeds, a backpack sprayer or low-volume controlled droplet applicator can be used. Extreme care needs to be exercised to avoid drift of herbicides (such as glyphosate–Roundup, oxyfluorfen–GoalTender, or paraquat–Gramoxone) to tree leaves or green stems.

MONITORING

Many different species of summer and winter annual and perennial weeds are found infesting California olive orchards. Weeds vary from area to area and year to year, even within orchards, so conduct weed surveys at least twice each year: once in late winter and again in late spring or summer to determine the spectrum of weeds present. These surveys are the basis for weed management decisions about herbicide choice or cultivation equipment and practices. Keep written records of survey results noting date and species observed. Use the pictures linked to the COMMON AND SCIENTIFIC NAMES table in the web version of this publication, the Weeds of California and Other Western States (UC ANR Publication 3488), or Weeds of the West, University of Wyoming, to help identify local weeds. The SUSCEPTIBILITY OF WEEDS TO HERBICIDE CONTROL tables will help you determine the best herbicide or combinations of herbicides to use for optimum control of individual weed species.

WEED MANAGEMENT BEFORE PLANTING

To reduce the competition from weeds during orchard establishment, control annual and perennial weeds before orchards are planted. It is especially important to control established stands of perennial weeds before trees are planted so that potential injury to young trees from herbicides can be avoided. Perennial weeds that can be especially troublesome are field bindweed, johnsongrass, dallisgrass, bermudagrass, and nutsedge.

Nonchemical controls. An especially effective method of weed control before planting is to cultivate, then irrigate to germinate new weeds, and shallowly cultivate again to destroy seedling weeds. Frequent cultivation lowers weed seed populations in the soil, thus reducing weed growth. At least two cycles of cultivation, irrigation, followed by a shallow cultivation are needed for a marked reduction in weed seedlings. Unfortunately, this method is not effective on established perennial weeds.

A method of control for perennial grasses such as bermudagrass and johnsongrass is to cultivate the soil when it is very dry. Cultivation cuts the rhizomes into small pieces so they can dry. Rework the soil frequently using spring-tooth harrows to pull new rhizomes to the surface so that they will dehydrate. If the soil is irrigated or rain occurs before total control of the perennial plant is achieved, the rhizome pieces will begin to grow and the effectiveness of this practice is reduced. By the same token, working the soil when wet can increase the population of perennial weeds, because each piece of cut rhizome can root and develop into a new plant.

Field bindweed growth can be reduced for 1 to 2 years by deep plowing or with a reclamation blade (a large V-shaped blade) to cut the roots 16 to 18 inches deep in dry soil. Populations of nutsedge can be reduced by deep plowing with large moldboard plows to bury the nutlets to a depth of at least 12 inches.

Seedlings of perennials can be controlled with repeated cultivation.

Soil solarization is a nonpesticidalmethod of controlling soilborne pests by placing clear plastic sheets on moist soil during periods of high ambient temperature. The plastic sheets allow the sun's radiant energy to be trapped in the soil, heating the upper levels. Solarization during the hot summer months can increase soil temperature to levels that kill many disease-causing organisms (pathogens), nematodes, and weed seeds. It leaves no toxic residues and can be easily used on a small or large scale. Soil solarization also improves soil structure and increases the availability of nitrogen (N) and other essential plant nutrients. (For additional information see UC ANR Publication 21377, Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds, available at your University of California Cooperative Extension)

Herbicides. Weed seedlings and established annual weeds can be controlled either with preemergent or postemergent herbicides before planting. Use a preemergent herbicide before planting an orchard only in conjunction with a rotation crop. Make sure the residual period of the herbicide is not long enough to preclude planting the trees. Postemergent herbicides generally have a short soil residual and are safer to use before planting trees. To avoid possible exposure of newly planted trees to herbicides in the backfill soil, many growers prefer to use preemergent herbicides only after trees have been planted.

WEED MANAGEMENT IN NEW ORCHARDS

Trees are most sensitive to weed or cover crop competition during the first few years of growth and where soil depth is limited. Weedy orchards may require several more years to become economically productive than weed-free orchards. Regardless of the method used to control weeds, be careful not to injure trees with chemicals, or to mechanically damage the trunk or roots. As trees become established, competition from weeds is lessened as shade from the tree canopy reduces weed growth.

Cultivation. Some growers prefer to manage weeds without herbicides for the first year or two after planting. This usually requires hoeing, cultivating, or using weed knives (less than 2 inches deep) around trees several times during spring and summer as well as cultivating or mowing between tree rows. This is best accomplished when weeds are still in the seedling stage; it becomes more difficult when weeds are allowed to get large. Hand tools are generally used close to the tree to minimize injury from mechanical cultivators, particularly when the trees are young. Mechanical cultivators available for use in the tree row include: weed knives, spyder cultivators, and rotary tillers. Rotary tillers such as a Weed Badger, Kimco, or Clements Hoe are most effective if used on loose soil that is not rocky. Hand-held mechanical flails (e.g., Weed Eaters) may be used, but can injure tree trunks. Disks, tillers, or mowers can be used between the rows. Mechanical control of weeds must be done repeatedly when weeds are immature. The equipment should be set to cut shallowly, to minimize damage to tree roots. As weeds mature, they are difficult to control, may clog equipment, and produce seed. When using any mechanical equipment around trees, be careful not to injure the feeder roots or trunk.

Cover crops. Planted cover crops can also be used to reduce weed populations between tree rows. With cover crops, the species selected and management will differ from one area of the stateto another. Be sure to select a cover crop such as fall-seeded cereal crops (wheat, oat, cereal rye, or barley), Blando bromegrass, Zorro fescue, rose clover, or subterranean clovers that will not become competitive with the trees. Examples of invasive cover crops include white clover, strawberry clover, and bermudagrass. Cultivation in preparation for planting a winter annual cover crop will also reduce weed growth. To preserve surface cover, mow the cover crop to the correct height recommended for that crop.

Mulches. Weeds in the tree row can also be controlled using mulches. Organic mulches (cereal straw, green waste, composted wood chips) or synthetic mulches of polyethylene, polypropylene, or polyester can be used around young trees. Shredded tree prunings also make good mulch. Always apply mulches when the soil surface is free of weeds. Mulches prevent the growth of weed seedlings by blocking light and preventing it from reaching the soil surface. They create more uniform moisture conditions, which in turn promotes young tree growth. However, mulches may also provide a good habitat for gophers, voles, field mice, and snakes or be a source of new weed seed that came with the mulch. Mulches do not control perennial weed growth unless all light can be excluded. Some woven fabric mulches offer excellent weed control for several years, but the initial cost of purchase and installation is high.

Herbicides. To control weeds after trees are planted and before bearing, apply a preemergent herbicide (e.g., flumioxazin, isoxaben, or oxyfluorfen) to either a square or circle around each tree (at least 4–6 feet across) or as a band down the tree row. Herbicides can also be applied to control weeds after they emerge. Selective herbicides are available for annual grass control and suppression of perennial grasses (e.g., sethoxydim, fluazifop, and clethodim), but to be effective they require the addition of an adjuvant (either a nonionic surfactant or a nonphytotoxic oil). These materials do not control nutsedge or broadleaf weeds and clethodim is the only one that will control annual bluegrass. Paraquat can be used to control weeds near young trees protected with shields or wraps. The nonselective herbicide glyphosate can control broadleaf weeds after emergence, but it should be used only around mature trees with brown bark and should not be allowed to contact tree leaves.

In conjunction with the use of herbicides in the tree row, mow or cultivate the weeds between the rows. Mow weeds when they get 6 to 8 inches high, usually about four to eight times a year. Cultivation is required when weed seeds germinate following each irrigation.

WEED MANAGEMENT IN ESTABLISHED ORCHARDS

It takes 3 to 4 years for an orchard to become established under normal growing conditions. Established trees are more tolerant of many herbicides than newly planted trees, thus increasing the options available for weed control. Generally weeds are controlled between tree rows by discing or mowing and a basal treatment of herbicide is applied around each tree or in a strip application down the tree row. For a detailed discussion of orchard floor management, see UC ANR Publication 8202, Orchard Floor Management Practices to Reduce Erosion and Protect Water Quality .

Cultivation. Cultivation can be used in established orchards to control annual and biennial weeds and seedlings of perennial weeds. Control seedlings of field bindweed, bermudagrass, and johnsongrass before they are 3 weeks old or they may form perennial structures such as rhizomes. Cultivating established perennials in an irrigated orchard often increases the weed problem. Cultivation also cuts and damages the roots of trees, reducing the ability of the tree to take up nutrients and allowing access to the tree of soil pathogens.

Flaming. Flaming is a method that can be used to control very young weed seedlings in established orchards. Use either a single flame that is directed to the base of the tree or several burners on a boom to flame the weeds between the tree rows. Flaming is effective only on newly emerged weed seedlings. Do not use flaming around young trees because it may damage the thin, green bark. Adjust equipment speed for desired weed injury without damaging the tree trunks. In mature orchards annual broadleaf weeds can be controlled with flaming but grasses are somewhat tolerant. Flaming is not intended to burn the weeds, but rather to kill the tiny seedling with heat. Properly flamed weeds should have a matt finish on the leaves and pressing your thumb and forefinger together on a leaf should leave a fingerprint. Never use flaming where there is dry, dead vegetation, leaves, or duff around the base of the tree. This material may ignite, causing a fire that girdles the trees. Flaming may also damage or ignite mulches in the orchard.

Mulches. Mulches can also be used for weed control as discussed in the section Weed Management in New Orchards. Because organic mulches degrade, they must be replenished annually. As mulches degrade they become a perfect growth medium for weed species such as common groundsel, prickly lettuce, common sowthistle, and panicle-leaf willowherb. Make certain that organic mulches are free of weed seed.

Herbicides. Preemergent herbicides can be applied either alone, in combinations of herbicides in fall after harvest, split into two applications (fall and spring), or in winter with a postemergent (foliar) herbicide. It may be most beneficial to delay the preemergent application in winter until most weeds have germinated. Then add a postemergent herbicide. This allows longer weed control into the summer yet does not allow much competition from weeds to the tree. For greatest safety, direct herbicide sprays only at the soil or at weed foliage, not at the tree leaves or 1- to 2-year-old wood. In orchards where tree rows are mulched or sprayed, there are often few weeds to treat and a sprayer with a weed detection system (e.g., WeedSeeker) can be used to reduce herbicide use.

Frequently, two or more herbicides need to be applied to obtain adequate weed control. It is critical to identify the weed species present in the orchard as described above in the section on Monitoring to determine which herbicide or combinations will provide the most effective control. Combinations may include two or more preemergent herbicides or a mixture of preemergent and postemergent herbicides. Read and follow label directions carefully before combining herbicides.

Cover crops. Cover crops are planted in some orchards to replace the resident weed vegetation on the orchard floor. These winter annual cover crops are fall-seeded cereal crops (wheat, oat, cereal rye, or barley), Blando bromegrass, Zorro fescue, rose clover, or subterranean clovers. These are seeded into a prepared seedbed between tree rows in late September through mid-November. Most plants will reseed themselves if mowed in January or early February and then allowed to regrow into April and May. Mowing after the seeds mature ensures seeds for the next season. Avoid invasive plants such as white clover and bermudagrass in a ground cover. Sometimes larger-seeded cover crops such as bell bean, purple or common vetch, or crimson clover are planted in orchards and tilled in as green manure. Perennial grasses (tall fescue, Berber orchardgrass, or perennial ryegrass) may also be grown but will require summer irrigation and may compete with tree growth. Keep cover crops away from the trees. Changing cover crop species reduces the potential for buildup of disease pathogens, weeds, rodents, and insect pests. For more information on cover crops, consult UC ANR Publication 21471, Covercrops for California Agriculture, or UC ANR Publication 3338, Cover Cropping in Vineyards: A Grower's Handbook.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Olive
UC ANR Publication 3452
Weeds
W. T. Lanini, Weed Science/Plant Sciences, UC Davis
Acknowledgment for contributions to Weeds:
C. L. Elmore, Vegetable Crops/Weed Science, UC Davis
D. W. Cudney, Botany and Plant Sciences, UC Riverside
D. R. Donaldson, UC Cooperative Extension, Napa County

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