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.
The first step to develop any weed management program is to identify the weeds infesting the orchard or planting site. Familiarity with each weed's growth and reproductive habits is crucial in order to choose the most effective management strategy. See weed photos linked to the weeds in the list of COMMON AND SCIENTIFIC NAMES OF WEEDS.
Weed management is part of an overall orchard management system; plants on the orchard floor can influence other pests such as insects, rodents, 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, sunlight, and rooting space. Competition is most severe during the first four to five years of the tree's life or where root growth is limited. Weeds, particularly grasses that form a dense cover 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 three 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, organic matter, or both influence what 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. Lower rates of preemergence herbicides are often required for weed control on sandy, light soils, but residual control may be shorter than on clay or clay loam soils. Split applications are sometimes used in sandy, light soils to reduce the risk of injury to olives and to extend the period of weed control. Clay soils, which dry more slowly than sandy loam soils, usually are not cultivated as frequently as the lighter soils because of the extra time needed to fully desiccate uprooted weeds in the heavier soils.
The irrigation method, amount of water applied, and pattern of rainfall affects the frequency and timing of cultivation. Irrigation practices also affect the performance and longevity of residual herbicides. Frequent wetting of the soil promotes more rapid herbicide degradation; 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 preemergence herbicide is moved into the soil; subsequent irrigation is less important for herbicide movement. The optimum amount of water for herbicide activity is from 0.5 to 1 inch. Greater amounts of water (3–6 inches) during a relatively short period of time could move the herbicide far enough in sandy soils to increase the risk of tree root uptake and injury.
When properly used, herbicides registered for use in olives can control most weed species. In many orchards, combinations 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 (preemergence herbicides) and those that are active on growing plants (postemergence herbicides). Some herbicides have both pre- and postemergence 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.
Preemergence herbicides are applied to bare soil and are incorporated 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 exposure to sunlight. Weeds that emerge while the herbicide is on the surface, before it is activated by rain or irrigation, usually 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. Efficacy may also be reduced if preemergence herbicides are applied to a soil surface covered with leaves, dried weeds, or other debris. Oryzalin (Surflan) is 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).
Postemergence herbicides are applied to control weeds already growing in the orchard. They may be contact herbicides, such as paraquat and oxyfluorfen. They may be translocated, systemic herbicides 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 completely effective on mature weeds.
Postemergence herbicides can be combined with preemergence herbicides when weeds are present at the time of treatment, or applied as spot treatments during the growing season. In newly planted orchards, selective postemergence 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 postemergence herbicides. 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 or nozzles that produce coarse or larger spray droplets, which are less prone to drift. Off center (OC) nozzles are often used on the end of the boom to treat 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.
The first step in developing any weed management program is to identify the weeds infesting the orchard or planting site. Familiarity with each weed's growth and reproductive habits is crucial in order to choose the most effective management strategy.
Many different species of summer and winter annual and perennial weeds are found in 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, the Weeds of California and Other Western States, 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 perennial weeds before trees are planted. Perennial weeds are more difficult to control and require more herbicide applications, higher concentrations of herbicides, or both. Herbicide applications to perennial weeds after planting may lead to the injury of young trees. Perennial weeds that can be especially troublesome are field bindweed, johnsongrass, dallisgrass, bermudagrass, and nutsedge.
An especially effective method of weed control before planting is to cultivate, then irrigate to encourage germination of new weeds, and shallowly cultivate again to destroy seedling weeds. This approach lowers the number of weed seed in the soil and thus, reduces weed populations in the subsequently planted orchard. At least two cycles of cultivation and irrigation followed by another shallow cultivation are needed for a marked reduction in weed seedlings. Although this method is effective on annual weeds and seedlings of perennial weeds, it is not effective on established perennial weeds. Where water is a limiting resource or too expensive to use in this manner, deep burial of weed seeds and perennial weed structures may be an option.
One 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, which are more sensitive to drying. Frequent reworking of the soil with spring-tooth harrows can pull additional rhizomes to the surface, subjecting them to dehydration. However, be aware that purple nutsedge may be spread within a field by certain cultivation equipment such as harrows.
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 greatly reduced. Working the soil when wet may actually increase perennial weeds because many pieces of cut rhizome can root and develop into new plants.
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. Nutsedge can be reduced by deep plowing with large moldboard plows to bury the nutlets to a depth of at least 12 inches.
Soil solarization is a nonpesticidal method 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 weed seeds. Additionally, it may also aid in decreasing disease-causing organisms (pathogens) and nematodes. 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 Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds, available at your University of California Cooperative Extension.
Weed seedlings and established annual weeds can be controlled either with preemergence or postemergence herbicides before planting. Use a preemergence 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. Postemergence 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 preemergence herbicides only after trees have been planted.
Weed Management In New Orchards
Olive 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 compared to 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.
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 Clemens 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. Discs, tillers, or mowers can be used between the rows.
Mechanical control of weeds should be done repeatedly when weeds are immature; established weeds are much more likely to survive or regrow after the operation. 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, reroot (re-establish), and produce seed. When using any mechanical equipment around trees, be careful not to injure the feeder roots or trunk.
Planted cover crops can also be used to reduce weeds between tree rows. With cover crops, the species selected and management will differ from one area of the state to 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.
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 relatively 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 can promote young tree growth. However, mulches may also provide a good habitat for gophers, voles, field mice, and snakes; can introduce new weed seeds; and may lower soil temperatures, which can reduce tree growth. 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.
To control weeds after trees are planted and before they reach bearing age, apply a preemergence 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 as 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 and thus have more 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 band application down the tree row. For a detailed discussion of orchard floor management, see Orchard Floor Management Practices to Reduce Erosion and Protect Water Quality.
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 soil pathogens access to the tree.
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 usually 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 matte 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 plastic drip tubing and sprinkler heads or ignite mulches in the orchard. A burning permit may be required in some areas to protect air quality.
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 can become a 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. Weeds that grow in or on the mulch can be controlled with spot treatments of herbicides, which can extend the useful life of the mulch.
Preemergence 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 postemergence (foliar) herbicide. It may be most beneficial to delay the preemergence application in winter until most weeds have germinated and then tank mix a preemergence and postemergence herbicide. This allows longer weed control into the summer yet does not allow much competition from weeds to the tree. For greatest safety, direct herbicides 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 preemergence herbicides or a mixture of preemergence and postemergence herbicides. Read and follow label directions carefully before combining herbicides.
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 in 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 Covercrops for California Agriculture (UC ANR Publication 21471), or Cover Cropping in Vineyards: A Grower's Handbook (UC ANR Publication 3338).