Agricultural pest management
Integrated Weed Management
(Reviewed 9/08, updated 9/08)
Weeds in citrus orchards compete with trees for nutrients, water, and light. Weeds also cause problems by contributing to arthropod pest problems, interfering with cultural operations, and increasing frost hazard. Competition from weeds is damaging to citrus trees when they are young because it slows tree growth and increases their susceptibility to insect and disease damage. Weeds around tree trunks may create a favorable environment for pathogens that infect the trunk and roots as well as provide shelter for field mice. However, as trees grow older, the tree canopies shade part of the orchard floor and reduce weed growth. Weed competition with mature trees can be more serious in drip- or microsprinkler-irrigated orchards because tree roots are concentrated in a smaller area than in furrow irrigation.
Careful management and good sanitation help limit weed infestations. To prevent the spread of weeds, make sure that irrigation canals and ditchbanks are free of weeds and weed seeds. Provide good drainage because high moisture in areas such as furrow bottoms, at furrow ends, and around stand pipes favors weed growth. Where furrow irrigation is used on slow draining soils, use shorter furrows or establish lateral furrows halfway into the tree rows to reduce the time water stands in the furrows. Discourage weed seedling establishment by letting the top 2 or 3 inches of soil dry completely between furrow or sprinkler irrigations. Do not allow weeds around the orchard perimeter to mature and produce seeds.
Herbicides can provide effective control of most weeds in a citrus orchard, facilitating irrigation and other cultural operations. Herbicides also create a relatively weed-free orchard floor with less frost hazard during winter because of the warming influence of the bare ground.
However, certain problems are associated with total reliance on herbicides. In orchards planted on slopes, complete weed control creates bare orchard floors that are prone to soil erosion. On certain sites, an orchard floor devoid of vegetation can become compacted and a silty surface layer may develop, impeding water penetration into the soil profile. Repeated shallow cultivation or the application of mulch may be needed to address these problems. If a particular herbicide is used repeatedly, species that are not susceptible to the herbicide may thrive and become dominant. In addition, repeated use of the same herbicide may lead to the development of herbicide resistance. Therefore, herbicide rotation is an important strategy in integrated weed management in citrus orchards.
To effectively manage vegetation, you must know the weed species present and their abundance and location in the orchard. Conduct a survey at least twice a year, in late winter and in summer, and keep records of your observations. Pay special attention to perennials and check fence rows and ditch banks. A map can be helpful in locating trouble spots infested with perennials or resistant species, moist areas favoring weed growth, or sources of reinfestation from surrounding land. Record results from your winter survey (example form— ) and summer survey (example form— ) and keep them as part of your permanent orchard records. Monitoring information collected over several years is invaluable in determining changes in weed species and adjusting management tactics.
WEED MANAGEMENT BEFORE PLANTING
Weed management starts before the orchard is planted. Site preparation is an important part of an orchard weed management program. In spring, survey the site for the weed species present and then disc the weeds under and level any irregularities in the grade, especially when planning furrow irrigation.
Perennials on the site, such as johnsongrass or bermudagrass, are easier and less expensive to control before you plant the trees. Established johnsongrass and bermudagrass can be destroyed by repeated discings in summer; the disced and exposed rhizomes and stolons will dehydrate. Or, during early fall when the perennials are still flowering, treat with glyphosate; repeat the treatment in spring to kill regrowth, and disc 2 to 3 weeks later to expose the root system to drying.
Before or after planting, usually in spring, a preemergent herbicide can be incorporated over the entire site or into 4- to 6-foot-wide strips where the trees are planted. Herbicides such as trifluralin (Treflan), oryzalin (Surflan), oxyfluorfen (Goal), and napropamide (Devrinol) can be safely used around young citrus trees. A preemergence application usually controls germinating seedlings throughout the summer. A given dosage of preemergent herbicide may be more toxic to trees in sandy soils or soils low in organic matter. Follow all label precautions carefully. Carefully calibrate and check the functioning of spray equipment.
Weeds that escape the preemergence treatment are often treated with postemergent herbicides. During summer, spot-treat summer annuals and perennials; such treatments are especially necessary in moist areas.
WEED MANAGEMENT IN NEWLY PLANTED ORCHARDS
Once trees are planted, disturb the soil as little as possible if you plan no-till management. For furrow irrigation, establish one or two narrow furrows along the planted trees. Perennial grasses can be controlled with sethoxydim (Poast) or fluazifop-p-butyl (Fusilade). Glyphosate (Roundup) suppresses nutsedges, bermudagrass, and perennial broadleaf weeds. To avoid injury to the trees, be sure not to spray citrus foliage or trunks with this herbicide. Although wrappers help to protect tree trunks from herbicides, they should not be relied on. Regular preemergence and postemergence treatments during the establishment years remove much of the competition by weeds and facilitate irrigation and other cultural practices. Synthetic fabric mulches that are resistant to UV degradation can be placed around the base of the tree. These fabrics allow moisture to penetrate but prevent weeds from emerging. Install fabric in a 4 x 4 foot square, centered around the tree. Use anchoring staples or nails to hold fabric in place. Woven fabrics last at least 5 years. Use in conjunction with tree wraps to prevent the fabric from contacting tree trunks because heat injury to the bark is possible.
WEED MANAGEMENT IN ESTABLISHED ORCHARDS
A ground cover is maintained in some citrus orchards, mainly in northern California and on hilly terrain. A ground cover of resident vegetation or a sown cover crop prevents soil erosion and improves water penetration and soil structure. Some ground covers can be managed by complete mowing or by mowing the row middles while keeping a strip along the tree rows free of weeds with herbicides. Repeated mowing favors the establishment of perennial weeds, which are deep rooted and more competitive with citrus than annuals.
Weeds are rarely managed in citrus with regular cultivation. Tillage destroys the feeder roots of citrus trees that are responsible for absorbing nutrients, water, and oxygen in the top soil. Disease organisms may enter through root systems that have been injured by tillage. Discing contributes to soil erosion, especially on sloping land, and to soil compaction. If the soil is dry, cultivation creates dust, which interferes with biological control of insect and mite pests. Discing may also increase the weed population by bringing buried seeds to the surface or spreading rhizomes, tubers, or stolons throughout the orchard.
In established orchards, weed management has to be adjusted to the irrigation method used. In orchards irrigated by furrows, weeds are a particular problem in furrow bottoms and at furrow ends where high moisture and extensive leaching of herbicides allow weeds to grow. Where herbicides are applied with cluster nozzles, one side of the furrow is in the shadow of the spray stream and remains untreated. Under a low-volume irrigation scheme, the permanently wet zone around emitters of sprinkler heads favors weed growth and promotes the breakdown of soil-residual herbicides.
Preemergent herbicides are used to control germinating weed seed; they do not control established plants. Spray preemergent herbicides onto the soil just before an irrigation or rainfall, so that the water carries the chemical into the soil where the weed seeds germinate. Preemergent herbicides can provide control for up to a year, depending on the solubility of the material, adsorption of the material to soil, weed species, and dosage applied. Leaching from the soil is more extensive on sandy than on clay soils. Prolonged moist conditions during heavy winter rains, in furrow bottoms, or around low-volume emitters during irrigation favor breakdown and leaching of herbicides. Bromacil (Krovar) leaches more rapidly than simazine (Princep), diuron (Karmex), and napropamide (Devrinol) and is thus less effective under frequent, low-volume irrigation. Splitting a treatment into two or more sequential applications can prolong the control provided by the herbicides.
Postemergent herbicides are used on established weeds. They act either by contact or by translocation throughout the plant. Contact herbicides, such as paraquat (Gramoxone Inteon), kill those parts of the plant that are actually sprayed, making good coverage and wetting essential. A single spray kills susceptible annual weeds; retreatment is necessary if regenerating perennials are present or if annual weeds reestablish themselves from seeds. Translocating herbicides, such as glyphosate (Roundup), move into the plant to kill it. Complete coverage of the plants with translocated herbicides is not necessary.
UC IPM Pest Management Guidelines: Citrus
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