Weeds in citrus orchards compete with trees for nutrients, water, and light. Weeds also cause problems by contributing to arthropod and rodent 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 and use screens to remove weed seeds from canal water. 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 or herbicides with the same mode of action may lead to the development of herbicide resistance. Therefore, herbicide rotation as well as mechanical control where possible is an important strategy in integrated weed management in citrus orchards. Allow adequate time for soil-residual herbicides to degrade before orchard removal and planting of new citrus or other crops. Refer to the appropriate herbicide label(s) for replant intervals to reduce risk to crop.
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, with winter and summer being most important, and keep records of your observations. Pay special attention to perennial weeds 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 prior to planting, survey the site for the weed species present and then disc the weeds under and level any irregularities in the grade.
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, spray with glyphosate; repeat the spray 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 preemergence herbicide can be incorporated over the entire site or into 4- to 6-foot-wide strips where the trees are planted. Herbicides like trifluralin (Treflan) can be disc-incorporated into the soil before planting the trees. This controls germinating grasses and some small-seeded broadleaf weeds to help reduce weed competition around newly planted trees. A given dosage of preemergence 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 sprays are often sprayed with postemergence herbicides or with shallow cultivation between rows. During summer, spot-treat summer annuals and perennials; such sprays are especially necessary in moist areas. Be careful to keep the spray off tender wood and foliage to reduce the risk of crop injury.
WEED MANAGEMENT IN NEWLY PLANTED ORCHARDS
Regular preemergence and postemergence sprays during the establishment years remove much of the competition by weeds and facilitate irrigation and other cultural practices. Refer to the Herbicide Treatment Table for a list of pre- and postemergence herbicides registered for use in newly planted citrus orchards.
Once trees are planted, disturb the soil as little as possible if you plan no-till management. 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. 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 4 x 4 square foot of fabric, 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
Mulching is a comon practice, mainly in the Coastal and Southern California region. 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.
- Use a layer of coarse organic material, about 4 to 6 inches thick, for the best weed control. Bark, greenwaste (residential yard trimmings), straw, and wood chips make good mulch.
- Apply organic mulch around newly planted trees over an area several feet wide. Take care to keep mulch thin near the 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.
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.
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 and can facilitate field operations under wet conditions. 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 numbers 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 microsprinkler irrigation scheme, the frequently wetted area around microsprinkler heads favors weed growth and promotes the breakdown of soil-residual herbicides.
Preemergence herbicides are used to control germinating weed seeds; most preemergence herbicides do not control established plants. Spray preemergence herbicides onto the soil just before an irrigation or rainfall, so that the water carries the herbicide into the soil zone where the weed seeds germinate. Preemergence herbicides can provide control for up to a year, depending on the herbicide's solubility, half-life, its adsorption to soil, weed species, and dosage applied.
Leaching from the soil is more extensive on sandy than on clay soils. Runoff can occur on sloping soils or soils high in clay. Prolonged moist conditions during heavy winter rains, in furrow bottoms, or around microsprinklers during irrigation favor breakdown and leaching of herbicides. Rimsulfuron (Matrix), oryzalin (Surflan), and mesotrione (Broadworks) are less likely to leach in sandier soils under frequent irrigation than bromacil (Krovar), simazine (Princep), or diuron (Karmex). Splitting an herbicide spray into two or more sequential applications can prolong the control provided by the herbicides.
Postemergence herbicides are used on emerged weeds and are more effective on young plants. They act either by contact or by translocation throughout the plant. Contact herbicides, such as paraquat (Gramoxone) or glufosinate (Rely 280), kill those parts of the plant that are actually sprayed, making thorough wetting of the weed foliage essential. A single spray normally kills susceptible annual weeds if they are young and tender; an additional spray or more is often necessary if they are large or droughty. Perennial weeds are not normally killed with a single contact-type spray, but require repeated treatments over time.
Systemic herbicides are transported inside the plant, where they move within the vascular tissue (xylem, phloem, or both) to kill plants. Complete coverage of weeds with systemic herbicides is not necessary. Glyphosate (Roundup) and sethoxydim (Poast) are examples of systemic herbicides used in citrus.
Tolerance and resistance to herbicides prevent some herbicides from controlling certain weeds. Tolerant plant species have a natural lack of susceptibility to certain herbicides. 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 (Poe annua), horseweed (Conyza canadensis), and hairy fleabane (Conyza bonariensis), and Palmer amaranth (Amaranthus palmeri) have developed resistance to glyphosate (the active ingredient in Roundup and similar products).
Minimize development of herbicide resistance by integrating multiple weed management methods such as herbicides, cultivation, mulching and hand-weeding. If herbicides are used, avoid repeatedly applying a single herbicide or herbicides with the same mode of action by rotating herbicide classes or tank-mixing with multiple herbicide classes. 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.
Repeated use of low rates of an herbicide can cause a shift in the weed population to plants that tolerate these low rates; careful monitoring is essential. Perennial or annual weeds not controlled by these low 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. Low-label rate or below-label rate applications may select for herbicide-resistant plants. Therefore, herbicide rotation within or between seasons is an important strategy in integrated weed management in citrus orchards. Allow adequate time for soil-residual herbicides to degrade before orchard removal and planting of new citrus or other crops. Refer to the appropriate herbicide label(s) for replant intervals to reduce risk to subsequent crop.
For more information on herbicide resistance, see Selection Pressure, Shifting Populations, and Herbicide Resistance and Tolerance, UC ANR Publication 8493.
For more information on monitoring and management of citrus weeds, see UC Ag Experts Talk: Management of Weeds in Citrus Orchards.