Integrated weed management involves the use of multiple strategies to manage weed populations in a manner that is economically and environmentally sound. Such strategies include cultural, mechanical, chemical, and biological methods. Peach orchards may be infested with a variety of annual and perennial weeds, each competing with the trees for water and nutrients. Competition for these resources is of greater concern with young trees because weeds can reduce their growth, vigor, and delay production.
Weeds also cause problems in older orchards because they can increase the risk of frost damage early in the season, harbor pests and pathogens, interfere with irrigation systems, compete with the trees for water and nutrients, and interfere with harvest.
Integrated weed management strategies vary from orchard to orchard and are influenced by location in the state, climatic conditions, soil texture and profile, irrigation practices, topography, cost, and grower preferences. There are several components to a good orchard weed management program. These include preventive strategies, orchard floor management, and weed monitoring. Further, proper use of pre- and postemergent herbicides and timely disking and cultivation are important factors in weed management.
Weeds are commonly controlled either chemically or mechanically in a 4- to 6-foot-wide strip in the tree row. Resident vegetation is generally permitted to grow in the areas between the tree rows, but it must be managed through repeated mowing, tillage, or chemical treatment. Mulches, subsurface irrigation, flamers, and feeding by geese or other animals can also be used to control weeds in orchards.
Preventing weeds from producing seed and preventing establishment of new weeds into the orchard are the most cost-effective methods of weed management. If possible, keep irrigation canals, ditch banks, and the irrigation systems free of weeds and weed seeds. A good drainage system is also essential as a preventive tactic. Fix leakages in the irrigation system and do not allow accumulation of water in low spots because moist sites encourage weed emergence and growth. Don't ignore weeds on the orchard margins because they produce seeds that disperse into the orchards. Therefore, control these weeds before they set seeds. When moving equipment from a weed-infested field, clean the undercarriage and tires of vehicles and equipment before entering the orchard because seeds and reproductive parts of weeds can be transported along with them.
Detection of new weeds and weeds that escaped previous control efforts is an important component of a weed management plan. For weed monitoring to be useful, you must correctly identify the weed species present, especially when they are in the seedling stage. It is easier to control annual weeds with chemical or mechanical tools when they are small and have not become established. If perennial weeds emerge from seed, control them with timely cultivation or effective herbicides before they produce reproductive structures. Established perennial weeds are most vulnerable to control during fall when they begin to go dormant and begin storing carbohydrates in their roots or reproductive structures for next year's growth. Herbicides applied at this stage can be translocated to the roots or rhizomes to better kill the weed.
Many herbicides are effective only against certain weed species. Regular monitoring will help to properly choose and time treatments. Follow-up monitoring allows you to assess if treatments are successful. Weeds often grow in patches and, therefore, it may not be necessary to spray postemergent herbicides or apply mechanical control in the whole orchard. A spot herbicide treatment or hand weeding may save time and money while achieving good weed control.
How to Monitor
Survey your orchard for weeds in late fall and again in late spring. Keep records on a survey form that includes a map. Pay particular attention to perennial weeds and other problem weeds and note their location on the map. Record weeds found in rows and middles separately. Weeds in tree rows must be managed, but annual weeds in row middles may be beneficial as a cover crop. Also keep records of weed management actions including timing, rates and dates of herbicide applications and cultivations. Survey information collected over a period of years tells you how weed populations may be changing and how effective your management operations have been over the long term.
Late fall weed survey.
Survey your orchard after the first rains of the fall when winter annuals have germinated. Monitoring weeds in fall will identify any summer species and perennial weeds that escaped the previous year's weed control program. Adjustments can be made to control these species in the next year. Fall monitoring will also identify any winter species that are emerging. Record your observations on the fall weed survey form and use the map to show areas of problem weeds.
Late spring weed survey.
Survey your orchard in late spring or early summer, after summer annuals have germinated. By surveying weeds at this time, you can identify any species that escape control from earlier management and know what perennials are present. If herbicides were used, monitoring identifies any need for changing to another herbicide. Pay particular attention to perennials and check for their regrowth a few weeks after cultivation. Record your observations on the late spring weed survey form and use the map to show areas of problem weeds.
ORCHARD FLOOR MANAGEMENT
A well-managed orchard floor cover between the tree rows has several benefits. It provides a stable surface upon which machinery can be operated under wet conditions that otherwise would prevent access to the orchard. The plants in the ground cover develop root channels that improve soil structure and water infiltration. Improved infiltration rates also reduce the risk of off-site movement of pesticides. Further, plant cover reduces soil compaction and the potential for erosion.
Although resident orchard-floor vegetation has several benefits, be sure that the vegetation does not invade the tree rows or it can result in a major problem, especially if the plants are difficult to control with herbicides. An example is hairy fleabane, which is difficult to control with the preemergent herbicides registered for peach plantings and is susceptible to postemergent sprays only when small. Also, its prolific production of wind-borne seed allows it to quickly invade tree rows. Other plants (e.g., burclover, curly dock, mustards, and certain clovers) host insect and mite pests such as lygus, Calocoris sp., thrips, and spider mites and are not good plants to maintain in the orchard floor vegetation.
Seeded Cover Crop
Planting a cover crop between the tree rows is an alternative to managing resident vegetation. Usually resident vegetation will produce less biomass (organic matter) than planted cover crops. Legume cover crops also fix nitrogen for tree use, often reducing the amount of commercial nitrogen fertilizer required. There are three main cover crop systems: winter green manure crops that are usually a mixture of legumes and large-seeded cereal grains seeded annually and typically cultivated in the spring for maximum nitrogen release; annual reseeding legumes and grasses planted initially in the fall and managed during spring and early summer in no-till orchards so they reestablish themselves in the fall by natural reseeding; and perennial sods planted in fall through spring in no-till orchards to provide year-round cover but require additional irrigation during the summer. Properly managed cover crops often reduce undesirable winter weeds and summer weeds depending on how the cover crop is managed. Tall cover crops or weeds, however, can increase the risk of frost damage in spring and should be mowed or disced appropriately based on cover crop system and species to reduce this risk. (For information on choosing and managing a cover crop, go to http://asi.ucdavis.edu/programs/sarep/research-initiatives/are/nutrient-mgmt/cover-crops or see Covercrops for California Agriculture, UC ANR Publication 21471 or Cover Crops for Walnut Orchards, UC ANR Publication 21627).
IRRIGATION SYSTEM CONSIDERATIONS
Design weed management programs so that they fit the irrigation system. For example, the dissipation of preemergent herbicides is slow in furrow and basin flood systems with raised berms because the irrigation water does not come in contact with the herbicide. However, in sprinkler, microsprinkler, and drip-irrigated orchards the irrigation water contacts the herbicide, thus increasing its dissipation.
Consideration of irrigation type is also important in selecting preemergent herbicides to prevent tree injury. Certain soil-residual herbicides, like diuron, norflurazon, and simazine, are prone to leaching in sandy-type soils that are frequently irrigated with low-volume sprinkler, mist, or drip irrigation. Under these conditions, these herbicides can leach into the tree root zone and cause injury or possibly leach into groundwater and contaminate it. Using these herbicides in orchards irrigated with furrow or basin flood irrigation would help reduce the likelihood of leaching and potential tree injury.
Weed control provided by the preemergent herbicides also breaks down sooner around sprinklers or emitters compared to the rest of the orchard. Areas around sprinklers and emitters require additional weed control measures, such as a postemergent herbicide or hand hoeing. For these treatments, using a sensor-controlled sprayer that applies herbicides only to the areas where weeds are growing, similar to a spot treatment, is a good choice because it can reduce herbicide use by 50% or more compared to a treatment where the entire orchard is treated.
SOIL TYPE CONSIDERATIONS
Consider the soil type in an orchard when selecting a weed management strategy. Sandy loams to loamy sands require less herbicide for effective weed control than clay loams. Labels for preemergent herbicides have specific application rates for different soil textures. Applying the rate of herbicide suggested for a clay loam soil to loamy sand not only wastes herbicide but may also cause crop injury.
Timing of cultivation is more flexible on loams and loamy sands than on soils high in clay because equipment moves easily through soil. Lighter soils are also generally easier to access for spraying and other operations during wet conditions than heavier soils.
WEED MANAGEMENT BEFORE PLANTING
It is easier and cheaper to control perennial weeds before planting the orchard than after, because there is a better selection of treatment options available when the ground is fallow. Established weeds can be controlled either chemically or mechanically. If the weeds are annuals, control them before they set seed by mowing, disking, or using herbicides. Perennial weeds are most effectively controlled with herbicides, or they can be mechanically controlled, suppressed by repeated diskings in summer, or controlled with a combination of the two techniques.
A good time to control perennial weeds such as dallisgrass, bermudagrass, field bindweed, nutsedge, and johnsongrass is the summer before planting. Apply glyphosate when these weeds are actively growing, and then cultivate 2 weeks after the herbicide is applied. Many underground plant structures can be controlled by cultivation alone, which brings them to the surface and causes them to desiccate, but the soil must be dry for root systems of the perennial plants to completely desiccate and die. Spring tooth harrows are especially effective in bringing underground parts to the surface for drying. Yellow nutsedge is easier to control with desiccation than purple nutsedge. Cultivation can actually spread perennial weeds if the root system isn't desiccated.
Many annual weeds can be effectively controlled by cultivating with a soil-inverting plow that buries seed into the soil profile where they desiccate or rot. Deep plowing of nutsedge provides suppression for about 6 to 8 weeks, but this approach is not effective for long-term control because purple nutsedge can emerge from depths of 18 inches or more.
Grading the Orchard
Grade a new orchard site to ensure even drainage and to eliminate low spots that tend to promote perennial weed growth. Also, proper drainage prevents formation of wet areas within the tree row. Constant wetting accelerates the dissipation of herbicides, which leads to weed growth.
Preparing Tree Rows
Although a preemergent herbicide such as trifluralin can be incorporated in the future tree row before planting, treated soil must not be placed around the roots at planting or tree injury may result. When planting the trees, place untreated soil directly around the roots and then cover them with a surface layer of treated soil. During the early years, maintain a weed-free strip that is at least 30 inches from trunk on each side of the tree to prevent weeds from competing with the developing tree. If planting holes are dug with an auger, use glyphosate before planting and then follow planting with an application of preemergent herbicide once the trees have settled into the soil.
WEED MANAGEMENT IN NEWLY PLANTED ORCHARDS
In orchards that have received an herbicide treatment, disturb the soil as little as possible once the trees are planted. In orchards that are furrow irrigated, establish one or two narrow furrows along the planted trees. Perennial grasses can be controlled with clethodim (Select Max), fluazifop-p-butyl (Fusilade), or sethoxydim (Poast). Glyphosate can be used to suppress nutsedges and perennial broadleaf weeds. Avoid spraying peach foliage or trunks with glyphosate. Plastic-coated wrappers may help to protect trunks from contact with herbicides, but there is no guarantee that injury will not occur. Regular preemergent and postemergent treatments during the establishment years remove much of the competition from weeds and facilitate irrigation and other cultural practices.
If herbicides were not applied before the trees were planted, weeds will need to be controlled. Cross-discing (cultivation both within and across the tree row) is an alternative to herbicide use, but be careful not to injure tree roots when discing near trees or suckering can result and cause long-term problems if herbicides will be used in the future to control weeds in the tree row. If mechanical control is used, additional control measures (hand hoeing or spot treatment with herbicides) will be needed for weeds growing adjacent to the trees that are not controlled with tillage operations.
WEED MANAGEMENT IN ESTABLISHED ORCHARDS
If vegetation (either resident vegetation or cover crop) has been maintained in the orchard middles, it can either be mechanically managed by mowing or chemically managed by applying low rates of a postemergent herbicide that stunt the plants. Though effective, the chemical mowing approach may eventually contribute to glyphosate resistance with some weed species. An alternative to mowing is to let the cover crop grow until it is nearly mature and then roll it with a ring-roller to press the vegetation down. This accelerates the senescence process but allows some seeds to mature. Also, the intact mulch blocks light that may prevent weed seeds from germinating. In early spring mow cover crops or resident vegetation to reduce the risk of spring frost damage.
Within the tree row, preemergent and postemergent herbicides are common management tools. For best results, most preemergent herbicides need to be sprayed onto the soil just before an irrigation or rainfall so that the water carries the chemical into the soil where the weed seeds are located. Check the pesticide label for specific application details. Some preemergent herbicides can provide control for up to a year, depending on the solubility of the material, adsorption of the material to soil, the weed species present, the dosage applied, and the amount of rainfall or irrigation that occurs. Herbicide leaching is greater on sandy than on clay soils. Prolonged, moist conditions during winter, in furrow bottoms, or around low-volume emitters during irrigation favor breakdown and leaching of herbicides. Because most preemergent herbicides do not last throughout the season and miss some weeds, postemergent herbicides are usually necessary to effectively manage orchard weeds.
Postemergent herbicides are used on established weeds. They act either by contact or by translocation throughout the plant. Contact herbicides, such as paraquat, kill those parts of the plant that are actually sprayed, making good coverage and wetting essential. A single spray kills susceptible annual weeds. Re-treatment is necessary for perennials or if annual weeds reestablish.
Translocated herbicides, such as glyphosate, move into the plant and are translocated to the underground portions of the plant and kill them. Glyphosate, however, does not translocate into mature nutsedge tubers. Complete coverage with translocated herbicides is not essential but does improve control. Complete control of established perennials is often difficult, because root structures are often extensive compared to the top growth. For optimum control of nutsedge, treat before the 5-leaf stage. Repeat applications are necessary.