Pest Management Guidelines
Integrated Weed Management
(Reviewed 5/06, updated 4/09)
Integrated weed management involves using all available strategies to manage weed populations in an economically and environmentally sound manner. Such strategies include cultural, mechanical, chemical, and biological methods. Plum orchards are infested with numerous annual and perennial weeds. These weeds compete with plum trees for water, nutrients, and light (in young orchards). The competition for these resources is of greater concern in young orchards because weeds can reduce the growth, vigor, and productivity of the trees. Weeds continue to cause problems in older orchards because they can increase the risk of frost damage early in the season, harbor pests and pathogens of the plum trees, interfere with irrigation systems, compete for water and nutrients, and cause problems at harvest. There are several components to a good orchard weed management program. These include preventive strategies, orchard floor management, and weed monitoring. It is equally important to design a weed management program based on the irrigation system and soil type of the orchard. Further, proper use of pre- and postemergent herbicides and timely discing and cultivation is necessary for a well-functioning integrated weed management system.
Integrated weed management strategies vary from orchard to orchard. Location in the state, climatic conditions, soils, irrigation practices, topography, and grower preferences influence orchard floor management decisions and the tools used. Weeds are commonly controlled either chemically or mechanically in a 4- to 6-foot-wide strip in the tree row. The area between the tree rows may be chemically treated, mechanically mowed, or tilled. Mulches, subsurface irrigation, flamers, and geese can also be used to control weeds in orchards. Often several weed management options are used in an orchard depending on the types of weeds present, age of the trees, soil conditions, and grower preference.
A good weed management strategy in plum orchards begins with prevention. Prevention is the most effective method of weed management. Keep irrigation canals, ditch banks, and the irrigation system free of weeds and weed seeds. A good drainage system is also essential as a preventive tactic. Leakages in the irrigation system and accumulation of water in low spots should be prevented because such sites can encourage weed emergence and growth. Also control weeds on the orchard margins because they produce seeds that may disperse into the orchard. Weeds that produce wind-dispersed seeds (e.g., marestail, hairy fleabane) may be a greater concern in field margins. Be sure to control weeds in the field margins before they set seeds. Clean the undercarriage and tires of vehicles and equipments before entering the orchards because seeds and reproductive parts of weeds can be transported along with them.
Detection of new weeds and weed escapes is essential for preventing weed establishment or shifts in weed populations. Regular monitoring is a very important component of an integrated weed management program. For weed monitoring to be effective, the weed species present in and around the orchard must be properly identified. Identify weeds in the seedling stage because it is easier to control annual weeds with chemical or mechanical tools when they are small and have not become established. Perennial weeds are more vulnerable to control at the early bud stage or during fall when the plants begin to go dormant. Herbicides applied at these stages can be translocated to the roots or rhizomes to better kill the weed.
Most herbicides are only effective against certain weed species. Regular monitoring will determine if your treatments are working. Weeds often grow in patches so it may not be necessary to spray postemergent herbicides or cultivate the whole orchard to manage them. A spot treatment may save time and money while achieving good weed control. A handheld GPS unit is useful for marking patches of troublesome weeds for spot treatment and subsequent monitoring.
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.
Survey your orchard after the first rains of the fall when winter annuals have germinated. Monitoring weeds in fall accomplishes several tasks. It 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.
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 cover between the tree rows has several benefits. It provides a stable surface upon which machinery can be operated under moist conditions that otherwise would be prevented access to the orchard. The cover plants develop root channels that improve soil structure and water infiltration. Improvement in infiltration rates also reduces the risk of off-site movement of pesticides. Further, plant cover reduces soil compaction and the potential for erosion. On the other hand, tall cover crops or weeds increase the risk of frost damage in spring. Mowing or discing the orchard floor, however, will reduce this risk.
Although resident orchard-floor vegetation has several benefits, it must not be allowed to invade the tree rows. Invasion into the tree rows is a serious problem if the invading plants are difficult to control with herbicides. For example, hairy fleabane, which is difficult to control with the preemergent herbicides registered in plums and is susceptible to postemergent sprays only when small, is a prolific production of wind-borne seed that allow it to quickly invade tree rows. Planting a cover crop instead of using resident vegetation between the tree rows is an alternative. Choose a cover crop mix with known properties such as mowing height and frequency requirements, time to seed set, and time to senescence. (For more information on choosing a cover crop, consult UC ANR Publication 21471, Covercrops for California Agriculture). Properly managed cover crops can prevent invasion of the orchard by weeds that cause problems.
Management Based on Irrigation System
Weed management programs must be adjusted to fit the irrigation system. For example, the dissipation of preemergent herbicides is slow in furrow and basin flood systems with 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. Consequently, weed control provided by the preemergent herbicides breaks down sooner around the sprinklers or emitters than in the rest of the orchard, and these areas require additional weed control measures, such as a postemergent herbicides or hand hoeing. The use of sensor-controlled sprayers that apply herbicides only to areas of weed growth can reduce postemergent herbicide use by 50% or more by treating only the weed-infested areas.
Management Based on Soil Type
Consider soil type when selecting a weed management strategy. Sandy loams to loamy sands require less herbicide for effective weed control than clay loams. Labels for some preemergent herbicides have specific application rates for different soil textures. Applying an amount 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 more easily over these soils when they are moist than over moist clay soils.
WEED MANAGEMENT BEFORE PLANTING
Perennial weeds are easiest to control before the orchard is planted. Perennial weeds not controlled before planting can cause problems for newly planted trees and are much more expensive to control. Established weeds can be controlled either chemically or mechanically. Annual weeds should also be clipped or controlled so that they do not produce seeds. Perennial weeds can be mechanically controlled by repeated discings in summer or controlled with herbicides. A good time to control weeds such as dallisgrass, bermudagrass, and johnsongrass is the summer before planting. Apply glyphosate when the grasses are actively growing. This should be followed with cultivation 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. The soil must be dry for root systems of the perennial plants to completely desiccate and die. Many other weeds, including nutsedges, can be effectively controlled by cultivating with soil-inverting plows.
Grade the orchard site to ensure even drainage. Low spots within the orchard promote perennial weed growth that can be difficult to control. Also, proper drainage prevents formation of wet areas within the tree row. Constant wetting accelerates the dissipation of herbicides, which leads to weed growth.
To control weeds in future tree rows, incorporate a preemergent herbicide like trifluralin into the soil before planting. When planting the trees, place untreated soil directly around the roots and then cover it with a surface layer of treated soil. 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.
WEED MANAGEMENT IN NEWLY PLANTED ORCHARDS
Weeds can significantly reduce young tree growth if not managed/controlled. Once trees are planted, disturb the soil as little as possible. 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 can be used to suppress nutsedges and perennial broadleaf weeds. Avoid spraying plum foliage or trunks with glyphosate. Wrappers may help to protect trunks from 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.
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. 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 for some seeds to mature. In addition, the intact mulch blocks light, which may prevent weed seeds from germinating. Manage cover crops and resident vegetation by mowing or discing to reduce the risk of early spring frost damage to the crop.
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 details. 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, and the dosage applied. 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.
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 if perennials that regrow from underground roots or other underground structures are present 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, as root structures are often extensive compared to the top growth.
UC IPM Pest Management Guidelines: Plum
WeedsA. Shrestha, Kearney Agricultural Center, Parlier
W. T. Lanini, Weed Science/Plant Sciences, UC Davis
F. J. A. Niederholzer, UC Cooperative Extension, Sutter-Yuba counties
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