Pest Management Guidelines
Integrated Weed Management
(Reviewed 3/09, updated 3/09)
Integrated weed management involves the use of all available strategies to manage weed populations in a manner that is economically and environmentally sound. Such strategies include cultural, mechanical, and chemical methods. Almond orchards may be infested with a variety of annual and perennial weeds. These weeds compete with almond trees for water and nutrients. Competition for these resources is of greater concern with young trees than older established ones, because weeds can reduce the growth, vigor, and productivity of young trees. Weeds not only use water, but they also interfere with the distribution of water by disrupting sprinkler spray patterns. Weeds that are still growing at harvest can cause problems with nut drying and gathering the crop from the orchard floor. Keep weeds away from the base of almond trees to avoid providing cover for small mammals that can damage tree trunks.
Almond orchards can benefit from a well-managed vegetative orchard floor cover that is limited to the area between the tree rows. The vegetative cover can provide a stable surface upon which machinery can be operated under moist conditions. The plants on the orchard floor develop root channels that improve soil structure and water infiltration. Better water 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 vegetation makes a good cover, some plants may invade the tree row. Invasion of the tree row by resident vegetation is a serious problem if these plants are difficult to control with herbicides. An example is hairy fleabane, which is difficult to control with the preemergence herbicides registered for almonds and is susceptible to postemergence sprays only when small. The prolific production of windborne seeds allows this weed to quickly invade tree rows.
Planting a cover crop in the area between the orchard rows is an alternative to using resident vegetation. Choose a cover crop mix with known properties, such as mowing height and frequency requirements, time to seed set, and time to senescence. (For information on choosing a cover crop, see Covercrops for California Agriculture. Properly managed cover crops can prevent invasion of the orchard by weeds that cause problems. The cost and management of a cover crop should be considered before deciding on a suitable cover.
Weed management programs must be compatible with the irrigation system. Preemergence herbicides dissipate slowly in furrow and basin flood systems that use berms in the tree row because the irrigation water does not come in contact with the herbicide. In sprinkler, microsprinkler, and drip-irrigated orchards, the irrigation water contacts the herbicide, thus increasing its dissipation. Consequently, weed control breaks down sooner around the sprinklers or emitters than in the rest of the orchard. Areas around sprinklers and emitters require additional weed control measures, such as a postemergence herbicide application or removal by hand. There are sensor-controlled sprayers that can be used to apply herbicides automatically to areas of weed growth, similar to a spot treatment. By treating only weeds in infested areas, these sprayers reduce postemergence herbicide use by at least 50%.
Soil type must also be considered when selecting a weed management strategy. Sandy loams to loamy sands require less herbicide for effectiveness than clay loams. Labels for preemergence herbicides have specific application rates for different soil textures. Applying an amount of herbicide suggested for a loam soil to a loamy sand not only wastes herbicide but may 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 the soil.
Monitoring for weeds is best done in fall and in late spring. Fall monitoring for weeds accomplishes two tasks. First, it will identify any summer species that escaped the current year's weed control program and adjustments can be made to control these species in the next year. Second, fall monitoring will identify winter species that are emerging.
Weed growth is directly influenced by rainfall or postharvest irrigation. Moisture in September and October induces germination of weeds at a time of year when temperatures are still relatively high. Weed growth is accelerated by the higher temperatures. If a postharvest irrigation is not applied and rain does not fall until November, then weed growth will not be as rapid because temperatures are generally lower. Early rains require that cultivation and flaming begin earlier. Early rains also mean a postemergence herbicide may have to be added to the preemergence herbicide that is applied in winter. The rate of postemergence herbicide required depends on the size of the weeds at application time. A good fall monitoring program will help ensure that the appropriate materials are applied at the most effective rates and timing.
Late spring monitoring identifies any species that escape control from earlier control practices. If herbicides were used, monitoring identifies any need for changing to another herbicide to control weeds that weren't controlled earlier. Use the susceptibility charts in this guideline to determine alternate herbicides to control the weeds that escaped previous herbicide treatments. Weed species that are not controlled by herbicides listed as effective against these weeds should be a warning to the manager: if there is no obvious pattern that could be attributed to sprayer malfunction, then herbicide resistance could be developing. See Herbicide Resistance: Definition and Management Strategies, for suggestions on how to adjust management to avoid development of herbicide resistance.
WEED MANAGEMENT BEFORE PLANTING
Control perennial weeds before planting. 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; follow this with a 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 in order for root systems of the perennial plants to completely desiccate and die. Many other weeds, including nutsedges, can be effectively controlled by cultivating with a plow such as the Kverneland, which is specifically designed to invert the soil profile to the plow depth.
Grade the orchard site to ensure an even drainage. Low spots within the orchard promote perennial weed growth that can be difficult to control and will cause continuing problems. Also, proper drainage keeps puddles from forming within the tree row. Puddles accelerate the dissipation of herbicides, which leads to weed growth that competes with the young trees.
To control weeds in the future tree row, incorporate a preemergence 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 at least 30 inches from the 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 preemergence herbicide.
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. Glyphosate suppresses nutsedges and perennial broadleaf weeds. Avoid spraying almond foliage or trunks with glyphosate. Wrappers help to protect trunks from herbicides but do not guarantee that injury will not occur. Regular preemergence and postemergence treatments during the establishment years remove much of the competition by weeds and facilitate irrigation and other cultural practices.
WEED MANAGEMENT IN ESTABLISHED ORCHARDS
Treating the tree row with herbicides and maintaining vegetation in the orchard middles are common practices. The vegetation consists of either resident plants or a planted cover crop, typically one that reseeds itself. The vegetation is mowed mechanically or treated with low rates of postemergence herbicide that stunt the plants rather than killing them. An alternative to mowing is to let a 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 seed maturation. In addition, the intact mulch shades the soil and may prevent some weed seeds from germinating. This mulch usually degrades by harvest, but if it hasn't degraded at least 3 weeks before harvest, a close mowing with a flail mower will chop the cover crop into pieces that will easily degrade.
Within the tree row, preemergence and postemergence herbicides are common chemical management tools. For best results, most preemergence 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 germinate. Preemergence 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. Leaching from the soil is more extensive 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.
Postemergence 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 themselves from seeds. 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 always necessary, but poor coverage will result in poor weed control.
Herbicide resistance is the inherited ability of weeds to survive and grow at herbicide dosages many times greater than usually needed for control of that species. The potential risk for the development of herbicide resistance is greatest when the same herbicide is used repeatedly, as is often done in orchards. To prevent the development of herbicide resistance, use a variety of weed-control strategies, including cultural practices and alternating herbicides with different modes of action. Failure to do this can result in the rapid loss of herbicides as an effective pest management tool, although cultivation remains an option. If resistant populations are observed, avoid moving resistant weeds from one field to another by cleaning equipment before moving out of a field with known herbicide-resistant weeds. Consider scheduling known resistant fields as the last ones to be harvested. Some populations of horseweed (Conyza canadensis), annual or Italian ryegrass (Lolium multiflorum), junglerice (Echinochloa colona), and hairy fleabane (Conyza bonariensis) have developed resistance to glyphosate in California.
The first step in preventing herbicide resistance is early detection. Be on the lookout when monitoring for patterns that indicate resistance. These include patches of dense weed population with less dense populations radiating out from the central patch, and escapes scattered in no particular pattern throughout the field.
Prevention and management
Important weed management strategies to help prevent the development of resistance to herbicides in orchards include:
If horseweed and hairy fleabane are already growing in the orchard, either treat them with a postemergence herbicide or use mechanical cultivation before they get larger than 18 to 21 leaves. In established orchards, use 2, 4-D or glufosinate (Rely) to control these early stages. Glyphosate formulations will also work well for the non-resistant horseweeds if rates are 1 to 2 lb a.i./acre. Mixing 2,4-D or glufosinate with glyphosate will improve control if resistance is suspected. Similarly, glyphosate-resistant ryegrass can be controlled with alternative herbicides such as paraquat (Gramoxone Inteon) or glufosinate. Closely monitor following treatment to assess its effectiveness.
UC IPM Pest Management Guidelines: Almond
WeedsA. Shrestha, Kearney Agricultural Center, Parlier
J. H. Connell, UC Cooperative Extension, Butte County
Acknowledgment for contributions to Weeds:
T. S. Prather, Department of Plant, Soil, and Entomological Sciences (PSES), University of Idaho