Integrated weed management involves the use of multiple strategies to manage weed populations in an economically and environmentally sound manner. Such strategies include cultural, mechanical, chemical, and biological methods. Apricot 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 newly planted 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 in some cases, 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 postemergence herbicides, timely discing and cultivation, and resident vegetation or cover crop management in the middles 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. However, it must be managed through repeated mowing, tillage, or chemical treatment. Mulches (synthetic and organic), 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 in 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 may disperse into the orchards. It is important to control these weeds before they set seeds.
- 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.
- 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.
- Manage established perennial weeds during fall when they begin to go dormant and store carbohydrates in their roots or reproductive structures for next year's growth. Systemic herbicides, like glyphosate, applied at this stage are 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 shortly after treatments are applied allows you to assess if treatments were successful. Weeds often grow in patches and, therefore, it may not be necessary to spray postemergence herbicides or apply mechanical control in the whole orchard. A spot treatment 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. It is also very helpful to survey fields shortly following treatment to determine effectiveness of the treatment.
- Keep records on a survey form that includes a map of the orchard.
- 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 species and numbers 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 emerged. Monitoring weeds in fall accomplishes several tasks. It will identify any summer species and perennial weeds that escaped the previous or current 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. For more detailed information, see UC ANR Publication 8202, Orchard Floor Management Practices to Reduce Erosion and Protect Water Quality.
Although maintaining resident vegetation on the orchard floor 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 preemergence herbicides registered for apricot plantings and is susceptible to postemergence sprays only when treated at a very young stage of growth. Also, its prolific production of wind-borne seed allows it to quickly invade tree rows. If perennial weeds invade the middles over time, alternative methods (including repeated discing) will be required for management.
Seeded cover crop
Planting a cover crop between the tree rows is an alternative to managing resident vegetation. Choose a cover crop mix with known properties such as mowing height and frequency, time to seed set, and time to senescence. (For information on choosing a cover crop, see Covercrops for California Agriculture, UC ANR Publication 21471). Properly managed cover crops can prevent invasion of the orchard by weeds that cause problems. Tall cover crops or weeds, however, increase the risk of frost damage in spring and should be mowed or disced to reduce this risk.
IRRIGATION SYSTEM CONSIDERATIONS
Design your weed management program so that it fits the irrigation system. For example, the dissipation of preemergence 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 preemergence herbicides to prevent tree injury. A soil-residual herbicide, like norflurazon is prone to leaching in sandy-type soils that are frequently irrigated with low-volume sprinkler, mist, or drip irrigation. Under these conditions, this herbicide can leach into the tree root zone and cause injury or leach into groundwater and contaminate it. Treatment with this or similar products in orchards irrigated with furrow or basin flood irrigation would help reduce the likelihood of leaching and potential tree injury.
Weed control provided by preemergence 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 postemergence 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 preemergence 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 can be moved through more easily in lighter soils. 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 are better treatment options available when the ground is fallow. Established weeds can be controlled chemically, mechanically, or with a combination of methods. If the weeds are annuals, control them before they set seed by mowing, discing, or using herbicides. Perennial weeds can be mechanically controlled by repeated discings in summer, controlled with herbicides, or controlled with a combination of the two techniques.
A good time to control perennial weeds such as dallisgrass, bermudagrass, and johnsongrass is the summer before planting. Apply glyphosate when the grasses 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 these plant parts 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. Cultivation can actually spread perennial weeds if the root system isn't desiccated. Many other weeds, including nutsedges, can be effectively controlled by cultivating with a soil-inverting plow that buries the underground tubers or nutlets at least 10 inches deep into the soil profile, where they may rot.
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 the preemergence herbicides trifluralin and pendimethalin can be incorporated in the 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 preemergence 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) in non-bearing orchards, while fluazifop-p-butyl (Fusilade DX), and sethoxydim (Poast) can be used in bearing and non-bearing orchards. Glyphosate can be used to suppress nutsedges and perennial broadleaf weeds. Avoid spraying apricot foliage or trunks with glyphosate. Plastic-coated wrappers may help to protect trunks from coming into contact with herbicides, but there is no guarantee that injury will not occur. Regular preemergence and postemergence 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 postemergence 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 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, preemergence and postemergence herbicides are common 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 moves the chemical into the soil and activates it at the depth where the weed seeds are located. Check the pesticide label for specific application details. 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, the dosage applied, and the amount of rainfall or irrigation that occurs after treatment.
Herbicide leaching is greater in sandy soils than in clay soils. To reduce the likelihood of herbicide leaching in sandy soils,
- split preemergence herbicide applications in early and late winter, or
- use a single application in late winter before the last predicted rainfall period.
Prolonged moist conditions during winter favor the breakdown and leaching of herbicides in furrow bottoms, or around low-volume emitters during irrigation.
Postemergence herbicides are used on seedling or established weeds. They act either by contact or by translocation throughout the plant. Contact herbicides, such as paraquat or carfentrazone, kill those parts of the plant that are actually sprayed, making good coverage and wetting essential. A single spray kills susceptible annual weeds as long as they are small. Retreatment 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 through the plant 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, particularly in some weed species that are hairy and woody (like hairy fleabane). Complete control of established perennials is often difficult, because root structures are often extensive compared to the top growth. It may be necessary to combine postemergence herbicides with different modes-of-action to control a wider array of weeds and reduce the likelihood of weed resistance.