Integrated weed management is the use of multiple strategies including cultural, mechanical, chemical, and biological methods to manage weed populations in a manner that is economically and environmentally sound. Nectarine 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 are also a problem in older orchards where 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 impede harvest operations.
Integrated weed management strategies vary from orchard to orchard and are influenced by geographic location, climatic conditions, soil texture and profile, irrigation practices, topography, cost, and grower preferences. A good orchard weed management program is composed of preventive strategies, orchard floor management, and weed monitoring. The proper use of pre- and postemergence herbicides and timely discing 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 must be managed through repeated mowing, tillage, or chemical treatment. Mulches, subsurface irrigation, flamers, and grazing by animals can also be used to control weeds in orchards. Check local regulations to determine how long before harvest the animals must be removed.
Preventing the establishment of new weeds and existing weeds from producing seed are the most cost-effective methods of weed management. Keep the irrigation system, canals, and ditch banks free of weeds and weed seeds. A good drainage system is essential. Fix leakages in the irrigation system. Do not allow accumulation of water in low spots; this will encourage weed emergence and growth. Control weeds on the orchard margins before they produce seeds that disperse into the orchard. When moving equipment from a weed-infested field clean the undercarriage and tires of vehicles and equipment before entering the orchard because seeds and other reproductive parts of weeds can be transported along with them.
Detection of new weeds and those that escaped previous control efforts is an important component of a weed management plan. Correct identification of weed species, especially in the seedling stage is essential. Annual weeds are easier to control with chemical or mechanical tools when they are small and have not become established. If perennial weeds emerge from seed, control them with cultivation or herbicides before they produce reproductive structures. Established perennial weeds are most vulnerable to control during fall when they begin to store carbohydrates in their roots or reproductive structures in preparation for dormancy. Herbicides applied at this stage will translocate to the roots or rhizomes and provide more effective control than at other times during the year.
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 postemergence herbicides or apply mechanical control in the whole orchard. The use of hand weeding or a spot treatment of herbicides 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. Record and map infestations, paying particular attention to perennial 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. Keep records of weed management actions including timing, rates and dates of herbicide applications, and cultivations. Survey information collected over a period of years will show the spectrum of weeds present and help determine the most effective control strategy.
Late Fall Weed Survey
Survey your orchard in fall after the first rains when winter annuals have begun to germinate. 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 weed species next year. Fall monitoring will also identify any winter species that are emerging. Record your observations on the fall-weed survey form 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 if there is a need to change 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 show areas of problem weeds.
Orchard Floor Management
Benefits of having a well-managed orchard floor cover between the tree rows include: timely access to the orchard for operations under wet conditions that otherwise would be prevented, as well as better soil structure and water infiltration as a result of the root channels created by the plants that cover the ground (resident vegetation or planted cover crop). Improved water infiltration reduces the risk of off-site movement of pesticides, soil compaction, and the potential for erosion.
Resident orchard-floor vegetation has several benefits, but the vegetation should not be allowed to invade tree rows. Not all resident plants are desirable growing in the orchard floor; certain difficult-to-control plants can result in major problems. An example is hairy fleabane, which is not readily controlled with the preemergence herbicides that are registered for nectarine plantings and is only susceptible to postemergence sprays when small. It is a prolific producer of wind-borne seeds that allow it to quickly invade tree rows. Other plants that should be avoided as resident vegetation, such as burclover, curly dock, mustards and certain clovers, can host insect and mite pests, such as lygus, Calocoris spp., thrips, and spider mites.
Irrigation System Considerations
Design weed management programs so that they fit your irrigation system. In furrow and basin flood systems with raised berms, the dissipation of preemergence herbicides is slow because the irrigation water does not come in contact with the herbicide. Dissipation is increased in sprinkler, microsprinkler, and drip-irrigated orchards where the irrigation water contacts the herbicide.
Irrigation type is an important consideration in selecting preemergence herbicides to prevent tree injury. Certain soil-residual herbicides, such as norflurazon, and simazine, have been found to leach in sandy-type soils that are irrigated frequently with low-volume sprinkler, mist, or drip irrigation. Under these conditions, they can leach into the tree root zone and cause injury and possibly leach into groundwater, causing contamination. 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 preemergence herbicides breaks down sooner around sprinklers or emitters when compared to the rest of the orchard. These areas require additional weed control measures, such as a postemergence herbicide or hand hoeing. The use of a sensor-controlled sprayer that applies herbicides only to the areas where weeds are growing may be a good choice, because it can reduce herbicide use by more than 50% when compared to a treatment to the entire orchard.
Soil Type Considerations2>
Soil type is an important consideration when selecting an orchard weed management strategy. Sandy loam to loamy sand soils require less herbicide for effective weed control than clay loam soils. Preemergence herbicide labels have specific application rates for different soil textures. Applying the suggested rate of herbicide for a clay loam soil to loamy sand not only wastes herbicide but may also cause crop injury.
Because equipment moves easily through loam and loamy sand soils, the timing of cultivation is more flexible than on soils high in clay. Lighter soils are also generally easier to access for spraying and other operations during wet conditions than are heavier soils.
Weed Management Before Planting
It is easier and cheaper to control perennial weeds before planting the orchard because there is a better selection of treatment options available when the ground is fallow. Annual weeds are best controlled before they set seed by mowing, discing, or using herbicides. Established weeds can be controlled either chemically or mechanically.
A good time to control perennial weeds such as bermudagrass and field bindweed is in the late summer or early fall. Apply glyphosate when the weeds are at about 20 to 30% flowering (if done after this, seed may have already developed in some flowers). For perennial grasses (johnsongrass or dallisgrass) or nutsedge, apply glyphosate when these weeds are actively growing and then cultivate 2 weeks later. For glyphosate to work properly, the plants must not be under severe water stress.
Many underground plant structures can be controlled by cultivation alone. A spring tooth harrow is especially effective. This operation brings root and reproductive structures to the surface and causes them to desiccate. For this system to work, the soil must be dry. Not all weeds are created equally: purple nutsedge is harder to control with desiccation than is yellow nutsedge. If this operation is done incorrectly or at the wrong time, cultivation can actually spread perennial weeds.
Cultivating with a soil-inverting plow so that seed is buried in the soil profile and desiccates or rots can effectively control many annual weeds. Deep plowing of nutsedge can provide suppression for 6 to 8 weeks. This method is not effective for long-term control of purple nutsedge because the plants can emerge from soil depths of 18 inches, or more.
Grading the Orchard
Grade a new orchard site to ensure even drainage and eliminate low spots that promote perennial weed growth. Proper drainage prevents formation of wet areas within the tree row. Constant wetting accelerates the dissipation of herbicides, which may also lead to weed growth.
Preparing Tree Rows
Before planting, a preemergence herbicide such as trifluralin can be incorporated into the area designated as the tree row. Tree injury may result if the treated soil is placed around the roots at planting. When planting, place untreated soil directly around the roots and then cover them 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 apply a preemergence herbicide once the trees have settled into the soil.
Weed Management in Newly Planted Orchards
Once the trees are planted, disturb the soil as little as possible in orchards that have been treated with a preemergence herbicide. In orchards that are to be 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 nectarine 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. During tree establishment regular preemergence and postemergence treatments will remove much of the competition from weeds and facilitate irrigation and other cultural practices.
Weeds will need to be controlled if herbicides were not applied before the trees were planted. Cross-discing (cultivation both within and across the tree row) is an alternative to herbicide use, but care must be taken not to injure tree roots when discing near trees or suckering can result. This may cause problems in the future if herbicides will be used to control weeds in the tree row. 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
Vegetation (either resident vegetation or cover crop) can be maintained in the orchard middles and managed by mechanical or chemical mowing (i.e., applying low rates of a postemergence herbicide to reduce plant growth). Chemical mowing can be effective but may contribute to resistance with some weed species. Using a ring-roller to press down the vegetation of a nearly mature mulch cover crop is an alternative to mowing. This accelerates the senescence process while allowing some seeds to mature. Leaving the mulch intact blocks light and may prevent weed seeds from germinating. In early spring, mow cover crops or resident vegetation to reduce the risk of spring frost damage.
Preemergence and postemergence herbicides are common management tools used within the tree row. Most preemergence herbicides should be applied onto the soil just before a light (¼ inch) irrigation or rainfall to be incorporated into the soil. Do not apply the herbicide if a heavy rain (more than ½ inch in a short period) is anticipated. Check the pesticide label for specific application details. Some preemergence herbicides can provide control for up to a year, depending on their solubility, adsorption to soil, weed species present, 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. Most preemergence herbicides do not last throughout the season and do not control all weeds. The application of postemergence herbicides is usually necessary to effectively manage weeds in the orchard.
Postemergence herbicides are used on established weeds. They act by contact or by translocation throughout the plant. Contact herbicides, such as paraquat, kill only those parts of the plant that they come in contact with. Good spray coverage and wetting to spread the herbicide are essential. A single application can kill susceptible annual weeds. Retreatment will be necessary if more seeds germinate. Perennial plants that regrow from underground roots or other underground structures will need to be re-treated.
If applied at the correct timing, systemic herbicides such as glyphosate will move into the plant and be translocated to the underground portions of the plant and kill them. Glyphosate will not translocate into mature nutsedge tubers. For optimum control of nutsedge, treat before the 5-leaf stage. Repeat applications are necessary. Complete coverage with translocated herbicides is not essential but may improve control. Complete control of established perennials is often difficult, because root structures are often much more extensive than the top growth.