Weeds Pages
Integrated weed management ideally 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.
Pear 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 delay production and reduce tree growth and vigor. Weeds cause problems in older orchards because they can increase the risk of frost damage early in the season, harbor pests and pathogens, and interfere with irrigation systems, and 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 preemergence and postemergence herbicides and timely 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 grazing by geese or other animals can also be used to control weeds in orchards.
See also Chapter 13, Orchard Floor Management, in the Pear Production and Handling Manual.
Prevention
- Preventing weeds from producing seed and eliminating the establishment of new weeds into the orchard are the most cost-effective methods of weed management.
- Keep irrigation canals, ditch banks, and the irrigation systems free of weeds and weed seeds. Also keep the areas immediately around these areas weed-free.
- Eliminate low spots in the orchard and maintain drainage systems to prevent standing water. Fix leaking irrigation systems. Moist sites encourage weed emergence and growth.
- Pay attention to weeds growing on the orchard margins; they should be controlled before they produce seeds that can disperse into the orchard.
- Clean the undercarriage and tires of vehicles and equipment before they enter the orchard to prevent seeds and reproductive parts of weeds from being transported from infested sites.
Monitoring
Detecting new weeds and weeds that escaped previous control efforts is an important component of an effective weed management plan. Correctly identify the weed species present, especially in the seedling stage. Become familiar with each weed's growth and reproductive habits in order to choose the most effective management options. See the weed photos in the Common and Scientific Names of Weeds section or use the University of California Weed Research and Information Center's Weed Identification Tool.
Monitor regularly to help choose the proper timing and treatments; most herbicides are effective against many, but not all, weed species. After application, assess if treatments were successful. Weeds often grow in patches, making postemergence herbicide applications or mechanical control to the whole orchard unnecessary. Spot-treating may save time and money while achieving acceptable weed control.
How to monitor
- Survey the orchard for weeds in late fall and again in late spring.
- Keep records, including a map.
- Pay particular attention to perennial and other problem weed species. Note their location on the map.
- Record weeds found in rows and middles separately. Weeds in tree rows must be managed, but some low-growing, non-aggressive 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 indicates how weed populations may be changing and how effective your long-term management operations have been.
Late-fall weed survey
Survey the orchard after the first fall rains after winter annual weeds 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 then be made to control these species the next year. Fall monitoring will also identify any winter species that are emerging. Record your observations (example weed survey form—).
Late-spring weed survey
Survey the orchard in late spring or early summer, after summer annuals have germinated. Surveying weeds at this time enables you to identify any species that escaped control from earlier management or perennial weed species present. If herbicides were used, late-spring monitoring can help identify whether to change to another herbicide. Pay particular attention to perennial and annual species you suspect may be resistant to herbicides (see Special Weed Problems). Check for weed regrowth a few weeks after cultivation. Record your observations (example weed survey form—).
Orchard Floor Management
A well-managed orchard floor cover between the tree rows has several benefits. It provides a stable surface for machinery to operate 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 additional information on orchard floor management, see Orchard Floor Management Practices to Reduce Erosion and Protect Water Quality.
Resident vegetation
Resident orchard-floor vegetation has several benefits if it does not include species that invade the tree rows or are difficult to control with herbicides. An example of an unacceptable ground cover is hairy fleabane, a prolific, wind-borne seed producer that is susceptible to postemergence herbicides only when small and is difficult to control with the preemergence herbicides registered for pear plantings.
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.) Properly managed cover crops can help prevent problem weed species from invading the orchard. Tall cover crops or weeds, however, increase the risk of frost damage and fruit russeting in spring and should be closely mowed in the Sacramento Valley or even eliminated using herbicides or discing on the North Coast.
Irrigation System Considerations
Design weed management programs so that they fit 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-treated soil on the berm. However, in sprinkler, microsprinkler, and drip-irrigated orchards the irrigation water contacts the herbicide, thus increasing its dissipation rate.
Weed control provided by preemergence herbicides also breaks down faster around sprinkler heads or drip emitters than in the rest of the orchard. Areas around sprinkler heads and microsprinkler or drip emitters require additional weed-control measures, such as a postemergence herbicide application or hand hoeing. A sensor-controlled sprayer that applies herbicides only to the areas where weeds are growing, similar to a spot treatment, can reduce herbicide use by 50% or more compared to a postemergence treatment to the entire orchard floor.
Soil Type Considerations
Consider soil type when selecting a weed management strategy. Sandy loams to loamy sands require lower rates of preemergence herbicides for effective weed control compared to clay loam soils. Labels for many preemergence herbicides have specific application rates for specific soil textures. Applying the rate of herbicide suggested for a clay loam soil to loamy sand not only wastes herbicide but may also lead to crop injury and contamination of ground or surface water.
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 generally easier to access for spraying and other field 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. Control annual weeds before they set seed by mowing, discing, or using herbicides. Depending on the species, perennial weeds can be controlled mechanically by discing repeatedly during the summer, using herbicides, or a combination of the two techniques.
The ideal time to control perennial grasses such as dallisgrass, bermudagrass, and johnsongrass is the summer before planting. Apply glyphosate when the grasses are actively growing, then cultivate 2 weeks later. If weeds resume growth after cultivation, consider a second broadcast or spot treatment with glyphosate or the grass-specific herbicides sethoxydim, fluazifop-p-butyl, or clethodim. Cultivation alone can bring many underground plant structures to the surface and cause them to desiccate. For this method to kill the perennial plants, the soil must be dry for the root systems to completely desiccate and die. If the soil is not dry enough, cultivation can have the unintended effect of spreading perennial weeds, such as field bindweed. Many other perennial weeds, including nutsedge, can be effectively controlled by cultivating with a soil-inverting plow that buries the underground tubers or nutlets deep into the soil profile where they desiccate or 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 a preemergence herbicide such as pendimethalin 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 of tree establishment, maintain a weed-free strip that is at least 30 inches from the trunk on each side of the tree to prevent weeds from competing for water and nutrients 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 soil has settled around the trees.
Weed Management in Newly Planted Orchards
In orchards that have received an herbicide treatment prior to planting, 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, fluazifop-p-butyl, or sethoxydim. Glyphosate can be used to suppress nutsedge and perennial broadleaf weeds. Avoid spraying pear foliage or trunks with glyphosate.
Plastic-coated wrappers may help to protect trunks from contact with herbicides. Care must be taken to prevent sunburn inside the wrappers, especially on trees from cold storage planted in late spring. If planting late, use tree wrappers designed to reduce the inside temperatures and paint the trunk all the way to the ground with white interior latex paint diluted with twice as much water (1:2 paint:water ratio).
Regular preemergence and postemergence treatments during the establishment years, especially during the active growing months between April and November, remove much of the competition from weeds and facilitate efficient irrigation and other cultural practices. Fabric mulches used in tree rows can eliminate the need to treat around young trees, thus reducing the risk of phytotoxicity.
Cross-discing (cultivation both within and across the tree row) is an alternative to herbicide use in some orchards. However, discing near trees can injure tree roots and lead to excessive suckering. Excessive suckers can result in phytotoxicity problems if systemic herbicides are applied to the tree row in the future. 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.
Weed Management in Established Groves
Vegetation (either resident vegetation or cover crop) maintained in the orchard middles can be managed by mowing. Where there is risk of frost damage, closely mow cover crops or resident vegetation in early spring. An alternative to mowing is to let the cover crop grow until it is nearly mature and then roll it with a ringroller to press the vegetation down. This accelerates the senescence process but allows some seeds to mature. The intact mulch blocks light and may help prevent weed seeds from germinating. Vegetation can also be managed chemically by applying low rates of a postemergence herbicide to stunt the plants (chemical mowing). If not well managed, this practice can lead to an orchard with predominantly herbicide-resistant weeds.
It is easier to control annual weeds with chemical or mechanical tools when they are small and not yet established. Control perennial weeds that emerge from seed with timely cultivation or effective herbicides before they produce reproductive structures, which can occur in as little as one month under ideal growing conditions. Established perennial weeds such as field bindweed are generally most vulnerable to control during early flowering when they start storing carbohydrates in their roots or reproductive structures for next year's growth. Systemic herbicides applied at this stage are translocated to the roots or rhizomes to better kill the weed. It is also important to apply systemic herbicides to vigorously growing plants as herbicide uptake and translocation is reduced in stressed plants.
Preemergence and postemergence herbicides are common management tools used within the tree row. For best results, most preemergence herbicides need to be sprayed onto the soil just before irrigation or rainfall so that the water carries the chemical into the soil where the weed seeds are located. Apply when 0.25 to 0.5 inch of precipitation is expected; do not apply if 1 inch or more is expected in a short period because excessive leaching or runoff of the herbicide may occur. Check the pesticide label for specific application details. Preemergence herbicides can provide control for 6 to 9 months, depending on the solubility of the material, adsorption of the material to soil particles, the weed species present, dosage applied, and amount of rainfall or irrigation that occurs. Herbicide leaching is greater on sandy than clay soils. Prolonged moist conditions during winter, in furrow bottoms, or around low-volume emitters during irrigation favor the breakdown and leaching of herbicides.
Postemergence herbicides are used on established weeds. They can have either contact activity or be translocated 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 application should kill susceptible annual weeds. Re-treatment may be necessary if coverage is poor, annual weeds reestablish, or for suppression of perennial weeds that regrow from roots or other underground reproductive structures.
Translocated herbicides, such as glyphosate, move into the plant and are translocated to the growing points, killing actively growing tissue above or below ground. 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 perennial weeds is often difficult, because their root structures are often extensive compared to the top growth. Regular applications over several years may be needed to achieve satisfactory control.