Integrated Weed Management

Integrated weed management is the use of multiple strategies to manage weeds in an economically and environmentally sound manner. These strategies include cultural, mechanical, chemical, and biological methods.

Weeds can cause a variety of problems for both young and mature orchards. They compete with cultivated trees for water, nutrients, and (when trees are smaller) light. The competition for these resources can reduce the growth, vigor, and productivity of trees, especially in young orchards.

Weeds continue to cause problems in established orchards because orchard floor vegetation can increase the risk of frost damage early in the season, harbor pests and pathogens, interfere with irrigation systems, compete for water and nutrients, and cause harvest problems.

Integrated weed management strategies vary from orchard to orchard, but the components of a weed management program include employing preventive strategies, orchard floor management, and weed monitoring. Location, climatic conditions, soil characteristics, topography, and grower preferences affect the tools and strategies which may be used and, in turn, influence decision making. Two factors of particular importance for developing a weed management program are the irrigation system and the orchard's soil texture. Further, proper use of pre- and postemergence herbicides, and timely mechanical operations such as discing and mowing are often important parts of a well-functioning integrated weed management system.

Weeds are commonly controlled either chemically or mechanically in a 6- to 12-foot-wide strip within the tree row. Resident vegetation is generally permitted to grow in the areas between the tree rows and later managed through mowing, tillage, or chemical treatment. Mulches, subsurface irrigation, flamers, and sheep or other grazing animals (such as geese) are other weed control options. Often several weed management choices are used in an orchard depending on the types of weeds present, the age of the trees, the soil conditions, and grower preference.

Glyphosate resistance is the main cause of difficult-to-control weeds in plum/prune orchards. Some populations of annual bluegrass, annual ryegrass, hairy fleabane, horseweed, Italian ryegrass, and junglerice have developed resistance to glyphosate and other herbicides in California.

To prevent the development of resistance, use a variety of weed control strategies, including cultural practices and alternating herbicides with different modes of action. Failure to do so can result in the rapid loss of herbicides as a 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 resistance. Consider scheduling fields with resistant ryegrass last for management operations, such as planting and harvesting. In addition, land planing or other practices that move surface soil within an orchard can also spread resistant weeds within a field.

Managers should be aware that weeds surviving herbicides labeled to control them could signal herbicide resistance. This is especially true if there's no clear reason like sprayer problems or misapplication. See Herbicide Resistance: Definition and Management Strategies, UC ANR Leaflet 8012 and Preventing and Managing Glyphosate-Resistant Weeds in Orchards and Vineyards, UC ANR Publication 8501, for suggestions on how to adjust management to avoid development of herbicide resistance.

PREVENTION

Preventing new weeds from establishing and preventing existing weeds from producing seed are the most cost-effective methods of weed management.

Keep irrigation canals, ditch banks, and irrigation systems free of weeds and weed seeds. Accumulation of water in low spots can encourage weed emergence and growth. Having good drainage and preventing irrigation leaks is essential in preventing the accumulation of water.

Control weeds on the orchard margins because they produce seeds that can disperse into the orchard. Weeds that produce wind-dispersed seeds, such as horseweed and hairy fleabane, may be a problem in field margins.

Clean the undercarriage and tires of vehicles and equipment like mowers before entering an orchard to prevent the transport of seeds and reproductive parts of weeds.

MONITORING

Regular monitoring is a critical component of an integrated weed management program. Detection of new weeds and weed escapes is essential for preventing weed establishment or shifts in weed populations. 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. It is easier to control both annual and perennial weeds with both chemical and mechanical tools when they are still small and not well established. Perennial weeds are more vulnerable to chemical control at the early bud stage or during fall as they start to go dormant. Systemic herbicides applied at these stages can be translocated to the roots or rhizomes to better kill the weed.

Most herbicides are only effective against a limited number of weed species. Regular monitoring will allow you to determine if your treatments are working.

Spot treatments 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 and problem weeds, and note their location on the map.

Record weeds found in rows and middles separately. Weeds within tree rows must be managed, but annual weeds in row middles (between the rows) may be beneficial as ground cover or for creating root channels for water infiltration. Also, keep records of weed management actions including timing, rates, and dates of herbicide applications, mowing, and cultivations. The survey information collected over a period of years can reveal how weed populations may be changing and how effective 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 identifies 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 also identifies any winter species that are emerging. Record your observations on the fall weed survey form and use the map to show areas with problem weeds.

Late spring weed survey

Survey your orchard in late spring or early summer, after summer annual weeds have emerged. By surveying weeds at this time, you can identify any species that escape control from earlier management and know what perennial weeds are present. If herbicides were used, monitoring identifies any need for switching to another herbicide. Pay particular attention to perennial weed species and check for their regrowth a few weeks after cultivation or herbicide application. Record your observations on the late spring weed survey form, and use the map to show areas of problem weeds.

ORCHARD FLOOR MANAGEMENT

Well-managed orchard-floor vegetation between the tree rows has several benefits. It provides a stable surface upon which machinery can be operated under wet conditions that would otherwise prevent access to the orchard. The cover plants can develop root channels that improve soil structure and water infiltration. Improvement in infiltration rates also reduces the risk of off-site movement of pesticides. Furthermore, 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 and should be mowed or disced to reduce this risk.

Although resident orchard-floor vegetation has several benefits, care should be taken to minimize its spread into the tree rows. Such spread can turn into a serious problem if the vegetation is difficult to control with herbicides. For example, hairy fleabane and threespike goosegrass are prolific seed producers and can quickly establish in tree rows. Additionally, these species are only susceptible to postemergence herbicides when they are small.

Planting a cover crop is an alternative to using resident vegetation between the tree rows. Choose a cover crop mix with known properties such as mowing frequency and height 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 and UC ANR Sustainable Agriculture Research and Education Program’s Cover Crops Database. Properly managed cover crops can reduce invasion of the orchard by weeds that cause problems but can require additional water beyond crop needs in some cases.

Management Based on Irrigation System

Design the weed management program to suit the irrigation system.

The breakdown of preemergence herbicides is slow in furrow and basin flood systems with berms because the irrigation water does not come in contact with the herbicide on the dry part of the berm.  Conversely, the preemergence herbicide breaks down more quickly in the furrows and basins where the soil is wet for longer periods of time. In sprinkler, microsprinkler, and drip-irrigated orchards, irrigation water contacts more of the herbicide, increasing the speed it breaks down in the regularly wetted area such as near the drippers. Consequently, weed control provided by the preemergence herbicide diminishes sooner around sprinklers or emitters than in the rest of the orchard. These areas require additional weed control measures, such as postemergence herbicides or hand hoeing. The use of sensor-controlled sprayers that apply herbicides only to areas of weed growth can reduce postemergence herbicide use by 50% or more by treating only the weed-infested areas.

If subsurface drip irrigation is used, free moisture near the soil surface can be limited and fewer summer annual weeds will germinate and establish if it does not rain. However, in finer-textured soils, when the water application rate exceeds the infiltration rate, water may still flow to surface and provide enough moisture for annual weed seeds to germinate. Cultivation, flaming, and mulches can all be used with a subsurface drip system.

Management Based on Soil Texture

Consider soil texture when developing a weed management strategy. Lighter-textured soil like sandy loams to loamy sands may require less herbicide for effective weed control than clay loams. Labels for some preemergence herbicides have specific application rate requirements or limitations for different soil textures.

Applying an herbicide at the rate for clay loam soil to a loamy sand may waste herbicide and can also increase the risk of crop injury. Cultivation timing is more flexible on soils with lower clay content because equipment is less likely to get stuck or compact the soil in wet conditions.

WEED MANAGEMENT BEFORE PLANTING

Perennial weeds are easiest to control before the orchard is planted because the most effective control options are available when the ground is fallow. Uncontrolled perennial weeds can cause problems for newly planted trees and become much more expensive to control later. Perennial weeds can be controlled mechanically by repeated discings in summer or controlled with herbicides. Annual weeds should be mowed or controlled so that they do not produce seed.

Control grassy perennial weeds such as bermudagrass, dallisgrass, and johnsongrass in the summer, before planting. Apply glyphosate or clethodim when they are actively growing and follow the application with tillage 2 weeks later.

Many weeds with underground plant structures, including nutsedges, can be effectively controlled by cultivating with soil-inverting plows. If the soil is completely dry, underground structures brought to the surface by plowing will dry out and die. Nutsedge tubers must be buried at least 10 inches below the soil surface and allowed to rot before inverting the soil again.

Grade the orchard site to ensure even drainage. Low spots that retain moisture within the orchard promote perennial weed growth that can be difficult to control. Proper drainage also prevents formation of wet areas within the tree row that can be detrimental to tree health. Constant wetting or long periods of excess moisture accelerates the breakdown of herbicides which can lead to weed growth in the low spots.

To control weeds in the future tree rows, incorporate a preemergence herbicide like trifluralin into the soil before planting. When planting trees, place untreated soil directly around the roots and then cover that with a surface layer of treated soil. Treated soil must not be placed around the roots at planting or tree injury may result.

If planting holes are dug with an auger, use glyphosate before planting and then follow planting with an application of preemergence herbicide registered for use in young trees.

WEED MANAGEMENT IN NEWLY PLANTED ORCHARDS

Weeds can significantly reduce young tree growth if not managed. 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 with the developing tree during the early years. Weeds in the tree rows of young orchards also provide shelter for damaging rodent pests such as voles and gophers. Once the trees are planted, disturb the soil as little as possible.

Preemergence herbicides can control weeds and may help control costs where repeated applications of postemergence herbicides would be needed for adequate weed control. Check with your PCA and read the label carefully as only a limited number of preemergence herbicides are registered for use in newly planted plum or prune orchards. These include pendimethalin, isoxaben, oxyfluorfen, and oryzalin. The efficacy of these herbicides varies depending on the specific weeds targeted.

Multiple postemergent herbicide options are available for use in newly planted plum/prune orchards. Many offer control of specific weed types. Perennial grasses can be controlled with clethodim, sethoxydim, or fluazifop-P-butyl. Glyphosate can be used to suppress nutsedges and perennial broadleaf weeds. Glufosinate is effective on glyphosate-resistant hairy fleabane. Avoid spraying trunks or foliage because these herbicides can also cause crop injury. Wrappers or cartons can help to protect trunks from herbicides, but injury may still occur if the spray reaches uncovered tissue. White paint, diluted half and half with water, does not adequately protect trunks from herbicide injury. Regular preemergence and postemergence herbicide applications during the establishment years remove much of the competition from weeds, improve irrigation, facilitate management of rodent pests, and reduce interference with cultural orchard management practices.

If herbicides were not applied before planting, weeds will need to be controlled by discing or herbicide application. Cross-discing (cultivation both within and across the tree row) is an alternative to herbicide use, especially when trees are planted flat (no berms or mounds). Be careful not to injure tree roots when discing near trees which can cause suckering. Suckering can result in long-term problems for future use of herbicides to control weeds in the tree row. Sucker foliage can absorb herbicides that can then translocate to different parts of the tree causing damage.  This is especially important for trees planted on Marianna 2624 rootstock and a few other plum rootstocks that are highly prone to suckering.

WEED MANAGEMENT IN ESTABLISHED ORCHARDS

If resident vegetation or a cover crop is maintained in orchard middles, it can be managed with mowing or cultivation. An alternative to mowing is to roll the annual cover crop with a ring-roller when it is nearly mature. This presses the cover crop down and accelerates the senescence process but allows for some seeds to mature. The pressed-down vegetation also acts as mulch, blocking light, which can prevent some weed seeds from germinating.

To reduce the risk of early spring frost damage to the crop, mow or disc cover crops and resident vegetation. Tall vegetation prevents the soil from reradiating heat during the night, so maintain a vegetation height of below 2 inches if frost is in the forecast.

Use preemergence and postemergence herbicides for weed management within tree rows. For best results, most preemergence herbicides need to be applied to the soil just before irrigating or rainfall. The water carries the herbicide into the soil where the weed seeds are. Check the product label for specific details.

Preemergence herbicides can provide control for up to a year, depending on the solubility of the herbicide, the binding of the herbicide to soil, weed species present, the application rate, and the amount of rainfall or irrigation. Herbicide leaching risk varies among herbicide chemistries but is also influenced by soil type with greater leaching potential in sandy or coarse-textured 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 weeds when they are visible and act either by contact or by translocation throughout the plant.

Contact herbicides, such as paraquat, glufosinate, or carfentrazone, kill only the parts of the plants that are directly exposed to the herbicide, making good coverage and wetting essential. A single application usually controls small susceptible annual weeds. However, repeated application often is necessary for established perennial weeds that can regrow from underground structures, or if annual weeds regrow.

Systemic herbicides, such as glyphosate, move throughout the plant including to underground organs such as roots and tubers and often provide the best control of established perennial weeds. Glyphosate, however, does not translocate well into mature nutsedge tubers. Although complete coverage with systemic herbicides is not as essential as with contact herbicides, good coverage often results in more reliable herbicide performance. Complete control of established perennial weeds is often difficult with single applications, as roots and other underground structures are often extensive compared to the top growth.