Pest Management Guidelines
Integrated Weed Management
(Reviewed 12/07, updated 12/07)
Weeds can cause a multitude of problems in walnut orchards if they are not well managed. First and foremost, they reduce the growth of young walnut trees as they compete for water, nutrients, and rooting area. Weeds also increase water use and pest problems, and may enhance the potential for disease. They make it more difficult to recover nuts from the orchard floor, increase management time and costs, and generally cause problems for the grower. Weeds must be kept away from the trunk because field mice (voles) infest weedy orchards and can damage trees by chewing away the bark and girdling trees, resulting in tree death or reduced growth and production. Crown rot in trees can also be a problem when weeds are allowed to remain around the trunks; and weeds that are allowed to grow around the base of the tree become a fire hazard if they dry out. Weeds must be controlled around the trunks, preferably without disks or other mechanical control that may cut roots or hit the trunks or main roots and cause wounding. These wounds are often an entry point for crown or root pathogens. Crown gall is of particular concern for Paradox rootstock. Weeds must also be controlled around sprinkler heads or they can impede water distribution and disrupt the normal application pattern.
Walnut orchards may benefit from plants growing between the tree rows or if they are carefully managed. These plants, in a well-maintained ground cover, can help increase water infiltration, reduce soil compaction, reduce soil erosion on sloping orchards, maintain soil organic matter content, and cool the orchard. Cover crops that include legumes also add nitrogen to the soil.
Planting a cover crop between the rows can reduce weed growth. The additional water and fertilizer requirements of the cover crop must be compensated for, however, or reduced tree growth can occur. Annual cover crops can be grown to maturity in years when there is sufficient rainfall, then removed before there is a need for additional irrigation. In drought years, the cover crop may not yield enough biomass to be useful. A sod cover will allow a firm base for equipment movement through the orchard. If there is danger of frost in spring, however, it should be mowed or eliminated with tillage or herbicides. Some cover crops will reseed themselves while others will need to be reseeded each fall.
To control weeds, herbicides are generally used only in the tree row. This reduces the total amount of herbicide needed and also prevents the surface roots in the tree row from being damaged by cultivation equipment. Most weed species in walnut orchards can be controlled by one or more of the registered herbicides. Preemergent herbicides are used to control weeds before they emerge. These herbicides can be applied in fall after harvest, split into two applications (fall and spring), or applied in the late winter with a postemergent herbicide. Most preemergent herbicides require rain or sprinkler irrigation to move them into the soil. If the orchard does not have sprinklers, time the application so it is followed by rain. Postemergent herbicides control weeds already growing in the orchard and should be applied whenever monitoring indicates a need. They may be combined with a fall or spring preemergent treatment or applied as spot treatments during the growing season.
Soil characteristics of the orchard have a marked effect on the weed spectrum, the number of cultivations and irrigations required, and the residual activity of herbicides. Sandy, light-textured soils require less preemergent herbicide for weed control, but residual control may be shorter than in clay or clay loam soils. Use low herbicide rates on sandy soils or those low in organic matter. Heavy soils take longer than sandy loam soils to dry out enough for effective cultivation and also require higher rates of preemergent herbicides for effective control.
The method of irrigation and the amount of irrigation or rainfall also affects chemical selection and the residual control of weeds. In nonirrigated orchards, the earlier weeds are controlled in spring the more water that is conserved and available to the trees. In irrigated orchards, heavy irrigation promotes faster herbicide degradation in the soil than light applications of water. In drip-irrigated orchards, degradation is most rapid around drip emitters, but in the dry areas between emitters weeds are less of a problem than in orchards with other types of irrigation. Preemergent herbicides can be incorporated with sprinkler irrigation but not with drip or furrow irrigation. Flood irrigation will only provide uniform incorporation of herbicides when the water distribution is uniform. Even when distribution is uniform, more water is usually applied by flood irrigation than is needed for herbicide incorporation; in sandy soils excess irrigation water may move the herbicide deeper in the soil than is desired for optimum weed control.
Before using any herbicide, monitor the orchard to determine what weed species need to be controlled and follow label directions carefully, especially information on soil texture and herbicide rates.
Detection of new weeds and weeds that escaped previous control efforts is an important component of a weed management plan. For weed monitoring to be useful, you must 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. Established perennial weeds are most vulnerable to control during fall when they begin to go dormant and begin storing carbohydrates in their roots or reproductive structures for next year's growth. This generally begins with the onset of flowering (1% bloom) for many species. Herbicides applied at this stage can be 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 allows you to assess if treatments are successful. Weeds often grow in patches and, therefore, it may not be necessary to spray postemergent 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. Keep records on a survey form that includes a map. 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 populations 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 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 be made to control these species in the next year. Fall monitoring will also identify any winter species that are emerging. Keep records of your observations and use the map to show areas of problem weeds (example weed survey form—
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. Keep records of your observations and use the map to show areas of problem weeds (example weed survey form—
Before planting trees, control as many weeds as possible on the orchard site, especially perennials. Perennials are not easy to control but there are more tools available to use before trees are planted than after. These include the use of nonselective herbicides and cultivation combined with drying the soil and soil solarization. A common preplant treatment to control perennial weeds such as dallisgrass, bermudagrass, and johnsongrass is to apply glyphosate (Roundup) in summer when the weeds are growing vigorously and then follow in 2 weeks with cultivation. If the soil and plant material can be dried after treatment, increased control is achieved. Field bindweed can be suppressed, but not eradicated with this method.
Bermudagrass and johnsongrass can also be controlled without the use of herbicides by cultivating the soil when the soil is dry. Cultivation will cut the rhizomes into small pieces so they will dry out. Rework the soil to pull new rhizomes to the surface and dry them out as well. If the soil is irrigated or rain occurs before total control of the perennial plant is achieved, the effectiveness of this practice will be reduced. If the soil is worked when wet, it will increase the population of these weeds because each piece of cut rhizome can root and develop a new plant. Even if all of the perennial parts of the weeds (i.e., rhizomes, stolons, etc.) are controlled, the seedlings of the plants must also be controlled when irrigation or rainfall occurs or these perennials will reestablish.
If irrigation is feasible, an especially effective method of annual weed control before planting trees is to cultivate, followed by an irrigation to germinate new weeds and then a shallow cultivation to destroy the weeds. This reduces the weed seed population in the soil, thus reducing weed growth. At least two cycles of cultivation, irrigation, followed by shallow cultivation must be carried out to markedly reduce new weed seedlings. This method is not effective on established perennial weeds, but can be on perennial seedlings if the time between irrigation and cultivation is less than one month.
Most annual weeds can be controlled in a strip down the tree row by using the selective preemergent herbicide trifluralin (Treflan) and mechanically incorporating it into the soil. When planting trees, be careful not to mix herbicide-treated soil into the planting hole or stunting can result.
Soil solarization. Soil solarization can significantly reduce weed populations in the planned tree rows. Soil solarization traps the sun's energy beneath a layer of clear plastic, increasing the temperature in the top 6 inches of soil to levels lethal to many weed seedlings as well as vegetative structures of perennial weeds. In areas like the Central Valley, soil solarization can heat the soil to more than 130°F at a 3-inch depth. The effect of solarization diminishes at greater depths and it does not control perennials as well as annuals. Seedlings of bermudagrass, johnsongrass, and field bindweed are controlled but not older plants. Yellow nutsedge is partially controlled while purple nutsedge is not significantly affected.
Effective soil solarization begins with preparing a smooth seed bed so that the plastic can be placed as close as possible to the soil surface. Disc to break up clods and then smooth the soil. Remove any material that will puncture or raise the plastic sheets such as rocks and weeds.
Irrigate before or after applying the plastic because wet soil conducts heat better than dry soil; in addition moist seeds are easier to kill than dry, hard seeds. Cover the soil with plastic as soon as possible after irrigating. It is possible to irrigate after laying the plastic by installing the drip system or the microsprinkler line (with only the spaghetti tubing) before planting. Furrow irrigation under the plastic is another option. (If the entire site is irrigated, weed growth will occur in the untarped centers and will be difficult to control without disturbing the plastic.) After irrigation, allow the soil to dry somewhat to avoid compaction by heavy equipment.
Use clear plastic that is 1.5 to 2 mils thick and impregnated with UV inhibitors to prevent premature breakdown of the material. Contact plastic suppliers well in advance so they can formulate plastic tailored to your needs. Cover the planned tree row with plastic from 6 to 10 feet wide. The width depends on the middles management program planned for the orchard. Bury the sides of the plastic to create a seal on the soil; this also helps prevent the plastic from being blown away by wind. Machines that lay down the plastic are available to automate the process.
Black plastic suppresses weed seed germination but will not heat the soil sufficiently for solarization. Black plastic can be used as a mulch to suppress nutsedges or common purslane.
In the Central Valley, the plastic should be in place from June through August and can remain in place until planting begins. Solarization may not be as effective in cooler coastal areas. In these areas, apply plastic in August and September or May and June. Cultivate solarized soil less than 3 inches deep to avoid bringing viable weed seeds to the surface where they germinate. For additional information on soil solarization, see Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds, UC ANR Publication 21377.
Competition from weeds is most severe during the first 6 to 8 years after planting; during this stage of orchard development, keep the area around the base of trees weed free. Regardless of the method used to remove weeds, be careful not to injure trees with chemicals or to mechanically damage the trunk or roots. As the orchard becomes established, competition from weeds is lessened as shade from the tree canopy reduces weed growth.
Weeds growing directly around the bases of trees can be controlled by a number of methods. A selective preemergent herbicide can be applied in a strip down the tree row or around the tree as soon as the soil has settled following planting. Do not let the spray contact tree leaves or the bark of trees less than 3 years old. Trees are most sensitive to herbicides when they are young. In orchards planted to Paradox rootstocks, the use of herbicides instead of mechanical control reduces the danger of wounding young trees with a disk or hoe and creating an entry for crown gall bacteria.
The area around young trees may also be hand hoed until the trees are 3 to 4 years old, when a swing mower can be used; mow or disc the weeds between the tree rows. Other mechanical tools available include disks, weed knives, cultivators, and rotary tillers. It is best to hoe when the weeds are a few inches tall; hoeing becomes difficult when weeds are allowed to get larger. Another alternative is the use of synthetic mulches made of polyethylene, polypropylene, or polyester around the base of trees to discourage weed growth; the weeds between the tree rows can be mowed or disced.
Once the orchard is established, the area around the base of the tree should continue to be kept weed free. By removing weeds from around the base of the tree, weed competition and the potential for rodent damage are reduced.
In walnut orchards, weeds are commonly controlled either mechanically or chemically in a 4- to 8-foot-wide strip down the tree row. Mechanical methods of weed control include hoeing or using weed knives in the row and cultivating between rows. Mechanical cultivators, such as a weed badger, will be effective if used on loose soil that does not contain rocks. These practices need to be done frequently when weeds are small to keep weeds from seeding and to reduce competition. If weeds are allowed to mature, not only do the plants become a fire hazard, but more importantly, they produce enough seeds for many years of weeds. One danger of cultivation is wounding the crowns, which may provide an entry for crown gall bacteria.
Young weeds growing at the bases of trees can also be controlled using a flamer; however, if the weeds have dried out or leaves have accumulated at the tree base, they may burn and girdle the young trees. Grasses are not controlled as well as broadleaf weeds with this method.
In orchards where weeds in the tree row are controlled with herbicides, the herbicides are commonly applied in strips 2 to 4 feet wide on either side of the tree down the tree row. The strip width selected is often coordinated with a disk or mower width for working between tree rows. The width of the strip can be reduced in older orchards to reduce herbicide use. The area between the tree rows is either mechanically or chemically mowed, tilled, or in a few instances, chemically treated. If weeds between the tree rows are controlled by mechanical mowing, use a flail or rotary mower. Mow before bloom if possible to minimize potential for frost damage. Mowing will normally need to be repeated every 4 to 6 weeks with up to eight mowings during the season depending upon weed pressure, species, and the length of season.
For additional information, see Orchard Floor Management to Reduce Erosion and Protect Water Quality, UC ANR Publication 8202.
Cover crops. In recent years there has been increasing use of cover crops in orchards. Two primary reasons for planting a cover crop are to enhance soil quality by adding organic matter and increasing soil nitrogen with legume cover crops. Other benefits cover crops can provide include:
Plant the cover crop in the area between tree rows. Resident vegetation in walnut orchards often does not usually grow uniformly enough to compete well with newly invading weeds. Planted cover crops generally compete better with invasive weeds and thus reduce weed infestations in the orchards over time. Plan for the additional water needs of the cover crop so that it does not compete with trees for available water, or, in the case of dryland orchards, plow under the cover crop in spring to maximize the amount of water available to the tree.
An annual cover crop that reseeds itself will compete against weeds and reduce the potential for problems in the future. To ensure success, plant cover crops in walnut soon after harvest, before leaves fall. Rainfall or irrigation water is needed for good germination and seedling growth.
Newly established cover crops may be seriously damaged by fall and winter orchard traffic during operations such as pruning, brush removal, chipping and shredding, and spraying. In orchards where these operations are planned, cover crops may be seeded in alternate middles and these operations carried out in the nonseeded middles. Or, plant cover crops in years when these operations are not planned for the orchard.
If there is a potential for frost and the cover crop is tall, mow once before bloom to minimize frost damage; the cover crop will regrow and flower later in the season. However, the cover crop will be most competitive if mowing can be delayed until after it flowers. After most species in the cover crop have produced seed, mow or roll it using a ringroller. The ringroller will allow more seed production and also create a surface mulch that will shade the soil, preventing germination of weed seeds. Close mowing before harvest will break the cover crop into small pieces to accelerate decomposition. For more information on choosing a cover crop, how and when to plant, and suggestions of cover crop mixes, see Cover Cropping in Walnut Orchards ANR Publication No. 21627.
Herbicides. The safest herbicides to use around a tree in an established orchard are preemergent herbicides. Postemergent herbicides may be used either alone or combined with a preemergent herbicide to kill the existing weeds plus give residual control of seedlings. Postemergent herbicides are also used to spot treat perennial weeds during the growing season. Spray postemergent herbicides only on weed foliage and directed to the base of trees.
Herbicide application equipment is set up to apply a given rate of material to the soil or young growing weeds. If leaves are present at the time a preemergent herbicide is to be applied, blow them off the soil just before spraying so the herbicide is sprayed on the soil and not on the leaves. If the herbicide is sprayed on leaves, wind may blow them out of the tree row and poor weed control will result. To apply herbicides in strips, a sprayer should be equipped with a short boom with low pressure flat fan nozzles. An off-center nozzle is often used on the end of the boom to apply the material in the tree row. This keeps the sprayer away from the tree. To avoid drift and maximize uniformity of application, keep the boom as low as possible to still maintain uniform application, and use low pressure (below 30 psi).
For spot treatments of small areas, use a backpack sprayer or a low volume applicator. When using any sprayer, and especially when using a low volume applicator, be very careful not to spray a material like glyphosate (Roundup, Touchdown) on the bark of young trees or let the material drift onto mature walnut leaves. Damage can last for more than one year and severely affect young tree growth.
UC IPM Pest Management Guidelines: Walnut
Acknowledgment for contributions to Weeds:
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