Agricultural pest management
Weed Management in Organic Orchards
(Reviewed 8/17, updated 8/17)
Weed control in organically managed orchards requires special attention to prevent problems before they start. Any method that reduces the amount of weed seed in the orchard will diminish weed numbers over time. One of the best ways to prevent weed problems is to control existing weeds before they go to seed. It is usually best to use conventional tactics, including synthetic herbicides, for one or two years after planting; this helps reduce the weed seed bank and weed numbers, and makes weed control by organically-approved means less expensive later. However, this approach will require three more years of not using synthetic herbicides in the orchard for it to be certified as organic. If the site is not already certified organic, herbicides can be used until the transition time to organic begins, which can be very helpful in ridding the area of these hard-to-control perennial weeds.
It is essential to correctly identify the diversity of weeds infesting the orchard or planting site. Become familiar with each weed's growth and reproductive habits in order to choose the most effective management options. See the weed photos linked to the weeds in the list of COMMON AND SCIENTIFIC NAMES OF WEEDS.
Transitioning mature, full-canopied trees to organic production will require less intensive weed management than starting out as a new organic orchard. Mature, shady orchards often have limited weed growth, whereas weeds can more effectively compete with trees in newly planted orchards where there is more sunlight available to the weeds.
WEED MANAGEMENT BEFORE PLANTING
The season before trees are planted is a critical period for weed management, so young trees can become established with reduced competition from weeds. Two methods of managing weeds at this time are cultivation and soil solarization.
Repeating a cycle of irrigation followed by cultivation several times to germinate and destroy young weeds can reduce the amount of weed seed in the orchard soil. Cultivation works well with summer annuals but not as well with perennial weeds such as nutsedge, field bindweed, bermudagrass, and johnsongrass. If the site is not already certified organic, herbicides can be used until the transition time to organic begins. This can be very helpful in ridding the area of these hard-to-control perennial species. Or, if most of the weed seeds are located in the surface 4 inches of soil, a soil-inverting plow can be used to bury them deeply so that they cannot germinate or successfully emerge. Use a soil-inverting plow such as a Kverneland plow because a standard moldboard plow will not sufficiently invert the soil in most cases.
Soil solarization can significantly reduce weed numbers in the planned tree rows. For more information on solarization, see the Soil Solarization section above.
WEED MANAGEMENT AFTER PLANTING
Controlling weeds while they are small is important for the most effective organic weed control.
During the non-bearing years, mulch may be used to control weeds in the orchards. Maintain the mulch layer throughout the year. In-row mulches of woven fabric or a 4-inch layer of organic materials including compost, newspaper, straw, hay, and wood chips control weeds by preventing light penetration necessary for weed growth. The weed fabrics typically last 4 to 5 years. Since organic mulches may reduce the soil temperature slightly, it is often better to apply these materials when the trees have been in the ground for at least one full year to avoid the possibility of reduced tree growth. Both organic and fabric mulches may need to be removed when production starts, as they can interfere with harvest operations. Be sure to monitor for voles and gophers, as mulches provide cover for them and these vertebrate pests can be severely damaging to young trees. Once the trees are established, weeds in the tree row may be managed with shallow in-row cultivation, cross discing, cross mowing, hand hoeing, flaming, organically acceptable herbicides, or mulches. The choice of method depends in part on costs, tree spacing, the use of berms, orchard floor management practices, and the type of irrigation system.
In-row cultivators are equipped with a sensor or trigger mechanism that pivots the cutting arm around the tree to avoid injury. Several companies make cultivation equipment; those that have performed well include equipment from Bezzerides, Kimco, and L & H Manufacturing. They are more effective on smaller weeds.
Flaming can effectively manage in-row weeds that are smaller than eight leaves.
When flaming is used repeatedly, grasses will eventually become the dominant weeds because their growing points are at or below soil level and are not readily killed with flaming. Also, perennial weeds can be suppressed, but usually are not controlled with flaming.
Check with the organic licensing organization to determine current status and any use restrictions for organically acceptable herbicides. As with any contact herbicides, good coverage is essential. In most cases, a spray volume of at least 60 gallons per acre will be required when using these products.
Broadcast application of organic herbicides is usually not economical. However, organic herbicides are useful for spot treatments, particularly to control weeds in mulches, because this will help to preserve the mulch and increase its useful life span.
Before using any animals, check federal, state, and local food safety regulations and comply with them. Consult the University of Idaho and University of Missouri websites for further information on grazing animals.
Geese can be used to manage grass weeds in orchards. Geese prefer grass species and will only eat other weeds and crops after the grasses are gone. If confined, they will even dig up and eat johnsongrass and bermudagrass rhizomes, which they prefer. These grasses are otherwise difficult to manage in organic systems.
Young geese are best because they eat larger quantities of food, although having at least one older goose helps to protect the younger birds. Generally, about four geese per acre are needed. Provide geese with drinking water and shade. Protect them from dogs and other predators; portable fencing works well. Consult the Metzer Farms website for further information on geese.
Sheep and goats are sometimes used in organic orchards as well. Sheep will effectively remove all weeds down to ground level. Goats are browsers and must be carefully managed to avoid damage, especially to young trees. Both sheep and goats are generally used during the time when trees are dormant and the chance of grazing damage is minimal. Because of the need to maintain animal health and condition, grazing animals generally suppress weeds rather than fully control them. Use of animals for weed control in tree nut orchards raises some concerns about food borne illness and should be considered and managed carefully if used.
UC IPM Pest Management Guidelines: Walnut
J. A. Roncoroni, UC Cooperative Extension, Napa County
Acknowledgment for contributions to Weeds:K. K. Anderson, UC Cooperative Extension, Stanislaus County
C. L. Elmore, Weed and Plant Sciences, UC Davis
J. A. Grant, UC Cooperative Extension, San Joaquin County
W. O. Reil, UC Cooperative Extension, Yolo County
T. S. Prather, Plant, Soil, and Entomological Sciences, University of Idaho, Moscow, ID
A. Shrestha, UC IPM Program, Kearney Agricultural Research and Extension Center, Parlier
G. S. Sibbett, UC Cooperative Extension, Tulare County
J. J. Stapleton, UC IPM Program, Kearney Agricultur-al Research and Extension Center, Parlier