Pest Management Guidelines
Weed Management in Organic Orchards
(Reviewed 3/09, updated 11/12)
Weed control in organically managed orchards requires special attention to preventing weed problems before they start. Cover crops planted in middles and mechanical control of weeds in the tree row are key components of an organic weed management program.
The season before trees are planted is critical, so young trees can establish with a reduced number of plant competitors. A summer fallow treatment of irrigation followed by tillage and then drying can reduce weed seed numbers in the soil. Repeat this cycle several times to further deplete seeds in the soil. If most of the weed seeds are located in the surface 4 inches, a soil-inverting plow can be used to bury them to depths from where they cannot emerge. A Kverneland plow can be used, but a standard moldboard plow does not sufficiently invert the soil.
Soil solarization of the planned tree row can also significantly reduce weed populations. The soil must be moist and the width of the solarized area should be at least 6 feet. All sides of the plastic must be buried 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.
Solarization must be done during summer and should be started at least by the beginning of August to have sufficient time to complete the process. Clear plastic or a plastic with a coating that suppresses seed germination can be used. Black plastic suppresses seed germination but will not heat the soil to the same temperature as clear plastic. Suppression is important for species that emerge through plastic; consequently, black plastic may be more effective than clear plastic if nutsedges or common purslane are significant problems on the site.
Similar to many conventionally managed orchards, weeds in the middles of organic orchards are commonly managed with cover crops and mowing. However, weeds in the tree row must be managed with in-row cultivation, cross discing, hand weeding, flaming, or with organically approved herbicides such as Matran or Weed Zap. These products are nonselective and provide broad-spectrum weed control but won't control all weed species present.
The choice of method depends in part on the type of irrigation system. Weed fabrics are not practical for almonds because nuts must be swept from the orchard floor and the sweeping may dislodge the weed fabric.
Furrow-irrigated orchards are amenable to in-row cultivation or mowing. Several companies make cultivation equipment; those that have performed well include equipment from Bezzerides, Kimco, and L&H Manufacturing. Cross-discing of young trees also is possible but requires furrowing after each discing.
In-row cultivators move in and out of the tree row to control weeds. The cultivator moves away from the tree using a sensing mechanism (normally mechanical) that triggers movement of the cultivator away from the trees. In-row cultivation may be possible if extra protection is provided to the sprinkler to ensure proper operation of the trigger mechanism on the cultivator so that the cultivator moves away from the sprinkler as it does for the tree.
Few options are available in organic production. In-row cultivators may damage irrigation lines and emitters. Hand weeding and possibly flaming could be used for weed control. Flaming may be effective on weeds that are typically smaller than 8 leaves. When flaming is used repeatedly, grasses will eventually become the dominant weeds because their growing points are close to the ground. Also, perennial weeds are not controlled with flaming. Protect the trunks of young trees from flamers to avoid injury to the cambium layer of the tree.
If resident vegetation does not grow uniformly enough to compete well with newly-invading weeds, consider planting a cover crop in the area between the tree rows. An annual cover crop, such as sub-clovers that reseed themselves, will compete against weeds, but may also require occasional removal of invading weeds to maintain a uniform cover crop stand. Mowing once before almond bloom to reduce frost hazard and eliminate flowering competition during almond pollination is possible. The cover crop will regrow and flower later in the season. The cover crop will be most competitive if mowing can be avoided. However, cover crops that are mowed infrequently in order to reseed also provide excellent cover for gophers. Gopher populations frequently build up in cover-cropped orchards, and during harvest they have nothing to feed on except tree roots. It is imperative that gopher control is maintained, regardless of the middles management system employed, but particularly if cover crops are utilized.
After most species in the cover crop have produced seed, it should be mowed or rolled using a ringroller. The ringroller will allow more seed production, it will also create a surface mulch that will shade the soil, preventing germination of weed seeds. Close mowing before harvest will break the cover crop residue into small pieces, which accelerates decomposition.
UC IPM Pest Management Guidelines: Almond
WeedsA. Shrestha, Kearney Agricultural Center, Parlier
J. H. Connell, UC Cooperative Extension, Butte County
Acknowledgment for contributions to Weeds:
T. S. Prather, Department of Plant, Soil, and Entomological Sciences (PSES), University of Idaho