Weeds Pages
The first step in developing a weed management program is to identify the 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.
Weed control in organically managed orchards requires special attention to preventing weed problems before they start. Any method that reduces the amount of weed seed in the orchard will diminish the number of weeds over time. One of the most effective ways to reduce future weed problems is to control existing weeds before they go to seed.
Weed control in the first few years after planting is the most difficult because the tree canopy is small and light and other resources are available for weeds. Diligent and regular control efforts are needed until the trees get large enough to compete more effectively with the weeds.
Some growers start orchards conventionally and transition to organic because using conventional herbicides during the first year or two after planting can efficiently control weeds which can lead to less seed returned to the weed seed bank in the soil and reduces the subsequent weed population. This makes weed control by organically-approved herbicides less expensive later in the orchard lifecycle. However, this approach requires a three-year waiting period when conventional herbicides must not be used, before the orchard can be certified as organic.
Similarly, transitioning mature, full-canopied trees to organic production requires less intensive weed management than starting a new organic orchard. Because of shading, mature orchards often have reduced weed growth compared with newly planted orchards; these established trees are also less sensitive to weed competition.
WEED MANAGEMENT BEFORE PLANTING
The season before trees are planted is a critical period for weed management so young trees establish with reduced competition from weeds. Two methods of managing weeds at this time are cultivation and soil solarization.
Cultivation
Repeating a cycle of irrigation followed by cultivation to germinate and destroy young weeds can reduce the amount of weed seed in the soil. Cultivation works well with summer annuals but not as well with perennial weeds such as bermudagrass, field bindweed, johnsongrass, and nutsedge.
If the site is not already certified organic, herbicides can be used until the transition period to organic begins, which can be very helpful in controlling hard-to-control perennial species such as field bindweed. Or, if most of the weed seeds on the site are in the top 4 inches of the soil, a soil-inverting plow can be used to bury them so deeply that they cannot germinate. Use a soil-inverting plow such as a Kverneland plow; a moldboard plow will not sufficiently invert the soil. Since seeds buried deep in the soil profile may remain viable, it is important not to invert this soil again, which would bring the seeds back near the surface where they can germinate and continue the problem.
Soil solarization
Soil solarization can significantly reduce weed numbers in planned tree rows. Soil solarization of moist soil traps the sun's energy beneath a layer of clear plastic, increasing the temperature in the top foot of soil to levels lethal to many weed seedlings as well as vegetative structures of perennial weeds. Solarization is more effective against annuals than perennials. Seedlings of bermudagrass, field bindweed, and johnsongrass are controlled but not mature plants. Yellow nutsedge can be partially controlled while purple nutsedge is not significantly affected.
Use clear plastic that is 1.5 to 2 mils thick and impregnated with ultraviolet inhibitors to prevent premature breakdown of the plastic. 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.
Although black plastic reduces weed seed germination and can suppress some species such as common purslane and nutsedges, it will not heat the soil sufficiently for solarization. Solarization leaves no toxic residues and can be used on a small or large scale; however, it creates a lot of plastic waste. Soil solarization may also improve soil structure and temporarily increases the availability of nitrogen (N) and other essential plant nutrients.
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 (e.g., with a roller).
- Remove any debris that could puncture or lift up the plastic sheets, such as rocks, old roots, and weeds.
- If irrigating before applying the plastic: cover the soil with plastic as soon as feasible after irrigating. After irrigation, allow the soil to dry somewhat to avoid compaction by heavy equipment.
- To irrigate after laying the plastic: install the drip system or microsprinkler line (with only the spaghetti tubing) before planting or use furrow irrigation under the plastic. (If the entire site is irrigated, weed growth will occur in the untarped centers and will be difficult to control without disturbing the plastic.) The edges of the plastic should be buried on all sides to create a seal on the soil and help prevent the plastic from being blown away by wind. Implements are available that assist in laying down the plastic and automate this otherwise labor-intensive process.
- Remove plastic before planting.
- Cultivate solarized soil less than 3 inches deep to avoid bringing viable weed seeds to the surface where they can germinate and establish.
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 from August to September or from May to June.
For more information see Soil Solarization, UC ANR publication 21377.
WEED MANAGEMENT AFTER PLANTING
In-row mulches of black plastic or a 4-inch layer of organic materials including compost, newspaper, straw, hay, and wood chips control weeds by preventing the light penetration necessary for weed growth. With synthetic or organic material, the mulch should be maintained throughout the year. With all mulches, a rodent management plan should be in place to minimize trunk and root damage from rodent feeding, particularly from voles.
Tree-row management
Similar to many conventionally managed orchards, weeds between rows of organic orchards are commonly managed with cover crops, mowing, or both. However, weeds within the tree row must be managed with organically approved herbicides, in-row cultivation, cross discing, mulches, hand hoeing, flaming, or weeder animals. The choice of method depends, in part, on the type of irrigation system.
In-row cultivation
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 in-row cultivation equipment.
- Sprinkler-irrigated orchards require extra precautions to ensure proper operation of the trigger mechanism on the cultivator so that it moves away from the sprinkler head in the same way as it does for the tree.
- Microsprinkler irrigation lines and emitters can be protected from damage by suspending the surface lines, usually with the microsprinklers pointed downward, in the trees or on stakes.
- Drip lines can be buried to prevent damage.
- Furrow-irrigated orchards allow in-row cultivation.
Flaming
Flaming and other thermal weeding methods can effectively manage in-row weeds that are smaller than eight leaves.
- Protect the trunks of young trees from flames to avoid injury to the cambium layer of the tree.
- Keep flames away from plastic irrigation tubing. To prevent damage to irrigation equipment, microsprinkler irrigation lines and emitters can be suspended in the trees or on stakes with the emitters pointed downwards, and drip lines can be buried.
- Flamers should be used on green vegetation, and not in orchards with a lot of dried vegetation, to avoid fires that may injure trees, damage irrigation systems, or spread out of control. The flamer is passed quickly over the green vegetation to damage cells; it is not necessary to actually burn the weeds.
When flaming is used repeatedly, grasses will eventually become the dominant weeds because their growing points are close to the ground. Perennial weeds are also suppressed, but generally not controlled well with flaming. Other thermal methods that have been tested or are available commercially include various hot air machines, steam weeders, and electrical weeders.
Organically Approved Herbicides
Check with the organic licensing organization to determine the current status and any use restrictions for herbicides acceptable in certified organic crops. As with any contact herbicide, good coverage is essential with organic herbicides. In most cases, a spray volume of at least 60 gallons per acre will be required when using these products.
- Repeated applications are necessary to control newly emerged weeds.
- Efficacy usually is greatest on seedlings or small weeds.
- Add an organically-acceptable surfactant to improve efficacy.
- Avoid spraying tree foliage to prevent injury to green tissue.
Broadcast application of organic herbicides is usually not economical. However, organic herbicides can be useful for spot treatments, particularly to control weeds in mulches, because this will help to preserve the mulch and increase its useful life span.
Weeder animals
Before using any animals, check federal, state, and local food safety regulations and comply with them. Consult the University of Idaho or University of Missouri Extension websites for more information on grazing animals.
Sheep and goats are sometimes used in organic orchards. 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.
WEED MANAGEMENT BETWEEN ROWS
There are several options for vegetation control in between rows, including cultivation, letting resident vegetation grow, and growing a cover crop.
Cultivation
Discing or other tillage practices are the traditional practice for managing vegetation between prune/plum tree rows. This eliminates weeds and helps control gophers. However, rising labor and fuel costs as well as limited orchard access following rain or irrigation have greatly reduced this practice in California.
Resident Vegetation
Resident vegetation does not usually grow uniformly enough to compete well with newly invading weeds. This is especially true for orchards with microirrigation where parts of the orchard floor receive no irrigation water. In addition, resident vegetation includes weed species that continually colonize the tree row. However, a focus on reducing costs has made the practice of retaining resident vegetation the most common in current prune/plum orchards. Growers commonly mow the resident vegetation to manage its height and to try and reduce seed production of certain weeds.
Cover Crops
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. Properly managed cover crops can reduce the establishment of weeds that cause problems, in addition to other benefits like reduced soil erosion, increased water infiltration, and increased nutrition (with leguminous cover crops). However, it is important to consider the additional water needs of the cover crop so that it does not compete with trees for available water. 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.
An annual cover crop that reseeds itself will compete against weeds and reduce the potential for problems in the future. To ensure success, plant the cover crops in plum/prune orchards soon after harvest, before leaves fall and when rainfall or irrigation water is available for germination and good seedling growth. Establishing a cover crop may be difficult in orchards with microirrigation systems during dry winters.
Newly established cover crops may be seriously damaged by fall and winter orchard traffic during operations such as pruning, brush removal, and spraying. Consider scheduling such damaging practices later in the winter months after the cover crop is established.
The cover crop will be most competitive to weed growth if mowing can be avoided. However, if there is a potential for frost and the cover crop is tall, mow it once before it blooms to minimize frost damage; the cover crop will regrow and flower later in the season. After most species in the cover crop have produced seed, mow or roll it using a crimper or ringroller. The roller will allow more seed production and create a surface mulch that will reduce emergence of weeds.