Weed control in organic eggplant production depends upon an integration of good cultural practices, careful cultivation, and hand labor. Preventing the production of weed seed in the field before planting will reduce subsequent weeding costs during crop production. The goal of organic weed control techniques is to reduce weed pressure and/or give the crop an advantage over the weeds in order to produce the crop as economically as possible.
The first step in developing a weed management program is survey the planting site and identify the weeds that are there. Become familiar with each weed's growth and reproductive habits in order to choose the most effective management options. For help in identifying common weeds, see the weed photo pages online that are linked to the weed list in the section COMMON AND SCIENTIFIC NAMES OF WEEDS.
Weed Management Before Planting the Crop
Crop Rotations and Field Sanitation
The previous crop can significantly affect weed pressure in the eggplant crop. A previous crop that has had excellent weed control generates fewer weed seeds that germinate in the eggplant crop. In addition, it is important to keep the areas surrounding the eggplant field free of weeds that have aerial dispersed seeds, such as groundsel and sowthistle. A general recommendation is to avoid fields, if possible, with infestations of field bindweed and yellow or purple nutsedge because these weeds will be expensive to control in organic eggplant production.
Preplant Irrigation to Stimulate Weed Emergence After Bed Shaping
The use of irrigation or rain to stimulate weed seed germination before transplanting eggplants can be very effective on already formed beds. The emerged seedlings are then killed by shallow cultivation, flaming, an organic herbicide, or a combination of these treatments. Pre-germinate as close as possible to the date of planting to assure that the weed spectrum does not change before transplanting. Changes in the weed spectrum may occur as a result of changes in the season or weather. The time of year, irrigation system, and the interval between irrigation and weed control all affect the efficacy of this technique. Waiting 14 days from the pre-irrigation to control weeds with shallow tillage can provide up to 50% weed control in the subsequent crop. If time permits, repeat the pre-germination process to further reduce weed populations.
Deep plowing is a tillage technique that buries weed seed or propagules of perennial plants below the depth at which they can germinate. The viability of buried weed seed declines over time and longer intervals between deep plowing and subsequent deep plowing (i.e. 3-5 years) is preferred in order to avoid bringing up large numbers of viable weed seed back to the soil surface.
The use of cover crops is a key cultural practice in organic production. Cover crops provide a variety of benefits to crop production but can potentially both increase or decrease weed pressure in vegetable production systems. Unfortunately, annual weeds frequently become established at the time of the cover crop, and depending upon the species of weed, they can grow and set seed unnoticed in cover crops. Often weed plants decompose before the end of the cover crop cycle making their detection difficult. In such cases, the cover crops act as nurse crops to weeds making substantial contribution to the seed bank.
Slow-growing winter cover crops, such as many legumes and cereal/legume mixes, can be particularly problematic in this manner as they allow substantial weed growth and seed set early in the growth cycle of the cover crop. Fast-growing winter cover crops, such as cereals and mustards, provide complete ground cover in the first 30 days of the cover crop cycle and are better able to compete with weeds. Competitive cereal and mustard cover crops varieties include Merced rye (Secale cereale), white mustard (Sinapis alba), and Indian mustard (Brassica juncea). An adequate seeding rate is also an important factor in providing for rapid ground cover. Be sure to monitor your cover crops, particularly in the first 40 days following seeding, to make sure that they are not creating a weed problem for the subsequent eggplant plantings. Certain cover crops, such as winter rye, and their residues possess allelopathic properties that can be harmful to seedlings or transplants of subsequent crops. In these cases, sufficient time should be allowed between cover crop plow-down and cash crop establishment to allow for complete decomposition of residues to occur. Alternatively, soil can be thoroughly leached with precipitation or irrigation water to preclude possible phytotoxicity.
Mulches are organic materials (i.e. straw), paper, or plastic films that are placed on the soil surface to inhibit weeds and provide other horticultural benefits to eggplant production. They prevent the light from penetrating to the soil surface so that the germination of weed seed is reduced. Dark-colored plastic mulches (i.e. black, brown, and green) are commonly used in eggplant production. Silver on black has also shown benefits in the San Joaquin Valley. However, weeds can emerge through the planting hole and in the furrows that are not covered by plastic. Yellow nutsedge has sharp leaves that can penetrate plastic film; research has shown that placing a layer of paper between the soil and plastic film can reduce emergence of nutsedge through plastic mulch.
Soil solarization can provide control of many soilborne diseases, nematodes, and weed pests. For further information, contact your local farm advisor or see UC ANR Publication 21377, Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds. (PDF)
Weed Management After Planting the Crop
Cultivation is one of the most effective cultural practices that can be carried out after transplanting. On double or single row, 40-inch beds it is possible to cultivate 80% of the bed (assuming a 4-inch wide uncultivated strip is left for each plant line and plastic mulch is not present). The first cultivation after transplanting cuts weeds with coulters and knives; a final cultivation is done just before canopy closure and is more aggressive than the first as soil is thrown to the base of the eggplant plants, thereby covering small weeds.
The goal of cultivation is to cut weed seedlings as close to the plant row as possible without disturbing the crop. New precision guidance systems for cultivation (i.e. EcoDan and Robocrop) can help improve the accuracy of cultivation operations. More precise cultivation allows for reducing the width of the uncultivated band and thereby removing a higher percentage of the weeds.
Removal of weeds from the plant line can be achieved by the use of specific weeding implements such as finger and torsion weeders. They will not generally remove all of the weeds but rather remove an increased percentage of the weeds that will make subsequent hand weeding operations more efficient.
Burying the drip irrigation tape 4-6 inches deep in the bed can reduce the amount of irrigation water that wets the soil surface and significantly reduce weed seed germination and subsequent weed problems in eggplants.
Hand hoeing or hand pulling is generally necessary in organic eggplants. Because of its long life cycle, multiple flushes of weeds germinate in an eggplant field. Early-season hand-weeding operations can be made more efficient with the techniques described above. Late-season weeds are particularly problematic and are always removed by hand. Even where dark-colored plastic mulches are used, hand weeding is required to remove weeds that emerge through the planting hole.
The only organically approved herbicide registered for use in eggplants is GreenMatch, a broad-spectrum postemergent-directed herbicide that burns down emerged weeds. This product only kills small, emerged weeds on contact and has no soil activity or residual.