Eggplants are produced during the warm season in a number of regions in the state. The earliest production district is in the Coachella Valley. There is also significant production in the Central Valley and the inland areas of the coastal production districts. Eggplants are planted in a variety of bed sizes that range from 40- to 60-inch centers. On narrower beds they are planted in one plant line per bed and on wider beds they are planted in one or two plant lines. Nearly all eggplants are transplanted in the state, which gives the crop some advantage over the weeds. Unfortunately eggplant transplants are slow to establish and initially compete poorly with weeds. Weeds germinating during the first 6 to 8 weeks after transplanting can have serious consequences. In some growing areas such as Central and Southern California, fresh market eggplants are grown on beds covered with black plastic mulch, which prevents germination of most weed species. After 8 to 10 weeks, the yield of eggplants is less affected by late-emerging weeds; however, weeds can interfere with harvest and produce weed seeds that can be troublesome in rotational crops.
Effective weed management in eggplants begins with proper field selection and identification of potential weed problems. It involves preirrigation and cultivation, proper land and bed preparation, sanitation, and proper selection of herbicides.
When combined with good cultural practices, available herbicides can control many of the weed species that are found in eggplant fields. The choice of herbicide depends upon the weed species that are present and the cultural practices followed by the grower.
Preemergent herbicides are applied to the soil and mechanically mixed with the soil or are irrigated into the soil before weeds emerge. They are effective against germinating seeds before they germinate; these materials usually give some residual control of 3-6 months. Postemergent herbicides are sprayed onto the foliage of the weeds after they have emerged. Certain postemergent herbicides are systemic and are absorbed by the leaves and stems of the weeds and translocated in the plant. Other postemergent herbicides are strictly contact and only kill the leaves/plants they come in contact with.
Herbicides work best if they are applied when soil moisture is adequate for plant growth. Preemergent herbicides are effective against germinating seeds, not dry seeds. Do not apply these materials to wet soils, however, as compaction can occur. Postemergent herbicides work best on nonstressed plants, which absorb and translocate the material more readily than stressed plants.
In eggplant production, some growers establish their beds in fall, but in Central and Southern California beds are created in February, drip lines and black plastic mulch applied to the soil, and beds are fumigated. In other areas of the state where beds are formed in fall, use either a rolling cultivator on rough beds or apply oxyfluorfen before weeds germinate. After weed seedlings emerge, glyphosate or paraquat can be used. In spring the beds can be worked and then fumigated with metam sodium. Alternately, shaped beds may be flamed or treated with preemergent herbicides; both methods require subsequent hand-weeding.
Herbicides may also be applied after planting but before crop emergence. Generally after eggplants emerge, growers rely on hand-hoeing to remove weeds from the seedline and cultivation to keep the rest of the bed tops and furrows weed-free. Herbicides may be needed for grass weed problems after eggplant transplanting or the crop emerges.
To plan a weed management program, it is essential to know which weed species are present and the relative abundance of each. Conduct weed surveys of each field at least twice a year: the first after planting but before weeding and the second just before harvest. It is important to keep a log of summer and winter weeds by field. Records from previous crops will indicate which weeds escaped control and will likely infest the eggplant crop. Also examine fencerows and ditch banks, as these are other sources for weed invasion. Pay special attention to where perennial weeds such as field bindweed and yellow nutsedge occur so that follow-up control measures can be taken.
Weeds are controlled best in the seedling stage; therefore, it is important to be able to identify weed seedlings. A good pictorial reference, such as the online weed gallery photos will help assist in identifying weeds.
Weed Management Before Planting
Field Selection and Preparation
Many major weed problems can be reduced by avoiding fields that are severely infested with weeds such as nightshade, cheeseweed, field bindweed, and nutsedge, which compete effectively with eggplants. Irrigation water can also be a source of weeds; keep canal banks free of weeds or install a weed screen on the inlets from canals. Avoid moving weed seed into fields on equipment. When equipment has been used in a weedy field, clean it before entering other fields.
Deep plowing (9 to 10 inches) with a moldboard plow before listing the beds can bury nutsedge tubers to a sufficient depth so that their emergence is slowed down during the crop establishment period. If a plastic mulch is not used, preirrigation coupled with light cultivation or flaming can also greatly reduce weed problems by helping to reduce the initial flush of weeds such as nightshades. Proper land and seedbed preparation also allow for more rapid growth of transplants.
Crop rotations can help to reduce weed problems, as well as disease such as Phytophthora root rot. Corn is a good rotational crop for eggplants because herbicides available for use in corn control nightshade and field bindweed. Alfalfa is a good choice for a rotational crop because the frequent cutting cycle reduces many weeds and available herbicides eliminate most other weeds. Other crops considered to be useful rotational crops with eggplants include beans, cereals, cotton, garlic, rice, onions, carrots, lettuce, cole crops and safflower. Avoid crops such as tomatoes, potatoes, and peppers; they are in the same family as eggplants (Solanaceae) and similar herbicides are used in their production, resulting in similar uncontrolled weeds.
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 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.
Oxyfluorfen (Goal), paraquat (Gramoxone SL 2.0), and glyphosate (Roundup) can be used to control weeds on winter beds. They help keep the beds weed-free during winter. Weeds can also be controlled by rolling cultivators or by discing.
In fields that are not fumigated or if a black plastic mulch is not used, apply preemergent herbicides just before transplanting. They are mechanically incorporated into the soil. The entire bed top may be treated or band treatments applied over the transplant row. Band treatments proportionally reduce the herbicide cost and may reduce the risk of herbicide carryover into the next crop. If weeds have already emerged, a postemergent treatment may also be necessary. When band treatments are used, the area between crop rows requires cultivation or some other method of weed control.
Solarizing soil the previous summer can significantly reduce levels of weed propagules before planting. Soil solarization 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 seeds, as well as soilborne pathogens and nematodes. For more information see Soil Solarization for Gardens and Landscapes or UC Publication 21377 Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds. (PDF)
Preplant soil fumigation for weed control using metam sodium can be very effective if done properly and will also control many soilborne diseases.
Weed Management After Planting
Cultivation and Hand-Weeding
Because eggplants are commonly transplanted into a black plastic mulch in the Central Valley, hand pulling of weeds is practiced instead of tractor cultivation. Hand-weeding can control most weeds, but some, such as nightshades, might be missed because they closely resemble eggplants.
The slow emergence of direct-seeded eggplants can be used as an advantage for weed control purposes. Weeds generally germinate earlier than eggplants and these weeds can be controlled by flaming the beds with propane burners. In order to kill the maximum amount of emerged weeds, flame when 1 to 3% of the eggplant plants begin to emerge. Timing is critical: only a 1- or 2-day window exists for flaming once the eggplant seedlings are ready to emerge because any emerged seedlings will be killed by the process. Flaming will control most broadleaf weeds when they are in the two to four true leaf stage; however, many grasses and volunteer cereals will not be effectively controlled. Eggplant seed that has been primed germinates too fast; as a result, fields planted with primed seed cannot safely be flamed.
Preventing weeds from setting seed helps reduce the population of weeds in the following crop; this also applies to areas adjacent to cropped fields. A program of roguing out nightshade plants late in the season can dramatically reduce the amount of seed that is deposited in the field at the end of the cropping cycle.
Buried drip irrigation can help reduce weed problems by keeping the tops of the beds dry. If conditions permit and water from the buried drip tape is sufficient to meet the needs of the crop but not ample enough to wet the bed tops, weed control can be satisfactory. Perennial weeds are likely to remain a problem with this system.
With furrow irrigation systems, maintaining deep furrows keeps the bed tops from becoming overly wet while providing adequate moisture for the crop. By keeping the bed tops drier, fewer weeds are likely to germinate in the soil surface.
The use of black, brown, or green plastic mulch can inhibit most weed growth on eggplant beds, except for yellow nutsedge. However, immediately around the eggplant plant, where it emerges through the plastic, weeds can also emerge and they will need to be removed by hand.
There is only one foliar-applied herbicide (halosulfuron-Sandea) that selectively controls some broadleaf weeds in eggplants, but clethodim (Select Max) and sethoxydim (Poast) can be applied to control most annual and perennial grasses. Clethodim and sethoxydim have good selectivity on eggplants; however, they may cause some phytotoxic symptoms if the temperatures are above 85° to 90°F.