How to Manage Pests

UC Pest Management Guidelines

Weeds reduce pepper yields by competing for light, water, and nutrients; delaying maturity; and by reducing the efficiency of harvest.

Peppers are a warm-season crop that need a long growing season. Peppers, whether transplanted or direct-seeded, are very poor competitors with weeds early in the season. Not only do pepper plants emerge slowly from the soil, but the seedlings grow slowly as well. As a result, weeds germinating during the first 6 to 8 weeks after crop emergence can have serious consequences. In some growing areas such as Southern California, fresh market peppers are grown on beds covered with black plastic mulch, which prevents germination of most weed species. After 8 to 10 weeks, the yield of peppers 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 peppers begins with proper field selection and identification of potential weed problems. It involves preirrigation and cultivation, proper land and seedbed 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 pepper fields. The choice of herbicide depends upon the weed species that are present and the cultural practices followed by the grower.

Preemergent herbicides are soil applied and mechanically mixed with the soil or are irrigated into the soil before weeds emerge. They are effective against germinating seeds before they emerge from the soil; these materials usually give residual control. Postemergent herbicides are sprayed onto the foliage of the weeds after they have emerged. They are absorbed by the leaves and stems of the weeds and translocated to the site of action; thus they work best when sprayed on unstressed plants. Some herbicides have both preemergent and postemergent activity.

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 pepper production, many growers establish their beds in fall, but in Southern California beds are created in April. To keep beds weed-free during winter, 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 handweeding.

Herbicides may also be applied after planting but before crop emergence. Generally after peppers 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 pepper transplanting or the crop emerges.

In the low-desert region some fields are broadcast fumigated with methyl bromide to control root diseases. The fumigation also controls most of the weed problems and, in this case, no herbicide applications are necessary.

MONITORING

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. Records from previous crops will indicate which weeds escaped control and will likely infest the pepper crop. Also examine fence rows 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.

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, nutsedge, and parasitic dodder, which compete effectively with peppers. 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 (9–10 inches) with a moldboard plow before listing the beds can bury nutsedge tubers to sufficient depth to prevent their emergence. 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.

A soil cap (2- to 4-inch mound of soil) over the seed line at planting can reduce the first flush of weeds competing with the crop seedling. The dirt mound also reduces soil moisture loss and precludes soil crusting after periods of rainfall. Disks or other implements form the soil cap just after planting. The cap is removed just after pepper seedlings germinate and before rapid elongation of the hypocotyl. Rains could delay cap removal, causing potential pepper stand loss. High temperatures during cap removal can lead to scalding of the emerging pepper seedlings, leading to stand loss. However, under good conditions, weed seeds that germinate in the soil cap are destroyed when the cap is removed and fast-growing weeds that germinate in the original bed are scraped off by the cap removal operation.

Proper land and seedbed preparation allow for more rapid and uniform germination of pepper seeds or for more rapid growth of transplants.

Crop rotations. Crop rotations help to reduce weed problems, as well as disease such as Phytophthora root rot. Corn is a good rotational crop for peppers because herbicides available for use in corn control nightshade and field bindweed. Also, corn is not a host for dodder. 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 peppers include beans, cereals, cotton, garlic, rice, onions, carrots, lettuce, cole crops and safflower. Avoid crops such as tomatoes, potatoes and eggplant; they are in the same family as peppers (Solanaceae) and similar herbicides are used in their production, resulting in similar uncontrolled weeds.

Herbicides. Oxyfluorfen (Goal), paraquat (Gramoxone Inteon) 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.

Preplant herbicides. In spring apply preemergent herbicides just before planting. They are incorporated into the soil either mechanically or by sprinkler irrigation. The entire bed top may be treated or band treatments applied over the seedling 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.

For direct-seeded peppers, soil-applied preemergent herbicides such as napropamide (Devrinol) and bensulide (Prefar) can be applied before planting to control grasses and many broadleaf weed species (except nightshades); only metolachlor controls nightshade. For transplanted peppers, trifluralin (Treflan) may be incorporated into the bed before transplanting. Other herbicides registered for use before transplanting are S-metolachlor (Dual Magnum) and pendimethalin (Prowl H2O).

In some areas, especially desert valleys, growers may choose to fumigate the soil with methyl bromide or metam sodium, which greatly reduces weeds as well as disease and nematodes.

WEED MANAGEMENT AFTER PLANTING

Cultivation and handweeding. After crop emergence, mechanically cultivate close to the seed line to remove weeds on the bed top, except for those in the seed line. Also cultivate the furrows. To avoid excessive competition with the peppers and to make removal easier, cultivate when weeds are small. (Using transplants may allow earlier cultivation than with direct-seeded peppers.) Removing weeds in the seedling stage permits a shallow cultivator setting, which helps to avoid bringing more weed seeds near the surface where they might germinate.

Close cultivation effectively controls many weeds in a pepper field with the exception of dodder and those growing in the seed line. Dodder does not require connection with the soil once it has attached to the pepper plant; therefore cultivation will not destroy it after attachment has occurred. Control weeds that can act as alternate hosts for dodder, allowing it to spread onto pepper plants. At thinning, eliminate pepper plants that have dodder attached to reduce dodder problems. Handweeding can control most weeds missed by cultivation, but some, such as nightshades, might be missed because they closely resemble peppers.

If fields with high potential weed populations are planted to peppers, transplanting the peppers provides the crop an initial growth advantage over the weeds. Subsequent cultivations can further reduce weed populations along the sides of the planted row.

Within the crop row, soil can be directed toward the base of the peppers to create a dust mulch. When pepper seedlings are about 5 inches tall, arrange cultivation tools so that a dry layer of soil (dust mulch) is created on the seed line to help prevent weed seeds from germinating and to smother small, emerged weeds. Escaped weeds in the seed line can be hand hoed at thinning. The expense of hand hoeing, however, is reduced by close cultivation before thinning, which allows the hand crew to move more quickly through the field. Hand-cultivate when weeds are small. Generally, one to three handweedings are needed to maintain peppers free of weeds through harvest.

Flaming. The slow emergence of direct-seeded peppers can be used as an advantage for weed control purposes. Weeds generally germinate earlier than peppers 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 pepper plants begin to emerge. Timing is critical: only a 1- or 2-day window exists for flaming once the pepper 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. Pepper seed that has been primed germinates too fast; as a result, fields planted with primed seed cannot safely be flamed.

Cultural practices. 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 rouging 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 peppers are direct-seeded, a second irrigation system (furrow or sprinkler) is needed to germinate the pepper seed. This process also favors germination of weed seeds, which can be flamed once they have emerged. The crop should be well established and free of emerged weeds before the subsurface drip irrigation is used. 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. However, if it is necessary to sprinkler-irrigate after peppers have emerged, subsequent weed growth will occur and require handweeding. 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, less weeds are likely to germinate in the soil surface.

The use of black, brown, or green plastic mulch can inhibit most weed growth on pepper beds, except for yellow nutsedge. However, immediately around the pepper plant, where it emerges through the plastic, weeds can also emerge and they will need to be removed by hand.

Herbicides. Paraquat (Gramoxone Inteon), carfentrazone (Shark), and pelargonic acid (Scythe) can be applied after direct seeding peppers, but before seedlings emerge, similar to the way flaming is used. There is only one foliar-applied herbicide (halosulfuron-Sandea) that selectively controls some broadleaf weeds in peppers, but clethodim (Select Max) and sethoxydim (Poast) can be applied to control most annual and perennial grasses. Clethodim and sethoxydim have good selectivity on peppers; however, they may cause some phytotoxic symptoms if the temperatures are above 85° to 90°F. Trifluralin (Treflan) and pendimethalin (Prowl H2O) are only labeled for use on transplanted peppers and must be incorporated into the soil; they do not control nightshade. Napropamide (Devrinol) can be applied over the top of transplants and then immediately sprinkler-irrigated into the soil. S-metolachlor (Dual Magnum) can be used pretransplant and controls nightshades and yellow nutsedge.

DCPA, pendimethalin, and S-metolachlor can be applied at layby when the plants are 5- to 7-inches tall. Treatments are usually applied as directed or shielded sprays on each side of the plant row and immediately incorporated by sprinkler irrigation or mechanical incorporation.

PUBLICATION

UC IPM Pest Management Guidelines: Peppers
UC ANR Publication 3460
Weeds
R. F. Smith, UC Cooperative Extension, Monterey County
O. Daugovish, UC Cooperative Extension, Ventura County
Acknowledgment for contributions to Weeds:
C. E. Bell, UC Cooperative Extension, Imperial County
W. T. Lanini, Weed Science/Plant Sciences, UC Davis

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