Pest Management Guidelines
Weed Management For Organic Production
(Reviewed 12/09, updated 12/09)
Weed control in organic pepper 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 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 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 pepper crop. A previous crop that has had excellent weed control generates fewer weed seeds that germinate in the pepper crop. In addition, it is important to keep the areas surrounding the pepper field free of weeds that have aerially 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 pepper production.
Pregermination of weeds before bed shaping
Pregermination involves the use of irrigation or rain to stimulate weed seed germination before planting peppers. The emerged seedlings are then killed by shallow cultivation, flaming, an organic herbicide, or a combination of these treatments. Pregerminate as close as possible to the date of planting to assure that the weed spectrum does not change before the vegetable crop is planted. 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 preirrigation to control weeds with shallow tillage can provide up to 50% weed control in the subsequent crop. If time permits, repeat the pregermination process to further reduce weed populations.
Pregermination of weeds after bed shaping
Once beds are shaped and ready to plant, water can be applied to stimulate a flush of weeds, thereby depleting the quantity of weed seed in the top inch of soil. The flush of weeds can be killed by shallow cultivation, flaming, or applications of organic herbicides. Be careful not to till too deeply or weed seed may be brought to the surface from deeper layers. The crop can then be planted immediately on these beds. This technique is called "stale" seedbed and it can provide substantial weed control.
WEED CONTROL AFTER PLANTING, BEFORE CROP EMERGES
Flaming or organic herbicide treatments can be used to kill the flush of weeds anytime between seeding the crop and its emergence. This technique is particularly effective on crops that have slow seed germination and has been widely used in direct-seeded peppers. Flaming and organic herbicides are effective on small (i.e. less than 2 true leaves) broadleaves but not effective on grass weeds.
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 a longer interval 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 crop 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 pepper plantings.
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 pepper 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 pepper production. 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 sterilization is generally accomplished by heat generated by soil solarization or by the use of steam. Both of these techniques may have promise for organic pepper production because of the long-term nature of this crop.
WEED MANAGEMENT AFTER PLANTING THE CROP
Cultivation is one of the most effective cultural practices that can be carried out after planting. On double-row 40-inch beds it is possible to cultivate 80% of the bed (assuming a 4-inch wide uncultivated strip is left for each seed line). 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 pepper plants, thereby covering small weeds.
The goal of cultivation is to cut weed seedlings as close to the seed 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 seed 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 peppers.
Hand hoeing is generally necessary in organic peppers. Because of peppers' long life cycles, multiple flushes of weeds germinate in a pepper 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.
UC IPM Pest Management Guidelines: Peppers
WeedsR. 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