Effective weed control in organic lettuce depends on an integration of sound 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. Growers that have a zero-tolerance for weeds in all crops prior to growing lettuce have lower weed pressure, and over time, the soil seedbank can be depleted. Certain crops such as fast maturing, high-density plantings of spinach and lettuce can also deplete the soil weed seed bank because these crops are often harvested before many weed seeds can mature – provided the field is tilled immediately after harvest.
The first step in developing a weed management program is to 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.
Lettuce is produced in three distinct production districts (coastal valleys, San Joaquin Valley, and the desert) during the cooler times of the year (San Joaquin Valley and desert) and from spring to fall along the coast. It is typically grown on double row, 40-inch wide beds, but there is a large amount of production on 80-inch wide beds with 5 to 6 seedlines. In addition, baby lettuce is grown on 80-inch wide beds with 24 to 32 seedlines on the bedtop.
It is important to locate plantings in fields with low weed pressure or to use weed management methods that reduce weed pressure before planting. It is also important to keep areas near production fields free from weeds with windblown seed. Some growers use transplants to give the lettuce crop a head start on the weeds. Transplants reduce the number of days to harvest and establish a canopy more quickly. They generally only need one weeding instead of two (as is done with direct-seeded lettuce) to get through the growth cycle. The goal of organic weed control techniques is to reduce weed pressure and give the crop an advantage over the weeds in order to produce the crop as economically as possible.
Weed Management Before Planting
Crop Rotations and Field Sanitation
The previous crop can significantly affect weed pressure in the lettuce crop. A previous crop that has had excellent weed control generates fewer weed seed that germinate in the lettuce crop. In addition, it is important to keep the areas surrounding the lettuce field free of weeds that have aerially dispersed seeds, such as groundsel and sowthistle.
Preplant Germination of Weeds Before Final Bed Shaping
Preplant germination of weeds (pregermination) involves the use of irrigation or rain to stimulate weed seed germination before planting lettuce. The emerged seedlings are then killed by shallow cultivation or flaming. Germinate and remove weeds as close as possible to the date of planting to assure that the weed spectrum does not change before planting. Changes in the weed spectrum may occur as a result of changes in the season or weather, such as from winter weeds to summer weeds. The time of year, irrigation system, and the interval between irrigation and weed control all affect how well this technique works. Waiting 14 days after the time of preplant irrigation allows weeds to emerge and for the field to dry enough to permit use of shallow tillage to control emerged weeds. Done properly, this method removes up to 50% of the weeds that would have otherwise emerged in the subsequent crop. If time permits, repeat the preplant process to further reduce weed numbers.
Preplant Germination of Weeds after Final 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 to not till too deeply or additional weed seed may be brought to the surface from deeper layers. The crop can be planted immediately on these beds. This technique is called "stale" seedbed weed control and can provide substantial control.
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 like onion. It is not widely used on lettuce because it is generally quick to germinate (i.e., 4-5 days in summer) and does not allow enough time to conduct pre-crop emergence weeding operations. In addition, flaming and organic herbicides are effective on small (i.e., less than 2 true leaves) broadleaves but not grass weed species.
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; longer intervals between deep plowing and subsequent deep plowing (i.e., 3–5 years) are 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 seeding, and depending upon the weed species, 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, and allow for substantial contributions to the seed bank. Slow-growing winter cover crops (legumes and cereal/legume mixes) can be particularly problematic; many allow significant 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. It is important to monitor the cover crops, particularly in the first 40 days following seeding, to make sure that they are not creating a weed problem for the subsequent lettuce plantings.
Solarization can be used in areas with sufficient solar radiation. Typically, the soil is irrigated and then covered with clear plastic for 4 to 6 weeks during the hot summer period. The effectiveness depends mainly on the heat that can be generated under the plastic during a short interval. Soil solarization can reduce amount of weed seed in the soil, as well as provide partial control of root knot nematode and soilborne fungal pathogens. 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.
Weed Management After Planting
Cultivation is one of the most effective postplant weed control practices that can be performed. 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 seedline). Direct-seeded lettuce is frequently cultivated at about the two to three leaf stage and, again, 2 weeks later. The first cultivation removes early emerging weeds and later cultivations remove weeds that germinate after thinning.
The goal of cultivation is to remove weed seedlings as close to the seed row as possible without damage to the crop. Precision guidance systems for cultivation (e.g., EcoDan Dan and Robocrop) can help improve the accuracy of cultivation operations. Precise cultivation allows the width of the uncultivated band to be narrowed and thereby remove more weeds. Weeds that remain in the seedline are removed by hand or other mechanical means. High-density plantings of baby lettuce on 80-inch beds with 24 to 32 seedlines are difficult to cultivate, and weed control in these systems relies on cultural practices to reduce weed pressure.
Machine removal of weeds from the seedline can be achieved by the use of specific weeding implements such as finger and torsion weeders, as well as intelligent cultivators (e.g., Robovator and IC Steketee). These devices are more suited to transplanted lettuce because they work best when the crop is larger than the weeds. These intelligent cultivators will not generally remove all of the weeds but rather remove a large percentage of the weeds thus increasing the efficiency of subsequent hand weeding operations.
Hand hoeing is essential in organic lettuce. Direct-seeded lettuce is thinned and weeded 30 to 40 days after seeding. At this time the plants are tender, and careful hand weeding is necessary in the seed row to safeguard the crop. Typically, the initial hand weeding and thinning operation is followed 2 weeks later by a final hand weeding before the crop canopy closes. The use of transplants will generally reduce the number of hand weedings to one time per crop. At present, there are no successful techniques that can substitute for careful hand weeding of direct-seeded crops, but successful employment of the above mentioned techniques can help make hand weeding operations less time consuming and more efficient.