Weeds can pose a serious problem to lettuce production, especially during the seedling stage when dense weed infestations reduce lettuce seedling vigor and uniformity, and make thinning difficult and costly. Weeds also harbor insects and diseases, further reducing lettuce yield and quality. Following thinning and cultivation, a second flush of weed growth may occur; if present at harvest, these weeds will interfere with harvest operations and may set seed that will infest subsequent crops.
Most lettuce is direct seeded and is not very competitive with weeds. Weed competition can reduce lettuce growth quickly. Lettuce is sometimes transplanted into weed-free beds. Transplanted seedlings grow rapidly and are more competitive with newly emerged weed seedlings than direct-seeded lettuce. Later in the growing season, weeds compete with lettuce for space in the row; heavy infestations can cause misshapen, small lettuce heads.
Accurate identification of weeds is essential for choosing effective control measures.
- Monitor fields before planting and use this information to develop an integrated weed management program that efficiently utilizes suitable cultural and chemical control measures.
- Keep records (PDF) of the weed species that occur in each field during the period of the year when lettuce will be grown. Not only are these records valuable in planning successful weed management strategies, but they help track the occurrence of hard-to-control weeds.
- Avoid fields that have high numbers of annual weeds such as sowthistle, little mallow, common groundsel, and prickly lettuce. These weeds can be problematic because they are not controlled by lettuce herbicides.
- Avoid fields with difficult to control perennial weeds, such as field bindweed and nutsedges, particularly for spring or summer-planted lettuce.
Weed Management Before Planting
Growers that have a strong zero-tolerance policy towards weeds can have lower weed seedbanks and develop fields with lower weed pressure. To be successful, growers must ensure that weeds are never allowed to set seed, including during the fallow periods. Manage weeds in previous crops or cover crops to avoid increasing weed seed numbers in the soil by cultivating weeds before seed set (which can occur in as little as 4 to 6 weeks after they emerge). After each crop is harvested, clean cultivate the field or plant a vigorous cover crop to avoid weed infestations. In areas with hot summers, soil solarization can reduce numbers of weed seed in the soil, as well as provide partial control of root knot nematode and soilborne fungal pathogens. Crop rotation can also help to prevent the buildup of high numbers of weeds that may be adapted to one crop system over another. For example, broccoli or celery can be rotated with lettuce so that weeds in the same family as lettuce, such as sowthistle and common groundsel, can be controlled by the herbicides registered for use on these crops.
Before planting the lettuce crop, preirrigate and cultivate to germinate and destroy weed seedlings. If this is done close to planting time, the weeds that germinate and are subsequently killed will be those that would have likely infested the lettuce crop. Have beds near final shape before preirrigation so that they can be prepared for planting with shallow cultivation. Cultivate very shallowly after preirrigation to avoid bringing up ungerminated weed seed from deeper soil layers. Cultivating implements that cut horizontally through the soil, such as harrows or lilliston cultivators, work better than discs, which cultivate vertically and miss many weeds. Done properly this creates a shallow layer of soil that is partially depleted of weed seed. This technique, when integrated with other weed control techniques after planting such as inter-row cultivation, hand weeding and herbicides, creates an effective weed management program for lettuce.
Paraquat (Gramoxone), pelargonic acid (Scythe), glyphosate (Roundup), and carfentrazone (Shark) can provide fallow bed weed control.
Metam sodium is available as a soil fumigant to control soilborne diseases and nematodes, but it can also be used to control weeds. It is most frequently used in baby lettuce production by blading it into shaped 80-inch wide beds. Check on buffer areas and restrictions regarding soil temperatures and waiting periods following application. Paraquat (Gramoxone), pelargonic acid (Scythe), and glyphosate (Roundup) are available for the control emerged weeds before planting. Glyphosate, as a preplant treatment, can be particularly helpful in controlling perennial weeds.
The preplant herbicide benefin (Balan) is mechanically incorporated into the top 2 to 3 inches of the lettuce bed. Because benefin remains in the soil after harvest, do not plant benefin-sensitive crops such as corn, sudangrass, sugarbeets, spinach, and sorghum, following a lettuce crop where benefin was used. Depth of incorporation is important to the performance of this herbicide. If it is incorporated too deeply it will dilute the herbicide, resulting in poor weed control efficacy. Mixing it too shallowly may reduce lettuce tolerance. Power driven incorporator, bed shaper units have given satisfactory results.
Soil tilth with minimum clods will provide best results for preplant incorporated herbicides. There are many types of soil incorporators. The ones with L-shaped teeth usually provide better mixing than those with straight teeth. For shallow incorporation, the straight tooth is effective. A ground speed of 2 to 3 miles per hour is recommended as excessive speed may reduce the herbicide-soil mixing process.
Avoid applying preplant incorporated herbicides in wet soils because a mechanical soil pan may develop that can interfere with seedling taproots and growth; a hardpan may also cause the herbicide to become layered, reducing crop selectivity. Poor herbicide performance has also resulted when treated soil has been moved into the furrows, or has become concentrated in the wrong area of the bed.
Seeding can be done immediately following treatment with any preplant herbicide. Irrigate within 2 to 3 days after planting to minimize volatility loss from the soil.
Weed Management At Planting
A preemergence herbicide can be applied immediately after seeding as an alternative to pre-plant incorporation. This allows growers to combine seeding and herbicide application in one operation, with a sprinkler irrigation following soon after application. With lettuce herbicides, a sprinkler irrigation or rainfall of 0.5 to 1 inch of water is considered adequate for herbicide movement into the zone of weed seed germination.
One of the preemergence herbicides available for use on lettuce is pronamide (Kerb), which controls a broad spectrum of weeds, including weeds in the mustard family. However, it will not control weeds in the composite family (e.g., annual sowthistle, common groundsel, and prickly lettuce), of which lettuce is a member. Excessive sprinkler irrigation (more than 2 inches) will leach pronamide too deeply in the soil to be effective for weed control. For this reason, pronamide results are often disappointing on early season lettuce crops (August to early October seedings) in the low desert where higher amounts of water are needed to establish a lettuce stand. In some counties, pronamide (Kerb) can also be applied by chemigation (as a delayed application 3 to 5 days after the first irrigation for germination is applied), which helps reduce leaching and improve efficacy. Do not plant small grains (e.g., wheat) and other pronamide-sensitive crops after a lettuce crop. Refer to the label for plantback times for rotational crops.
One weed control strategy is to apply benefin before planting followed by a treatment of pronamide at planting to greatly extend the spectrum of weed control. However, this combination also severely limits rotational crop alternatives.
Bensulide (Prefar) is also registered for use on lettuce and controls some of the small-seeded annual grasses such as annual bluegrass, barnyardgrass, crabgrass, and some broadleaf weeds such as pigweed and purslane. A combination of bensulide and pronamide as a preemergence application enhances the control of purslane, pigweed, lambsquarters, and nettleleaf goosefoot during warm season plantings. This application eliminates the need for soil incorporation and is effective under sprinkler irrigation systems. Corn, sudangrass, and sorghum are sensitive to soil residues of bensulide so be sure to read the label for specific plantback restrictions.
Weed Management After Planting
Plant or transplant lettuce into uniform beds with a precision planting system to obtain a uniform crop that allows accurate machine cultivation next to the lettuce seedline. With precision cultivation, fewer weeds remain and handweeding costs are reduced. Seeded lettuce is machine thinned about 2 to 3 weeks after planting or hand thinned about 3 to 4 weeks after planting. Many, but not all, weeds in the seedlines are removed in the thinning process. Fields are hand weeded 10 to 14 days following thinning to remove weeds and any double lettuce plants missed in the thinning operation. Typically, lettuce is cultivated two times during the growing season utilizing sweeps, coulter disks, top-knives and side-knives that are mounted on a cultivator. Camera-guided cultivation systems allow for greater precision in cultivation operations. Intelligent cultivators that use camera and electronic guidance are now available for the second cultivation (following thinning). These machines can remove weeds in the crop row much like a hand hoeing
Buried drip irrigation is also useful for lettuce weed control. Under this system, the lettuce is precision planted and sprinkler irrigated to germinate the crop. After emergence, fields are cultivated and the lettuce is thinned and handweeded within the rows. Subsequent irrigations using the subsurface drip system minimize soil surface moisture and weed seed germination.
Sethoxydim (Poast) or clethodim (Select Max) can be used for controlling small seedling annual grasses and some perennial grasses. Sethoxydim does not control annual bluegrass, but clethodim does. Effectiveness is reduced when grasses are under moisture stress. Later growth stages of annual grasses are more difficult to control. Clethodim is registered on leaf lettuce only.