Weed control in cool-season vegetables

• Short-season crops such as lettuce and spinach that allow for rapid turnover of the crops (e.g. 30 to 65 days), frequent cultivation (lettuce) and/or complete hand removal of weeds prior to mechanical harvest (clipped spinach and baby lettuce)
• High value of the crops allows for the use of intensive hand-removal of weeds, often prior to seed set
• Small production blocks that allow for careful observation and intensive management
• ll of these strategies have basically made some of the most troublesome weeds such as field bindweed and yellow nutsedge non-issues in the parts of the Salinas Valley where the cool season vegetables dominate. Rotations with baby lettuce and spinach are particularly useful in reducing weed pressure because the short days to harvest (25-30 days during the summer) do not allow weeds to set seeds and the weed seed bank declines with each production cycle. We have observed extremely low numbers of weeds in the soil seedbank on ranches that specialize in baby greens and spinach production.

Farther south in the valley growers rotate lettuce and spinach with other crops and weed control is more of an issue. This is particularly true in areas that have had a history of peppers or tomatoes in the rotations. In these situations, nightshades, as well as other weed species that are difficult to effectively control in pepper production, set seed and become problematic in subsequent rotations. The use of preplant bed fumigation with potassium N-methyldithiocarbamate (KPAM) was a common technique to help growers knock down weed populations prior to planting spinach. It is still used to some extent, but the size of buffer strips and costs have made it more challenging for growers to routinely include this practice in their management schemes. Organic growers typically include one to two preirrigations followed by shallow tillage to reduce the level of weed pressure prior to planting. This practice has been shown to reduce weed pressure by about 50 percent with each cycle of pre-irrigation, and thus is an effective weed control tool.

Chemical approaches to controlling weeds in cool-season vegetables are important, but there have been few new additions to the list of available herbicides for many years. The effectiveness of weed control programs on the various crops varies. Celery has a particularly strong program, but recent changes in the prometryn label included a 12-month plant back to lettuce, which added a difficulty to the use of this material. Pronamide (Kerb) was removed from the leaf lettuce label several years ago, which has increased weeding costs for these crops. However, Dow AgroSciences has been working to reregister pronamide for use on leaf lettuce. They have made significant strides in working with EPA on reregistration, which may occur in the next year or so. This registration will help lettuce growers in all the production regions, but in particular in areas and times of year when shepherd’s purse and burning nettle are particularly problematic.

New mechanical tools such as the automated thinners and automated weeders offer powerful new tools that may benefit all vegetable crops. Automated thinners remove weeds while thinning the lettuce. They do not remove weeds in the “safe zone” around the keeper lettuce plants. However, research is underway to improve the accuracy of detecting the crop and weed plants and new developments in this technology can be expected in the future.

The availability of labor to conduct weed control operations in cool-season vegetables is a growing concern. All growers express concern over not being able to get weeding and other operations done in a timely fashion. As we move forward, it will be interesting to see which technologies or combinations of technologies and practices will be of most help to growers control weeds in their production fields.

– Richard Smith, University of California