Salinas Valley growers face a tough challenge in managing weeds in high-density vegetables planted to 80-inch beds, but a long-term approach tailored to weed biology will be the solution, says Monterey County farm advisor Richard Smith.

That means keeping the weed seed bank in the soil as low as possible through a combination of diligent and timely cultural practices and careful selection of the most effective herbicides, Smith told growers and PCAs at a recent weed school gathering in Salinas, Calif.

“We are seeing quite a transition to 80-inch beds in vegetable production systems in the valley. In the case of high-density plantings there are fantastic opportunities, but it has also created some new weed control problems,” Smith said.

The 80-inch bed configuration has been attractive to many growers, both conventional and organic, because it requires fewer tractor passes across the field than 40-inch beds.

The wider beds, containing 24 to 32 seedlines, also accommodate mechanical harvesting of spinach, specialty lettuce mixes and other vegetables. But, Smith pointed out, an issue with mechanical harvesting is a very low tolerance for weeds in the crop.

In listing critical cultural practices, Smith said fields with known high-populations of weeds – particularly during June through September when purslane is emerging – should be avoided. Methods of causing weeds to germinate for destruction before planting are alternatives. Some growers have selected more vigorous crop varieties to better compete with emerging weeds.

Important too are measures to control aerially-dispersed weeds, such as groundsel and marestail, around fields. “We’d all like to keep weeds from setting seeds and that is hard to do, but many growers are successfully making it part of their program,” Smith said.

Although cultivation is a basic practice of weed control, it is virtually impossible with the multiple seed rows, and with 80-inch beds only the furrows can be cultivated. That means growers have to rely on combinations of hand weeding, fumigation and herbicides.

As one attractive alternative to labor-intensive hand weeding, growers are fumigating several vegetable crops preplant with Vapam bladed in a few inches under the surface, rolling the bed, and immediately sprinkler irrigating to seal in the fumigant.

Smith said progress is being made with herbicides that fit into the 80-inch bed practice. For green baby lettuce, for example, the existing 35-day pre-harvest interval for Kerb has been an issue because it does not fit the crop’s growing cycle, but data has been gathered in support of reducing the interval to 25 days in hopes of revising labeling for the material.

Another innovation being researched is postplant, pre-emergence chemigation with Kerb on baby lettuce to prevent the herbicide from leaching below the effective level for weed control. “The thing that was really noticeable was significant control of nightshade with chemigation versus ground applied Kerb,” Smith said.

Much of the cilantro, which needs considerable hand weeding, in the Salinas Valley has gone to 80-inch beds, and Smith said the IR-4 Program has conducted residue trials for prometryn (Caparol) for a 30-day pre-harvest interval for cilantro. He said the data will hopefully provide data for a Section 18 registration.

Smith said trials have been done with Basamid, a granular soil fumigant used in weed control on turf, and point to some promise for it as an alternative to Vapam. It does, however, require close management for proper irrigation for three days after application.

In an account of recent weed management research on 80-inch beds, Smith said, “In intensive vegetable production areas, the most common weeds are those that have strategies for setting enough seed to persist in spite of quick rotations and intensive cultivation.”