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.”

For most weed species, he continued, the bulk of the seed in the seedbank is dormant and only a small percentage is able to germinate at a given time. And of that amount, only a percentage is close enough to the soil surface, or within an inch, to germinate.

“That is an important concept that can be taken advantage of in cultural weed control strategies.”

As one example of a cultural practice that can be used by conventional and organic growers, Smith said planting dates can be manipulated to avoid some weed problems. “For instance, purslane requires warm soil temperatures of more than 60 degrees F. and is therefore principally a problem from late spring to fall.”

Thus, high density plantings made in the early spring can avoid problems with the weed, but those made during June to August coincide with the peak of purslane emergence and will be expensive to control.

Pre-germination of weeds prior to bed shaping is another possibility. It amounts to using irrigation or rain to stimulate weed seed germination prior to planting the crop. The emerged seedlings can then be killed by shallow cultivation.

The technique, dependent on irrigation method and time of year, should be used as close as possible to the date of planting to assure that the weed spectrum does not change prior to planting the crop.

Smith said a two-year study at Salinas showed how this works with pre-irrigating by sprinklers or furrow and waiting seven to 14 days before shallow tilling.

It reduced densities of weeds in the subsequent lettuce crop by 33 percent to 65 percent and reduced hand weeding time by as much as three hours per acre.

Pre-germination of weeds following bed shaping is the “stale” seedbed technique. After beds are shaped and ready to plant, an irrigation is applied to create a flush of weeds.

Weeds close to the soil surface are stimulated to germinate and are then killed by shallow cultivation or other means, thereby depleting the quantity of weed seed in the surface layer of soil of the shaped bed. Then the crop can be planted.

Cover crops that compete with weeds may also have some value in a long-range weed suppression program, but Smith said it is important to monitor cover crops during the first 40 days to make sure they are not creating a weed problem.

“Tools such as the rotary hoe can be used to cultivate weeds in cover crops. Rotary hoeing reduced weed seed production by chickweed and shepherd's purse by 80 percent to 95 percent in a legume/cereal cover crop during a two-year study in Salinas,” he said.