Vegetable magic with clear plastic sheets

In northern Wisconsin, where winter seems to hang on long after spring flowers have gone to seed almost everywhere else, some folks are harvesting slow-maturing vegetables that simply do not belong, in such northern latitudes, outside a greenhouse.

In addition, they're also boosting the production of such warm-weather lovers as tomatoes, peppers, cucumbers, and eggplant -- and are even getting frost-tolerant species off to an earlier start.

The simple secret lies in spreading clear plastic film, the type you can buy at any hardware store, over the garden soil early in the spring. The effect of the plastic is to warm up the soil to well above normal temperatures for so early in the year.

Black plastic, spread over the soil as a mulch, has long been recommended to gardeners. It eliminates weeds, for example, and absorbs the sun's heat, radiating it back around the plant with beneficial results on frosty spring nights. It warms the soil beneath it by only a degree or two, however.

In contrast, clear plastic film allows the sun's rays to penetrate and strike the soil directly. And because it prevents much of the heat from radiating straight back into the air (the greenhouse effect), the soil warms up considerably. In the warmer latitudes a clear plastic mulch might heat up the soil too much (unless it was used in winter), but ''it is most effective on Wisconsin soils,'' according to Thomas D. Syverud, superintendent of the University of Wisconsin's experimental farm at Ashland. In fact, Ashland experiments show soil temperatures under clear plastic to rise as much as 20 degrees F. higher than the surrounding soil.

Warm spring soil stimulates rapid growth. The biological life of the soil is also more active in warmer temperatures, converting organic matter in the soil into the nutrients that plants need.

In northern Wisconsin the problem of a short growing season is often compounded by clay soils that take a long time to warm up. By rapidly warming up the soil, the clear plastic lengthens the growing season so that slow-to-mature crops such as hubbard squash and watermelon have time to ripen.

At Madison, soil in the growing beds is first dug, fertilized, and raked smooth. Then it is covered with clear plastic, which is held in place by burying the edges in the soil. The plastic is buried at least 6 inches deep to anchor it firmly. Mr. Syverud suggests using film with a thickness of 4 or 6 mils. Thinner films tear too easily, he says, while thicker ones are unnecessarily expensive, particularly as clear plastic tends to become too brittle to be used for more than one year.

Plants are set out in holes cut in the plastic (take a knife and cut an X in the film, folding back the points to make a suitable hole). Rain falling on the plants tends to be funneled down the stem to the roots.

If you feel there is a need to get more water to the plants, note where the water collects in the plastic after a rain. Using a pointed stick, punch a small hole through the film in the center of the puddles and the water will drain into the soil below.

These are some of the results obtained at Ashland during a three-year test, comparing plots covered with clear plastic with those left uncovered:

Early tomatoes -- Fruit did not mature earlier, but there were half again as many tomatoes, while the total weight of the ripe tomatoes was double that of the check plots.

Late tomatoes -- The number of ripe fruit increased 1 1/4 times and the total weight 1 1/2 times that of the check plots. First fruit did not ripen earlier.

Cucumbers -- Maturity advanced by about one week. The weight of pickling cucumbers was 2 1/4 times greater under the clear plastic, while the weight of slicing cucumbers was up 3 1/2 times and number of individual fruits up 2 3/4 times.

Winter squash -- Maturity hastened with all varieties tested; even Hubbards reached maturity. Squash was up both in numbers and size.

Peppers -- Value of clear plastic mulch was less obvious. Benefits resulted in two of the three test seasons, when the total weight of the peppers was up by 25 percent.

Sweet corn -- Harvest of early-maturing varieties was advanced 10 to 14 days. More ears and larger ears were obtained by using the clear plastic mulch.

Other pluses of the plastic mulch: Slug damage was reduced, and there was less rotting of the tomatoes lying on the wet plastic rather than on bare soil.

Clear plastic does not prevent weed seeds from sprouting, but the physical barrier provided by the plastic does prevent the weeds from taking over.

It is important to plant frost-tender varieties only after all danger of frost has passed. Because the plastic locks most of the heat into the soil, less heat is radiated back around the plant after the sun goes down, making the plants more vulnerable to frost.