Gardening With Plastic

Black polyethylene sheeting is used to retain soil moisture and prevent weed growth around high-value crops at moderate and low latitudes. First introduced in the pineapple fields of Hawaii, the black sheeting now is often laid down using specially-built machines at the time plants are set out.

Whereas the black sheeting works well elsewhere, it is not suited to growing crops at high latitude where the soil is cold and summer days are long. Clear polyethylene sheeting placed around crops such as corn and squash that need warm soil will allow the sun's rays to reach the ground and warm it during the hours of sunlight. At night, the clear plastic will allow more soil heat to escape than would black plastic sheeting, so for those few hours the black actually would be more desirable.

However, Dr. Don Dinkel of the University of Alaska Experiment Station at Fairbanks says that the use of clear polyethylene sheeting will raise the average soil temperature by as much as 20°C (38°F). That much increase will cause substantially faster growth of many plants.

While black plastic itself absorbs heat from the sun, Dr. Dinkel indicates that the plastic usually is not in good enough contact with the ground to transfer the absorbed heat to the soil. Consequently, the black plastic will not create more than minor warming of the soil

The important thing for many crops the northern gardener wants to cultivate is to make the soil as warm as possible. Warm soil fosters rapid uptake of moisture and nutrients through the plant roots. Therefore, use clear plastic on northern gardens for those crops that prefer warm soil.