CORVALLIS, Ore. – There’s probably nothing more exasperating than working hard in the vegetable garden all summer, only to end up with hard green tomatoes, unripe melons, and sweet peppers the size of robin’s eggs.

Oregon ‘s cool springs and cool summer nights can prevent tomatoes and peppers from flowering and setting fruit early enough to grow to a decent size before the frosts of fall arrive.

Ross Penhallegon, horticulturist with the Oregon State University Extension Service, recommends using plastic sheeting to warm the soil. The sheeting comes in various colors including black, green and even red. Black seems to be the most popular color with vegetable gardeners.

“Lay sheets of black plastic on the soil where those heat-loving vegetables will be growing,” Penhallegon said. “The plastic absorbs heat during the day and keeps the soil warm during the night.”

Black polyethylene film in rolls three or four feet wide and 1 to 1.5 mils thick works fine, but Penhallegon recommends using thicker plastic if you want to use it for more than one season. If you want something that will decompose by the end of the year, look for black paper mulch.

Before planting peppers, melons and tomatoes, lay the plastic on the ground where the plants will grow. Cut six-inch holes in the plastic every two feet. If you use drip irrigation in your garden, put drip lines under the plastic.

If you use sprinklers to water, dampen the soil before you lay the plastic down and cut the holes larger to ensure that enough moisture enters the ground from the sprinklers.

Dig a hole for each transplant where there is a hole in the plastic. Then plant your plants. Be sure there’s enough plastic on all sides to cover a mature root system.

After everything’s planted, take a pitchfork and punch some holes in the plastic so water will soak through, but avoid the drip hose. Then weigh down the edges of the plastic with a few mounds of soil.