Most gardeners are aware that summer vegetables can be killed at 32øF , so time their spring planting to avoid frosts. If you're one of those gardeners who like to plant early, be aware that summer vegetables can be adversely affected by cold temperatures above freezing also.

When temperatures are cold, even though they may be above freezing (32øF), many plants may not grow and many seeds will not germinate. Most of our summer garden vegetables that originated in the tropics are injured by temperatures slightly above 32øF. Cucumbers, melons, peppers and tomatoes are all injured somewhat if the temperature drops below 40øF for any length of time. This means that if temperatures remain cold after you've set your vegetables plants out, you may loose any advantage of planting early.

One way to protect your early plantings against cold weather injury is to use physical barriers such as hot caps and row covers. Not only do these barriers provide frost protection and speed growth, they also help keep out migrating pests such as cucumber beetles and flea beetles, and whiteflies and aphids that can vector virus diseases.

For years, gardeners have made their own paper hot caps or plant cages with wooden or wire frames covered with muslin or nylon. Once the cage was made, it could be used for several years. More recently, various synthetic materials have become available, including vented polyethylene, spunbonded polyester, point-bonded polypropylene and woven plastics. Among the easiest to use are the spunbonded or floating row covers that can often simply be placed on top of the bed with no frame or hoops--the crop itself will lift the fabric up as it grows. You can use these floating row covers on sturdy crops that do not grow too tall, such as cucumbers. Sensitive growing tips on plants that grow upright can be injured by floating row covers, so you may need to use wire hoops to hold up the covers.

Both hot caps and polyethylene row covers keep the air temperature around the plant higher during the day, but have little effect on the temperature at night or in the soil. Normally, the plant covers are kept on just during the sensitive seedling stage. When the plants start to fill the air space inside the covering, gradually harden the plants by opening the hot caps or row covers slightly on the north side. Gradually increase the amount of plant exposure over a week or two. Once the plants get big enough to tolerate some damage, you can remove the covers completely. By mid-May or June the temperatures beneath plastic tunnels or other covers that trap heat may get too hot and damage plants, so you'll need to remove the covers then. Floating row covers that are spunbonded or nonwoven usually allow for air movement through many small pores, and don't usually reach temperatures that injure warm season crops. If you use floating row covers on crops that require insect pollination, such as melons, squash, cucumbers and pumpkins, you will need to remove the covers when flowers appear in order to allow bees and other insects to pollinate the crops.

When row covers are kept on for a long time, weed control can become a problem. You may need to carefully lift the row covers off the wire hoops or plants to do handweeding or hoeing. Watering vegetables under row covers can also be tricky. Drip or furrow systems probably work best under the covers.