Plant covers can protect individual plants or whole rows of plants from cold, the elements or insects. They collect sun warmth during the day and prevent loss of that warmth at night. They also raise the humidity level and reduce wind flow.
Plant covers work like little greenhouses, and need the same maintenance as greenhouses—they need to be ventilated or removed during hot weather, and closed again before nightfall.
Individual Plant Covers
Commercially-available “hotcaps” are waxed paper cones that protect single plants from late frosts.
Cloches are individual covers made of glass and shaped like bells. Like other plant covers, they are placed over single plants to protect them.
To make your own cloches, cut the bottoms from translucent plastic milk containers; the hole at the top provides ventilation. Cut off part of the handle if you want to push a stake through the carton to anchor it.
Smaller homemade plant covers can be made of plastic margarine tubs, glass jars, foam coffee cups, or other translucent disposable containers.
Row Covers
Row covers are plant covers that cover a row of plants. They are most commonly used in vegetable gardens, where plants are often planted in rows. The cover can be supported on a framework, or “floating,” and supported by the plants themselves. See for more information.
Cold Frames and Hotbeds
These miniature greenhouses are often used for the same purpose as plant covers—to start vegetables or flowers earlier in the spring than would be possible without them. See .
Managing Plant Covers
Plant covers trap warmth from the soil to keep the air within a couple of degrees warmer than the outside air. They are most effective if used without a mulch inside them. A mulch insulates the soil, keeping it from warming the air at night. The soil collects and gives up heat most readily when it is moist and bare. If you mulch your vegetable garden, do so after the danger of frost is past, and the cover is no longer needed to keep the plants warm at night.
Most plant covers need ventilation on sunny days. Porous covers, such as the spun material used for floating row covers, ventilate themselves. Unventilated plant covers, however, heat up in the sun to temperatures that can kill plants. Cloches are ventilated by tilting them to one side and propping them open with a rock or stick. Hotcaps made from milk jugs can be ventilated by removing the lid.
Plant covers that require daytime ventilation don’t protect the plants from insects, of course. Most insects find food by following its scent, and are good at locating ventilation openings to enter.
In the Fall
Plant covers can be used to protect plants from the first fall frost and extend the growing season in that direction, also. Plants are larger in the fall, however, so the types of covers that were used the spring no longer fit over the plants. Protect tender tomatoes, summer squash, or eggplants by draping a floating row cover over each plant, or by building a simple frame over them and covering it with any fabric, such as old sheets or polyethylene sheeting.
Opaque sheeting must be removed during the day to give the plants light; translucent fabric can be left on during the day if ventilated at the bottom to keep the plant from overheating.