Whether you fell under the spell of some eye-catching color at the garden center or just want to get an early jump on the gardening season, planting too early often exposes transplants to deadly frosts or cold weather. Even if you do plant at the right time, frost dates are just averages. You should be ready for early or late frosts and be prepared to protect young transplants and other plants.
There are various methods to protect plants when it looks like the thermometer will dip into dangerous territory. But you need to be ready ahead of time to guard plants against threatening temperatures.
Know the Risks
Good gardeners need to be good weather watchers and know when nighttime temperatures will threaten their plants. Weather reports on the evening news are a good place to start, but you should also be aware of the microclimates in your yard that may be colder or warmer than surrounding areas. Purchases one or more high-low thermometers from a local nursery or garden center and place them in various locations around your yard. Over a period of time, record highs and lows. Compare them to each other and with predictions on the nightly news. That will tell you a lot about different areas of your yard and an how compares to your overall area.
Frost sensitive seedlings, such as tomatoes and peppers, will usually be damaged or killed when temperatures dip to 32-33°F. Cool season annuals and vegetables can take more cold; usually down, at least, into the mid-twenties. Tropical plants are very cold sensitive. Some will be killed if temperatures fall to 40°F; others crumble at 35°F. Many plants, especially deciduous trees and shrubs, are just hardy by nature and can withstand temperatures well into minus territories. Bottom line, plant hardiness varies and you need to know what to expect. Look to good garden books and online resources for information on plant hardiness and get to know how to use USDA Plant Hardiness Zones.
Quick Fixes for Threatening Cold
Move the plants. The easiest method to protect plants from cold is to move them out of harm's way. This is easy with seedlings in flats and small potted plants. Larger potted plants can be kept on wheeled platforms. Moving the plants under a deck, into a garage or shed, or onto a covered porch will often provide enough protection. Smaller plants and seedlings can also be brought indoors.
Use Water. Dry plants are more susceptible to cold, so make sure plants are properly watered in winter. When cold weather is expected, wet the soil early in the day so it can be warmed by the sun. That heat will be released at night and raise overnight air temperature around plants as the soil cools. Fill plastic gallon jugs or buckets with water and place them in a sunny spot during the day. At night, put them around endangered plants. The water will moderate air temperatures; as it cools, it will release heat. For greatest benefit, paint a few water-holding containers black to maximize heat absorbed during the day.
Move the air. Cold, still air is especially damaging to plants. Stir a gentle breeze all night with an electric fan to keep frost from forming on the leaves. Always protect electrical connections from moisture.
Cover the plants. Covering plants with sheets, towels, blankets, cardboard, plastic sheeting or a tarp will trap heat around them and provide protection. You can also use inverted baskets, boxes or any container with a solid bottom for protection. Put the covers on before dark to trap warmer air produced when the sun is up. Ideally, coverings shouldn't touch foliage, which can be damaged as the frost settles on the material. Use stakes or other props to support heavy materials and to keep them off the foliage. If it will be windy, use stakes to anchor fabrics.
The next morning, remove coverings when temperatures rise to safe levels and the frost dissipates. Heat from the sun can build beneath solid coverings, and high temperatures could damage the plants.
Use frost blankets. Keep a supply of frost blankets or floating row covers, on hand. These covers are made from synthetic fibers or plastic, and allow water, and some light and air to pass through. You can lay row covers directly on plants or transplants, or create a tunnel by suspending them over a bed using stakes or wire hoops. Some frost blankets are made like bags and slipped over the top of plants. Check with your local nursery or garden center for options.
Heat with lights. Incandescent light bulbs generate sufficient heat to raise nearby air temperature enough to protect a plant from even hard freezes, especially if you can safely combine them with some type of cover. Some gardeners use strings of old fashion Christmas lights. Bulbs must be close to plant for this to work well. Fluorescent bulbs don't generate enough heat.
Protecting individual plants. Hot caps – rigid plastic or paper cones with venting holes – can protect individual seedlings at planting time. Hot caps act like mini-greenhouses, but venting holes eliminate the daily chore of placing and removing the covering. You can make your own hot caps using plastic two-liter bottles or gallon jugs with bottoms cut off and lids removed (but saved). Replace lids an hour or two before the sun goes down.
The Wall O'Water tepee, is a very effective twist on the hot cap idea. It encircles individual plants with a plastic sleeve of water-filled tubes. The water absorbs the sun's heat during daylight. At night, as the water slowly cools, it releases the stored radiant heat of the sun, keeping air inside the tepee warmer and frost-free.