When freezing temperatures threaten tender plants in your yard and you do not have the option of moving them indoors, covering them may prevent frost damage. Depending on how cold it gets and how long the temperatures stay below freezing, as well as the type of plant you are trying to protect, there may be times when covers alone are not enough.
Types of Covers to Use
Never use plastic of any kind, including black plastic garbage bags, to cover plants, as plastic conducts cold to the leaves and will increase the likelihood of damage to the plant. Old sheets, blankets, drop cloths and special frost protection blankets (called Reemay cloth or floating row covers) work best. Be aware, however, that even winter-weight floating row covers only protect down to about 22 degrees Fahrenheit. Check the package label.
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The point of covering plants is not to protect the leaves from the freezing temperatures. In fact, the point is to keep the warmth coming up from the soil contained around the plant. In order to do this effectively, position the cover like a tent over the plant. If this is a plant that you cover regularly in the winter, you may want to build a frame around it to support the cover. The cover should drape completely over the plant with room to secure it on the ground. Do not wrap the cover around the trunk.
When Covers Are Not Enough
If the forecast is for temperatures well below freezing, a cover might not be sufficient, especially for frost-tender trees like citrus. In this case, you will need to provide extra warmth. Using Christmas lights under the cover will help, but make sure you are using the old fashioned incandescent lights, as LED and mini-lights do not give off enough heat to make any difference. If you do not have these kinds of lights, which are becoming hard to buy, use a drop light with a 75-watt bulb, being careful to not place it where it can burn the trunk or branches.
Keep Plants Hydrated
Plants can survive freezes better if they are well watered prior to a freezing event. Before covering, soak the plant thoroughly if a hard freeze is predicted.
Remove Covers During the Day
If the hard freeze is predicted for more than one night in a row, remove the covers during the day to allow for sun warming of the soil, and re-cover in the late afternoon.