Are you the risk-taking gardener who tries to get a head start on your tender summer crops by planting them a few weeks before your local last-frost date? Or are you the type of gardener that plays by the rules and waits to plant your tomatoes and peppers? Whether you plant early or late, fickle spring weather can still bring a dreaded late frost that can destroy or damage many of your tender young starts. The last-frost date is not absolute; it is an average date by which your region typically experiences the last frost. This means that each spring, gardeners need to keep a close eye on the forecast to watch out for potential late-season frosts.
If meteorologists do predict a late frost for your area, there some plant protection techniques available to gardeners that can help prevent or at least lessen the damage from a frost or light freeze.
The most successful and cost effective technique is to cover your temperature-sensitive crops. Covering crops works to protect your plants by trapping the radiant heat of the soil beneath your covering material. So even though air temperatures might reach the mid to low 30s, the humid air around your plants will be several degrees warmer, preventing damage from the frost. The best covering materials are made of woven cloth (like old sheets) or paper (cardboard boxes) rather than plastic sheeting. Commercial frost blankets are also available and are effective at protecting plants from light freezes.
Draped covers should not actually touch the foliage because it will increase the chance of temperature damage. Instead raise covers above your plants on framework or a series of stakes. Covering the foliage of tender plants alone will not protect them. Cloth covers must be draped all the way to the ground and weighed down to capture the radiant heat of the soil. Adding moisture to the soil will help the earth retain more heat than dry soil, so consider watering your garden the day before a forecasted frost and then covering your crops that evening.
Be aware that covering container plants will not protect them from a freeze. If you are concerned about tender plants in containers, they should be relocated into warm sheltered spaces, like a garage, as the limited soil in your containers will not radiate enough heat to offer frost protection.
As a word of warning, I would suggest avoiding commercial sprays that promise to defend against frost damage by creating some type of thin protective barrier. The efficacy of these sprays in general is questionable and results are very mixed between brands and solutions. The best preventive measure for frost damage is a good plant covering plan that follows the basic rules discussed here.