Frost protection for plants

After a hard winter, the last thing you want is to lose plants to a late spring frost. Knowing how to provide frost protection for plants is your first defence.

Simply put, most light frost is frozen dew. It forms on vegetative surfaces and can damage the cells of plants that are actively growing. These include emerging and transplanted perennials, newly planted vegetables, and shrubs and trees leafing out and flowering.

It’s difficult to imagine how a tree hardy enough to withstand wintry weather can be damaged by a little spring frost. Plants prepare for winter by accumulating sugars and certain amino acids that act like antifreeze in their cells, drying tissues to make them more cold resistant. Once warm spring weather arrives, plant tissues begin to absorb water, becoming succulent again. This tender growth freezes at higher temperatures than does dormant tissue, making it more susceptible to frost damage.

Full morning sunlight causes most damage. Light frost freezes the cells just enough to injure the membrane, but not kill the cells. If the plants have an opportunity to repair that damage before they receive full sunlight, the damage will be minimized. That’s why it’s important to shade plants from strong morning sun if frost strikes at night.

Unfortunately, flower buds are more vulnerable to frost than other vegetative growth, so try to site early-flowering trees and shrubs on a northeastern-facing slope where they won’t be vulnerable to the huge temperature swings that occur under harsher, southwestern exposures.

Frost tolerance varies from plant to plant, too. Peas, onions, spinach and lettuce, for example, are better adapted to cooler conditions than are tender crops such as beans, squash, corn and tomatoes, which can be damaged even by temperatures as high as 10° C if sustained for more than two hours. The difference is in the chemical composition of the cell membranes: variations in sugar levels and cell structure can affect a plant’s ability to withstand frosts or cold weather. That’s why gardeners shouldn’t worry about the early appearance of spring bulbs like crocuses, winter aconite, species tulips and some daffodils — these have good frost resistance.

If frost threatens, there are several ways to provide frost protections for plants.

• Row covers provide a thermal buffer, just like a blanket. Depending on their thickness (medium-weight versions are best for spring), they can raise temperatures by as much as 4°C. Keep the fabric from touching plants, as this can lower surface temperatures. Row covers allow sun, water and air to permeate and may be left on during the day.
• Polytunnels act much like row covers, and the plastic can be stretched over hoops to protect tender crops. Be sure to raise the sides for ventilation during the day to prevent disease caused by warmer temperatures and increased humidity.
• To reduce heat loss, apply a mulch of straw, tar paper or newspapers in the evening, before frost occurs, to trap the heat in the soil and plants. Or, cover plants with cardboard boxes or old bed sheets; place rocks or bricks on top of the boxes and on the corners of the sheets to keep coverings in place. Remove coverings and mulch when the threat of frost has passed.
• A cold frame — a bottomless box with a clear cover that sits on the ground — is a versatile season extender, protecting tender plants from frost and cold in spring and fall. It can also be used to harden off seedlings started indoors so they slowly acclimatize to the cold. Raise the lid during the day for ventilation.
• Cloches, whether homemade from plastic bottles with their bottoms cut off or store-bought glass ones, act as mini-greenhouses for individual plants that need coddling against the cold. Remove cloches in the morning.
• Mist plants overnight. As water freezes, it gives off heat, raising the surrounding air temperature. In the morning, turn off the mister once the ice melts from plant surfaces.
• Know your last expected spring frost date. The most recent data is the Canadian Climate Normals, 1981 to 2010, published by the federal government online. You can search for your location and click on the tab for climate normals.