How to Protect Plants from Frost and Cold Snaps

Frost protection is a practical skill that saves plants that would otherwise be killed or damaged by temperatures that drop below freezing. Some frost protection is structural and prepared before the cold season arrives: mulching root zones, moving tender container plants indoors, and hardening off plants before exposing them to outdoor temperatures. Other frost protection is reactive: responding quickly to an unexpected frost forecast with temporary covers.

Understanding Frost Damage

Frost kills plant tissue by causing the water inside cells to freeze, which forms ice crystals that rupture the cell walls. The severity of damage depends on how far below freezing the temperature drops, how long it stays there, and whether the plant has been through a hardening-off process that allows cells to tolerate some freezing. Plants that have been brought out of a greenhouse or grown in a protected position are more vulnerable to a given frost temperature than the same species growing in an exposed garden position that experiences regular temperature fluctuations.

Mulching Root Zones

A deep mulch layer over the root zone insulates the soil against temperature fluctuations, slowing the rate at which soil freezes in winter and moderating the freeze-thaw cycling that heaves shallow-rooted plants from the ground. Apply 10 to 15 centimeters of straw, shredded leaves, or wood chip mulch over the root zone of borderline-hardy plants before the first frost of winter. Keep the mulch clear of stems and crowns to prevent the rot that occurs when organic material is in prolonged contact with plant tissue.

Frost Cloth and Row Covers

Frost cloth (horticultural fleece, row cover) is a lightweight, permeable fabric that traps the heat radiating from the soil around the plant, raising the local temperature by several degrees. It is the most practical temporary frost protection for tender plants in place in the garden. Drape it loosely over the plant and secure the edges with stakes or rocks to prevent wind from removing it overnight. Remove during the day to allow light, air circulation, and access for insects.

Multiple layers of frost cloth provide greater protection than a single layer: two layers of standard weight frost cloth can protect plants to approximately minus 4 degrees Celsius, depending on starting temperature and wind.

Cold Frames

A cold frame is a low, unheated structure with a clear lid (glass or polycarbonate) that creates a protected microclimate for vulnerable plants. It works by trapping solar heat during the day and retaining it through the night, providing protection well in excess of frost cloth alone. Cold frames extend the growing season significantly at both ends and allow tender perennials, annual seedlings, and cuttings to be overwintered with minimal additional heating.

Responding to Unexpected Frost Forecasts

When a frost is forecast and no preparation has been made, the fastest effective response is to cover plants with whatever is available: old bedsheets, upturned buckets, terracotta pots, or cardboard. Even imperfect coverage provides meaningful protection. Any covering is better than none. Remove all coverings as soon as temperatures rise above freezing the following morning to prevent overheating and to restore light access.