Frost damage occurs when moisture within plant cells freezes, forming ice crystals that rupture the cell walls. This cellular disruption leads to the collapse of tissue, often causing foliage to blacken and wilt. Because this damage can be devastating to tender plants, quick action is necessary when facing a cold forecast. Protecting plants involves understanding the thermal properties of the environment and using simple materials to create a temporary, warmer microclimate.
Recognizing the Threat
The need for garden protection is triggered by specific temperature drops, not just a chill. Most tender plants sustain damage when air temperatures fall to or just below 32°F (0°C). Monitoring local weather forecasts for a “Frost Advisory” or “Freeze Watch” provides a necessary window for preparation. A light frost occurs when temperatures briefly dip to the freezing point, which can kill sensitive annuals like tomatoes and peppers.
A hard freeze is a more serious threat, defined as four or more consecutive hours of air temperatures sustained at or below 28°F (-2°C). This level of cold is destructive to nearly all but the hardiest garden plants. It significantly lowers the soil temperature and makes protection less effective, guiding the intensity of protective measures needed.
Choosing the Right Protective Material
Effective materials for frost protection trap heat radiating from the soil while remaining lightweight and breathable. Floating row covers, made from spun-bonded polypropylene fabric, are an excellent choice because they permit light and water to pass through while offering several degrees of insulation. Old sheets, blankets, or burlap also work well for temporary use, providing thermal mass to hold in warmth. Choose thicker materials like quilts or blankets for greater insulating power against a severe cold snap.
Impermeable materials, such as plastic sheeting or tarps, offer a higher temperature increase but must be used with caution. Plastic should never directly touch the plant foliage, as this contact point can transfer cold and cause freeze damage, especially if moisture is trapped.
Inverted containers like opaque plastic pots or buckets provide good insulation for individual, smaller plants. These must completely cover the foliage down to the soil line. Any covering, especially heavy blankets, must be supported to prevent crushing the foliage underneath.
Step-by-Step Covering Techniques
Proper preparation begins hours before the temperature is expected to drop below freezing. Thoroughly watering the soil in the afternoon before the frost is predicted leverages the high specific heat of water. Moist soil absorbs and retains more heat during the day, slowly releasing this stored warmth throughout the night. This keeps the root zone warmer, and the heat is then captured by the protective cover placed over the plant.
For garden beds or larger plants, establishing a support structure is necessary to keep the insulating material off the foliage. Hoops made from PVC pipe, wire, or simple wooden stakes taller than the plants can create a protective canopy. Draping the cover over this framework prevents direct contact with leaves and stems, avoiding contact freeze damage. The covers must extend all the way to the ground on all sides to effectively seal in the ground heat.
Securing the edges of the cover creates a closed, warmer environment. Use heavy objects like bricks, stones, or mounds of soil to anchor the material firmly to the ground, preventing wind from lifting the cover and allowing cold air underneath. For tall shrubs or small trees, the material can be wrapped around the canopy and gently tied with soft twine, extending to the base to trap warmth. This sealed barrier creates a pocket of air several degrees warmer than the outside temperature, often preventing damage from a light frost.
Post-Frost Care and Removal
Once the immediate threat of freezing temperatures has passed, covers should be taken off promptly, ideally in the mid-morning after the ambient temperature has risen above 32°F. Leaving covers on, particularly opaque or plastic materials, once the sun is bright can cause rapid heat buildup underneath. This heat can scorch the foliage and cause more damage than the frost itself. If a subsequent night of frost is expected, remove the covers during the day to allow the soil to re-warm, and then replace them just before sundown.
If foliage appears blackened, wilted, or mushy after the frost event, resist the urge to immediately prune off the damaged parts. The damaged tissue provides a layer of insulation for the rest of the plant, protecting the stems and crown from future cold snaps. Wait until the threat of frost is completely over and the plant begins to show new, healthy growth in the spring to determine the full extent of the damage. At that time, prune back to the healthy, green tissue to encourage vigorous recovery.