Protecting garden plants from cold weather is a common concern during transitional seasons. A sudden drop in temperature, especially with clear skies and still air, can lead to frost formation that severely damages or kills tender vegetation. When cold weather is forecast, proactive measures are necessary to shield susceptible plants from the effects of freezing.
The Critical Temperature Threshold
The exact temperature requiring action is often debated, but 32°F (0°C) is the general danger zone. A “light freeze” is defined as air temperatures falling between 32°F and 29°F, cold enough to kill tender annuals and delicate new growth.
More significant damage occurs during a “hard freeze,” where temperatures drop into the range of 28°F to 25°F. This colder range causes widespread destruction to most unprotected annuals and can harm less hardy perennials. Since air temperature can be several degrees warmer than the temperature directly on plant surfaces, it is prudent to apply protection when the forecast calls for temperatures of 36°F or lower, especially if a clear, calm night is expected.
Understanding Cold Damage and Plant Vulnerability
Plant damage from freezing is caused by the formation of ice crystals within plant tissues, not the cold itself. When water freezes outside the plant cells in the intercellular spaces, it draws water out through osmosis. This results in cellular dehydration and collapse, which is the most common cause of frost injury.
If the temperature drops too rapidly, ice crystals may form directly inside the cells, causing a fatal mechanical disruption of the cell membranes and organelles. Newly sprouted annuals, tropical plants, and soft-stemmed vegetables like tomatoes and peppers are highly susceptible. Their cells lack the natural adaptations to prevent dehydration or internal freezing.
In contrast, cool-season vegetables such as kale and cabbage, or woody perennials, possess mechanisms that allow them to tolerate temperatures several degrees below freezing. Young seedlings, even of hardy varieties, are always more vulnerable than mature plants due to their higher water content and thinner cell walls.
Choosing Materials and Covering Techniques
Effective protection depends on trapping the heat radiating from the ground while preventing direct contact with the freezing air. It is important to cover plants before sunset, as this is when the ground begins to lose its stored warmth.
The best materials for insulation are breathable fabrics like commercial frost cloth (row covers), burlap, old blankets, or cotton sheets. These materials allow air and moisture to escape, preventing condensation from forming and freezing directly onto the foliage. Thicker frost cloths can offer protection down to the mid-twenties, while lighter options guard against a light freeze.
Plastic sheeting is ineffective and potentially harmful if it touches the foliage. Plastic transfers cold easily and causes frost burn where it meets the plant tissue. If the sun hits the plastic the next morning, it can create a rapid greenhouse effect, causing the plant to overheat. To use plastic safely, or any non-breathable material, you must erect a frame using stakes or hoops to keep the material from resting on the plants.
The cover must extend all the way to the ground to trap the residual heat rising from the soil, creating a protective microclimate. Securely anchor the edges of the material with bricks, rocks, or soil to prevent wind from blowing the cover off or letting cold air seep in. For larger plants, creating a supportive structure with stakes ensures the weight of the cover does not crush delicate branches or leaves.
Post-Frost Care and Uncovering
Once the threat of frost has passed, the protective cover must be removed promptly. Leaving covers on too long, especially plastic or thick blankets, will cause the temperature underneath to soar, stressing the plants or encouraging fungal growth. Remove the covers mid-morning, after the air temperature has risen well above freezing and any visible surface frost has melted.
After uncovering, assess the damage by looking for blackened, water-soaked, or mushy foliage, which indicates ruptured cells. Resist the urge to immediately prune this damaged material until the danger of subsequent frosts has completely passed. The dead foliage and stems act as a layer of insulation, protecting the underlying plant structure and any new buds from future cold snaps. Removing this protective layer prematurely can stimulate tender new growth that is highly vulnerable to a late-season freeze.