Plants must often be covered to protect them from frost damage, which occurs when the water inside plant cells freezes and expands, causing the cell walls to rupture. This internal damage results in the characteristic browning and wilting of tender foliage. The need for action is often dictated not just by the air temperature forecast but by the moisture level and stillness of the air near the ground. Understanding the specific temperature points that trigger a risk is necessary to safeguard sensitive plants. Protection methods work by insulating the plant and trapping the heat naturally radiating from the soil surface.
Temperature Thresholds for Protection
The decision to cover plants must be based on the forecast minimum temperature, as different ranges present varying degrees of risk. When the air temperature is expected to remain above 40°F, most common garden plants are safe from damage. However, extremely sensitive tropical or subtropical plants may suffer injury at this temperature, and their protection should be considered preventative.
The most common call to action occurs when the forecast predicts temperatures between 32°F and 36°F, which typically leads to a light frost. Damage to tender annuals, vegetables, and new spring growth can still happen even if the air temperature remains slightly above the freezing point. This is due to radiant cooling, where the plant tissue surface loses heat to the clear night sky and cools faster than the surrounding air. Covering plants in this range helps mitigate this radiant heat loss.
Temperatures below 32°F signal a freeze warning, requiring a more robust and insulated approach. A moderate freeze, characterized by temperatures dipping into the 29°F to 32°F range, is sufficient to kill most unprotected tender plants. When the temperature is expected to fall to 28°F or below for several hours, this constitutes a hard freeze, which is destructive to most vegetation. At these low temperatures, simple covers are less effective, and thicker, heavier materials are required for sufficient insulation.
Essential Coverage Techniques and Materials
The primary goal of covering plants is to create a temporary, warmer microclimate by trapping the heat that radiates from the soil. Therefore, any material used must extend to the ground and be anchored there to prevent heat from escaping. Effective covering materials include:
- Old bed sheets
- Blankets
- Burlap
- Specially designed commercial frost blankets
Woven fabrics are preferred because they allow the plant to breathe while still providing insulation.
It is important to avoid letting the covering material touch the plant’s leaves, especially when using plastic sheeting. Plastic can transfer cold directly to the plant tissue at every point of contact, potentially causing more damage than leaving the plant uncovered. If plastic is used for protection against wind or moisture, it must be draped over a frame or staked supports. Structural supports like tomato cages, stakes, or wooden frames should be used to create a tent-like structure that prevents the material from crushing delicate foliage.
When facing a hard freeze, an extra layer of insulation can be provided by combining materials. For example, a fabric sheet draped over the plant can be topped with plastic sheeting to increase the thermal barrier. The plastic must still be elevated to ensure it never rests directly on the plant’s surface. Properly securing the cover at the base using bricks, rocks, or soil is important, as a sealed enclosure maximizes the capture of ground heat.
Post-Frost Care and Timing the Unveiling
Removing the protective covers at the correct time is necessary to prevent damage. Covers must be removed once the morning temperature has risen above freezing and the frost has dissipated. Leaving covers on too long, particularly those made of plastic or dark, heavy material, can cause rapid heat buildup and superheating once the sun hits them, scorching the foliage.
The ideal time to unveil a plant is after the sun has been up for a while but before the day reaches its warmest point. This timing allows for a gradual transition back to ambient conditions and prevents sun scald on leaves that were shielded overnight. If the forecast calls for a return to freezing temperatures the following night, the covers must still be removed during the day to allow for light exposure and to vent trapped moisture.
After a frost, avoid immediately pruning any tissue that appears damaged, as the full extent of the injury is not always clear right away. Wait several days, or until the last expected frost date has passed and new growth emerges, before cutting back the browned or blackened leaves. Providing water to the soil before the cold event helps the ground store more heat and aids the plant in recovering from any desiccation.