Protecting garden plants from unexpected cold is a concern for gardeners, especially during seasonal transitions. When temperatures drop, water inside plant cells can freeze, leading to two main types of cellular damage. Ice crystals forming outside the cells cause desiccation by drawing water out, dehydrating the tissue. If ice forms inside the cells, it causes mechanical rupture of cell membranes, leading to irreparable damage and a wilted, dark appearance upon thawing.
The Universal Frost Danger Threshold
The critical temperature for plant protection is 32°F (0°C), the point at which water freezes. Covering plants should begin when the forecast predicts temperatures will approach this threshold. Frost can form on plant surfaces even when the official air temperature is as high as 36°F, especially on clear, calm nights.
A “light freeze” is defined as temperatures between 32°F and 29°F, which kills most tender vegetation. A “hard freeze” occurs when the temperature drops to 28°F or below for at least four consecutive hours, which is destructive to nearly all but the hardiest plants. Acting when the temperature is predicted to dip below 36°F provides a necessary buffer.
Plant Hardiness and Varying Cold Tolerance
The 32°F rule is a baseline, but a plant’s inherent hardiness dictates its vulnerability. The most susceptible are tender plants, including most summer annuals, tropical foliage, and vegetables like tomatoes, peppers, and squash. These plants suffer “chill injury” at temperatures sometimes as high as 40°F and are killed by a light frost.
A second category is frost-tolerant, or semi-hardy, which includes cool-season vegetables like beets, carrots, and celery. These crops can withstand temperatures down to 29°F without significant injury. They require covering only when a hard freeze is predicted.
Hardy or dormant plants, such as established perennials, trees, and robust greens like kale and spinach, are the most cold-resilient. These organisms have evolved mechanisms, such as undergoing dormancy, to tolerate temperatures well below 28°F. These established plants rarely require covering unless temperatures fall to severe, prolonged lows.
Effective Methods for Protecting Plants
The goal of covering is to trap the residual heat radiating from the soil and create a warmer microclimate. The most effective materials insulate but also breathe, such as polypropylene frost cloth, old sheets, or thick blankets. These materials should extend all the way to the ground to capture the warmth released from the earth.
Avoid letting the cover touch the foliage, as the material can wick away heat and transfer cold directly to the plant tissue, causing localized damage. Creating a tent or using stakes to suspend the fabric a few inches above the plant is a better technique. While plastic sheeting can be used, it should never rest directly on the plant and can trap damaging moisture. Securing the edges of the covering with rocks or soil is necessary to prevent cold air from entering and keep the cover from blowing away.
Timing the Removal of Covers
Removing the covers promptly after the cold threat has passed is important. Covers should be taken off once the temperature has risen above 32°F and any visible frost has dissipated, typically in the mid-to-late morning. Leaving covers on too long can cause temperatures underneath to spike rapidly due to solar radiation, creating a greenhouse effect that can scorch or stress the plants.
Continuous covering for multiple days without removal can lead to a buildup of excessive moisture and humidity. This stagnant environment increases the risk of fungal diseases and prevents the plant from receiving essential sunlight. Removing the cover allows the sun to warm the soil again and releases trapped moisture, ensuring the plants remain healthy.