Protecting garden plants from unexpected dips in temperature is a common challenge for gardeners hoping to extend their growing season or save delicate plants. When cold air is forecasted, covering plants creates a small, insulated microclimate that traps warmth radiating from the soil. This temporary shelter prevents the formation of ice crystals on plant tissue, which causes cellular damage and death during a freeze event. Knowing the temperature threshold for your specific plants and the proper covering technique can prevent a total loss.
Identifying Critical Temperature Thresholds
The temperature that prompts action depends heavily on the specific plant’s hardiness level. Tender plants, such as tropical varieties, basil, and tomatoes, are the most vulnerable and often require protection below 40°F, or certainly when a frost advisory is issued (36°F to 32°F). Ice crystals can form on plant surfaces even when the air temperature is slightly above freezing, especially under clear, calm, and dry conditions.
A light freeze (29°F to 32°F) typically kills tender annuals and causes cosmetic damage to semi-hardy plants. Protection is strongly recommended at this range to prevent ice formation on leaves, which ruptures cell walls. The real danger comes with a hard freeze, defined as temperatures dropping to 28°F and below for five hours or more.
At 28°F and colder, damage becomes widely destructive to most vegetation. While hardy vegetables like kale and spinach can tolerate temperatures down to about 20°F without cover, most garden plants need insulating measures to survive a freeze. A cover can elevate the temperature around the plant by 4 to 8°F, which is often enough to keep temperatures above the light freeze threshold.
Selecting and Applying Protective Materials
The material chosen for covering plants should be breathable and extend to the ground to effectively trap geothermal heat. Commercial frost blankets or row covers, often lightweight polypropylene fabric, are ideal as they allow for light transmission and air exchange. Ordinary cotton sheets, burlap, or heavy blankets also work well, providing more insulation than thin plastic, especially for larger shrubs or trees.
When applying the cover, create a tent-like structure that prevents the material from directly touching the foliage. Direct contact can conduct cold to the plant tissue, potentially causing damage where the material rests on the leaves. Stakes, tomato cages, or hoops can be used to support the material and maintain an insulating airspace around the plant.
The cover must be fully secured to the ground using rocks, soil, or pins to prevent warm air from escaping and cold air from infiltrating. Plastic sheeting should be avoided if it touches the plant, as it traps moisture that increases freeze damage. If plastic is used, drape it over a primary fabric layer or a rigid frame, ensuring it does not press against the plant material.
Timing the Installation and Removal of Covers
Covers must be placed over plants before sunset to capture the heat the soil absorbed throughout the day. The ground radiates this stored warmth upward during the night, and the cover acts as a barrier to keep heat near the plants. Installation in the early evening is also beneficial because diminishing wind speeds make the task easier and allow the cover to be secured effectively.
Removing the covers the following morning is necessary once temperatures have risen above freezing and any frost on the cover has dissipated. Leaving the covers on too long after sunrise can cause temperatures underneath to spike, leading to rapid heat build-up that can stress or “cook” the plants.
Removing covers allows plants to receive necessary light and prevents excessive moisture buildup under a sealed covering. If a series of cold nights is expected, remove the covers during the day to allow the soil to reheat and ensure the plants receive light and air. This timely removal prevents damage from overheating or suffocation.