Protecting outdoor plants from unexpected weather is crucial for their survival, preventing damage from environmental stress and the destruction of cellular structure. Many vulnerable species, particularly tropicals and tender annuals, lack the natural defenses to cope with sudden temperature drops or severe weather events. Knowing the specific conditions that require immediate coverage allows gardeners to extend the growing season and safeguard their investments. The decision to cover is based on the type of hazard, the predicted duration of the event, and temperature.
Temperature Thresholds for Action
The need to cover plants begins with understanding the difference between a light frost and a hard freeze. A light frost occurs when air temperatures fall into the range of 32°F to 29°F (0°C to -1.6°C). This level is sufficient to kill tender annuals and tropical plants, whose cells cannot withstand the formation of ice crystals.
A hard freeze is defined by temperatures of 28°F (-2.2°C) or below, which is widely destructive to most exposed vegetation. Hardier plants, such as late-season brassicas, can tolerate a light frost but will suffer significant damage during a hard freeze. Most plants freeze when temperatures remain at 28°F for five hours or more.
It is most effective to place coverings over plants well before sunset to trap the residual warmth radiating from the ground. The soil, warmed by the day’s sun, releases heat upward during the night, and the covering acts as an insulator. This trapped heat can provide a few degrees of protection, often enough to prevent damage during a light frost.
For a more precise prediction of frost formation, check the dew point temperature, not just the ambient air temperature forecast. Frost forms when the air temperature cools to the dew point, causing water vapor to condense as ice crystals on surfaces. If the dew point is low, temperatures may drop rapidly, sometimes resulting in a “black frost” where tissue freezes without visible white crystals. Strong winds can complicate protection; while a breeze may prevent frost on calm nights, high winds in below-freezing conditions increase desiccation and evaporative cooling.
Timing Coverage for Non-Cold Hazards
While cold is the most frequent trigger for covering plants, other severe weather events also necessitate immediate protection. Hail is a mechanical hazard that requires covering plants immediately upon forecast, as the impact can shred leaves, break stems, and bruise fruit. Covers used for hail must be durable, such as rigid plastic containers or thick tarps, rather than light frost cloth, to absorb the force of the ice.
Extreme, sustained high winds can cause significant desiccation, especially in evergreens or newly planted specimens, by stripping moisture from the leaves. Covering plants with a windbreak material, like burlap, is necessary when forecasts call for high winds combined with dry, cold air. Potted plants are particularly vulnerable to tipping and should be moved to a sheltered location or laid on their side to avoid physical damage.
Sudden intense sun exposure, often occurring after cloudy weather or when tender plants are first moved outside, can cause sun shock or scorch. This is especially true if plants have been overwintered indoors or are newly transplanted. Covering with a sheer cloth should be applied during the most intense part of the day to diffuse direct sunlight and prevent the rapid breakdown of chlorophyll.
How Long Plants Should Remain Covered
The timing of cover removal is as important as the timing of application to prevent secondary damage. Generally, covers should be removed once the air temperature rises above 32°F in the morning and the frost has melted. Leaving the covering on too long, especially non-breathable plastic, can cause the temperature underneath to rapidly increase when the sun hits it. This greenhouse effect can stress the plants and cause the foliage to overheat.
Covers must be removed during the day to allow plants access to sunlight for photosynthesis and to prevent the buildup of humidity. Excessive humidity and trapped moisture create an ideal environment for the proliferation of fungal diseases, such as mold and mildew. If an extended cold snap requires covering for multiple days, covers should still be removed for a few hours each day to ventilate the air and allow the foliage to dry.
Heavy, thick blankets or insulated wraps must be removed daily to prevent light deprivation and permit proper air circulation. The only exception is highly breathable frost cloth, which can sometimes be left on for a few consecutive days if the weather remains consistently cold and overcast. Monitoring the plants for condensation and signs of overheating is necessary for any prolonged period of coverage.