The temperature that causes cold damage is a variable threshold dependent on the plant’s species, growth stage, and the duration of the cold event. Tender annuals and tropical varieties suffer injury well above freezing, while hardy plants survive temperatures far below zero. Understanding the physical processes of cold damage and specific temperature benchmarks helps gardeners determine how to protect vulnerable greenery. Plant tolerance to cooling temperatures is built over time through a process called hardening.
The Mechanics of Cold Injury
Cold temperatures harm plants primarily by disrupting cellular structure through ice formation and subsequent dehydration. Water typically freezes in the extracellular spaces (outside the cells). This extracellular ice formation draws water out of the cells due to an osmotic gradient, causing freeze-induced dehydration as the cell shrinks and collapses.
This desiccation is the most common cause of cold-related plant death. A more destructive, though less frequent, event is intracellular freezing, where ice crystals form directly inside the cell’s protoplasm. Intracellular ice formation is fatal because the sharp crystals mechanically rupture cell membranes, leading to immediate tissue death.
Rapid cooling increases the likelihood of destructive intracellular freezing. Hardy plants prevent this by allowing water to move slowly into the extracellular space, concentrating solutes inside the cell to lower its freezing point. The extent of damage is determined by the minimum temperature reached and the duration of the freezing conditions.
Categorizing Dangerous Temperature Zones
The risk of damage is categorized by specific temperature ranges, with duration amplifying the severity. The first warning sign is frost, which is the formation of ice crystals on plant surfaces even when the air temperature remains slightly above \(32^\circ\text{F}\) (\(33^\circ\text{F}\) to \(36^\circ\text{F}\)). Frost damage is usually superficial, affecting only tender foliage.
A temperature drop into the range of \(29^\circ\text{F}\) to \(32^\circ\text{F}\) is a light freeze, sufficient to kill tender plants and new growth. This level causes minor damage to hardier vegetation and signals the point where plant tissue water begins to freeze. Sustained temperatures between \(25^\circ\text{F}\) and \(28^\circ\text{F}\) constitute a moderate freeze, destroying most annuals and significantly damaging semi-hardy plants.
Temperatures falling below \(24^\circ\text{F}\) are a hard or severe freeze, resulting in heavy damage to most plants. At this level, even the woody tissues of some shrubs and trees can be compromised, especially during prolonged exposure. The longer the temperature remains below freezing, the greater the penetration of ice formation and the likelihood of widespread cellular death.
Proactive Steps to Lessen Cold Stress
Preparing plants for a cold snap involves insulating them and maintaining a warmer microclimate around the foliage and roots. One effective action is to thoroughly water the soil the day before the freeze. Moist soil holds significantly more heat than dry soil, and this stored thermal energy radiates upward during the night, providing a protective thermal blanket.
Container plants lack the insulation of surrounding earth and should be moved to a protected location, such as a garage, shed, or porch. If moving is not possible, wrap the pots in burlap or bubble wrap to insulate the vulnerable root zone. For in-ground plants, temporary covers like bedsheets, blankets, or horticultural fleece can be draped over the foliage.
Coverings should extend to the ground and be secured to trap rising heat, maintaining a slightly higher air temperature beneath. Support the material with stakes or hoops to prevent direct contact with leaves, as this can transmit cold and cause localized damage. For temperatures severely below freezing, a cover alone may be insufficient; a small heat source, like a string of incandescent holiday lights, can be placed underneath.
Remove covers promptly the following morning once temperatures rise above freezing to prevent overheating and allow air circulation. Specialized row covers made of thick fabric provide several degrees of protection against light frost. These materials create a barrier against cold air and capture radiant heat escaping the earth after sunset.
Assessing and Managing Cold-Damaged Tissue
After a cold event, injured foliage often appears wilted, water-soaked, or mushy, quickly turning brown or black as the tissue dies. It is a mistake to immediately prune this damaged growth; the best course of action is to wait. The dead leaves and branches insulate the underlying, potentially still-living parts of the plant from subsequent freezes.
Removing dead tissue prematurely exposes healthy wood and buds to future cold snaps, resulting in greater overall damage. Delay hard pruning until the danger of frost has completely passed, typically in the late spring when new growth emerges. At that time, the true extent of the damage becomes clear, allowing for precise pruning.
To determine which parts are still alive, perform a simple scratch test on the stems or branches. If the tissue immediately beneath the bark is bright green, the wood is viable. If it is brown or black, the material is dead and can be safely removed. Waiting for new growth ensures gardeners only cut away tissue that is truly lost, maximizing the plant’s capacity for recovery.