Frost occurs when the temperature of the air and surfaces drops to \(32^\circ\text{F}\) (\(0^\circ\text{C}\)) or lower, causing ice crystals to form on plant tissue. This sudden cold is a serious threat to non-hardy plants, especially those with high water content, as the ice formation disrupts their cellular structure. Understanding the physical mechanisms of this damage is the first step in protecting vulnerable plants. This knowledge allows gardeners to implement effective preventative measures and manage recovery after a cold event.
How Freezing Damages Plant Cells
The destruction caused by freezing temperatures begins at the microscopic level, primarily through two distinct mechanisms involving the water within the plant. The most common form of injury is extracellular freezing, where ice crystals first form in the spaces between the cells, known as the apoplast. This external ice formation draws water out of the cells due to a difference in water potential, effectively dehydrating the cell.
This process causes the plant cell to shrink and collapse, similar to severe drought stress. If the dehydration is too rapid or extreme, the cell membrane can be permanently damaged, leading to the leakage of cellular contents and ultimately cell death. The loss of turgor pressure from the water migration results in the limp, wilted appearance seen in frost-damaged foliage.
The second, often lethal mechanism is intracellular freezing, where ice crystals form directly inside the cell’s cytoplasm. This occurs when the temperature drops very quickly or very low, overcoming the cell’s natural ability to supercool or dehydrate slowly. Ice crystals forming inside the cell puncture the delicate cell membranes and organelles, causing instant and irreversible rupture and death of the tissue.
Recognizing the Signs of Frost Injury
Identifying frost damage is relatively straightforward, as the symptoms are visible. The most immediate and common visual signs are a water-soaked or translucent appearance of the foliage, which quickly turns dark brown or black upon thawing. This “cooked” look is the result of the ruptured cells releasing their contents and losing structural integrity.
New growth, tender shoots, and flower buds are the most susceptible parts of the plant, often showing damage first with blackened tips and edges. Buds, especially those of fruit trees that bloom early, can be inspected by cutting them open; a brown or black discoloration inside the bud indicates that the tissue has been killed by the cold. In woody plants, the severity of the damage can be gauged by scraping the bark on a stem; healthy tissue underneath will appear green, while cold-injured wood will be brown or black.
Protecting Plants During Cold Events
Proactive protection is the most effective way to prevent the cellular damage caused by frost. One simple measure is to water plants thoroughly before an anticipated cold event. Wet soil holds heat much better than dry soil, radiating warmth upward throughout the night and helping to keep the air immediately surrounding the plants slightly warmer.
Covering vulnerable plants is a primary defense against frost, as this traps the heat radiating from the ground and prevents direct contact with freezing air. Materials like horticultural fleece, old blankets, or sheets work well. Ensure the covering extends all the way to the ground to trap the maximum amount of heat. If using plastic, it must not touch the plant tissue, as this can worsen the damage by holding moisture against the leaves.
Container plants should be moved to a protected location, such as a garage, porch, or indoors, before the temperature drops. For in-ground plants that are too large to cover, a thick layer of dry mulch piled around the base can insulate the roots and lower stems from deeper cold penetration. Using incandescent Christmas lights strung through the branches can provide enough localized heat to keep the temperature just above the freezing point underneath a cover.
Caring for Frost-Damaged Plants
Resist the urge to immediately prune away the damaged, discolored foliage. The dead material, while unsightly, can actually provide an insulating layer that protects the underlying healthy wood and buds from subsequent, deeper frosts.
Wait until the danger of any further frost has completely passed and new growth begins to emerge before trimming any damaged parts. At that point, you can clearly see which stems are truly dead and prune back to a healthy bud or green tissue. For herbaceous plants that have collapsed and turned mushy, it is best to remove the dead material to prevent the development of fungal or bacterial diseases.
Avoid applying fertilizer to damaged plants in the immediate aftermath of a frost event. Fertilizer encourages the plant to produce soft, new growth, which is even more susceptible to cold damage if another frost occurs. Focus on maintaining proper watering to support the plant’s natural recovery process. Once the weather has stabilized and new growth is evident, a balanced application of fertilizer can then help fuel the recovery.