Cold damage occurs when temperatures drop below a plant’s tolerance threshold, often resulting from an unexpected frost or freeze. This exposure causes significant cellular disruption, especially in non-hardy species. When water inside plant tissues freezes, the resulting ice crystals expand and puncture cell walls, leading to the collapse of the plant’s structure.
Identifying Cold Injury
The signs of cold stress can manifest quickly, often appearing within hours of a freezing event. The most common indication of severe damage is a water-soaked or translucent appearance in leaves, which rapidly turns brown or black as the compromised cells die off. This discoloration is a direct result of cell contents leaking after the cellular rupture.
In herbaceous plants, stems may become mushy and wilted, losing structural support. Woody plants, such as shrubs and trees, may exhibit split bark, which happens when the trunk expands and contracts rapidly in response to a sudden temperature drop.
A simple diagnostic test for woody stems is the “scratch test,” where a small section of the outer bark is scraped away. If the tissue underneath is green, the stem is still viable; if it is brown or black, the tissue is dead.
Immediate Care After Damage
The immediate response following a cold snap focuses on stabilizing the plant and preventing further shock. If the plant is in a pot, move it to a protected location, such as a garage or a sheltered porch, as soon as possible. Avoid moving it directly into a hot environment, as this rapid change can cause severe heat shock and accelerate damage.
Water management is another immediate concern. Cold temperatures can desiccate the soil, but a stressed plant cannot handle excess moisture. Check the soil moisture and water only if the soil is dry; this can help thaw frozen soil and make moisture accessible to the roots. Do not overwater, as cold-damaged roots are less efficient at water uptake, increasing the risk of root rot.
A common mistake is to prune away damaged foliage immediately. The dead or blackened leaves and stems serve an insulating function, providing protection against subsequent cold events. Furthermore, the full extent of the damage may not be apparent for several days or weeks, and removing tissue prematurely makes it impossible to determine where the live wood ends.
Long-Term Recovery and Pruning
Once the threat of recurring frost has passed and the plant has stabilized, typically in the spring, the long-term recovery phase begins. This is when the definitive assessment of damage and subsequent pruning should occur. The goal of pruning is to remove non-viable tissue to encourage new growth from healthy parts.
Use clean, sharp pruning tools to cut back damaged stems to a point where the scratch test reveals green, living tissue. For plants that have suffered severe dieback, look for new buds or shoots emerging from the base or the root crown, indicating that the root system has survived.
Removing the dead material allows the plant to redirect energy away from trying to repair unsalvageable parts and toward developing these new growth points.
Fertilizer should be withheld during the initial recovery period because encouraging new, tender growth too early leaves it vulnerable to late cold snaps. Once the plant shows consistent, vigorous new growth and the season has fully warmed, a light application of a balanced, slow-release fertilizer can be applied. Continued, appropriate watering throughout the spring and summer will also aid the plant’s recovery.