A hard frost causes water inside plant cells to freeze, forming ice crystals that puncture and rupture the cell walls. This damage, known as freeze injury, results in the mushy, blackened foliage seen after a freeze and compromises the plant’s ability to transport water and nutrients. However, it does not always mean the entire plant is lost. With patience and a strategic approach, many frost-damaged plants can be nursed back to health, allowing their root systems to push out new growth.
Identifying Frost Damage
The initial signs of frost damage appear within hours of the temperature rising above freezing. Affected leaves and stems look water-soaked, wilted, and translucent, eventually turning dark brown or black as the compromised tissue dies. This appearance is caused by the internal cellular structure collapsing after the ice has melted.
Differentiate between superficial leaf injury and harm to the main structure. For woody plants like shrubs and trees, a simple scratch test reveals where the living tissue ends. Gently scratch the outer bark on a stem or branch until the layer beneath is exposed. A bright green, moist layer indicates the plant is alive at that point, while a brown, dry, or discolored layer signifies that the tissue is dead.
Immediate Actions Post-Freeze
The actions taken immediately following a freeze focus on stabilizing the plant and preventing further shock. Avoid pruning off the damaged foliage right away, as this material acts as a natural insulator. It shields underlying stems and buds from subsequent cold snaps, and removing it prematurely exposes potentially healthy tissue.
If the plant is still frozen, protect it from the morning sun to ensure a slow, gentle thaw, since rapid temperature changes can worsen cell damage. Once the temperature is consistently above freezing, provide the plant with adequate water. Roots cannot absorb water from frozen ground, which leaves the plant vulnerable to dehydration. A deep watering near the root zone helps thaw the soil and replenishes moisture lost during the freezing event.
Pruning and Long-Term Recovery
Pruning should be delayed until the threat of frost has completely passed for the season. This waiting period, which may be several weeks or months, allows the plant to clearly delineate which parts are truly dead and which are merely injured. New growth will emerge from the living sections, providing a clear map for where to make cuts.
When the time for pruning arrives, use clean, sharp tools to cut back dead material to a point just above a healthy leaf node or where green tissue is visible. For severely damaged woody plants, this may involve a renewal prune, cutting back major stems to within a few inches of the ground. Herbaceous perennials that have turned into a soft, mushy mass should be cut back to the ground to prevent rot and disease.
Once new growth is well underway, typically in mid-to-late spring, a light application of balanced, slow-release fertilizer can encourage vigor. Avoid over-fertilizing or applying high-nitrogen products too early, as this stimulates tender new growth highly susceptible to future cold damage. The plant’s energy should focus on repairing its internal systems before being forced to produce new foliage.
Preventing Future Cold Weather Injury
Planning ahead reduces the risk of future cold weather injury to sensitive plants. Use protective covers strategically when a freeze is forecasted. Lightweight materials like horticultural fleece, burlap, or old blankets should be draped over plants, ensuring the cover extends all the way to the ground to trap residual warmth from the soil.
For plants in the ground, heap a thick layer of organic mulch, such as straw or wood chips, around the base. This mulch mound protects the plant’s crown, the area where stems meet the roots, which is the most likely location for recovery. Container plants are vulnerable because their roots are exposed on all sides, so they should be moved to a sheltered location like a garage or placed close to a warm wall.