Orchids are primarily tropical and subtropical plants that evolved in environments with consistently warm temperatures and high humidity. This delicate heritage makes them vulnerable to cold, requiring proactive protection. Unlike temperate plants, orchids lack the physiological mechanisms to withstand freezing temperatures. Even exposure to temperatures slightly above freezing can cause significant, often irreversible, cellular damage, making prevention the only reliable way to ensure their long-term health.
Understanding Cold Sensitivity
The concept of “cold” is relative and depends entirely on the specific orchid genus. Damage is categorized into two forms: chilling injury and true freeze damage. Chilling injury occurs above freezing, typically below 50°F (10°C), where cellular functions break down without ice formation. True freeze damage occurs at or below 32°F (0°C), resulting in ice crystals that physically rupture the cell walls.
Temperature tolerance varies widely, making species identification necessary for winter planning. Warm-growing orchids, such as Phalaenopsis, are highly intolerant of cold, preferring nighttime temperatures above 60°F (15.6°C) and risking damage if exposed to the low 50s. Intermediate-growing orchids, like Cattleya and Oncidium hybrids, can tolerate brief dips into the mid-50s F (12–13°C). Cool-growing types, such as Cymbidiums, are the most cold-tolerant, sometimes accepting temperatures down into the low 30s F (1–4°C) if protected from frost.
Preparatory Measures for Winterizing
The most effective long-term defense involves planned relocation and environmental controls well before the first frost warning. Orchids kept outdoors should be moved inside when nighttime temperatures consistently fall below their minimum tolerance threshold.
The ideal indoor spot must maintain a stable temperature, avoiding areas near drafty windows, exterior doors, or heating and cooling vents that cause rapid temperature fluctuations. Since forced-air heating reduces indoor humidity, the environment must be managed to replicate the orchid’s native conditions. Placing pots on humidity trays—shallow trays filled with water and pebbles, ensuring the pot base does not touch the water—can help maintain moisture. Running a room humidifier is also an effective way to increase humidity levels.
Adjusting the watering schedule offers a degree of cellular protection. Plants are more susceptible to cold damage when their growing media is saturated with water. Allowing the potting mix to become dry just before an anticipated cold spell minimizes internal moisture available to freeze, mitigating potential damage. Reducing fertilizer application is also recommended during the winter, especially if the plant enters a period of reduced growth.
Immediate Action During Cold Snaps
When an unexpected cold snap occurs, or if plants cannot be moved to a permanent warm location, rapid response mitigation is necessary. Temporary supplemental heat sources can protect plants housed in semi-protected areas like garages or greenhouses. Space heaters should be used with caution, positioned to warm the ambient air rather than blowing directly onto the plant, which causes desiccation.
For orchids in a greenhouse or on a covered porch, temporary insulation techniques can raise the temperature by several degrees. Lining the interior walls with bubble wrap creates an insulating layer that traps heat. Banking pots together tightly also helps, as the combined mass of the pots and media retains heat better than individual plants.
Outdoor orchids that cannot be moved must be wrapped to create a physical barrier against cold air and frost. Covering the plants entirely with materials like burlap, frost cloth, or newspaper helps trap ground heat and prevent frost formation on the leaf surface. The protective covering should extend down to the ground to ensure maximum heat retention. The covering must be removed promptly once daytime temperatures rise and the threat of freezing has passed to prevent overheating or fungal issues.
Identifying and Treating Cold Damage
Once the cold event has concluded, a careful assessment of the plants is necessary. Cold injury symptoms often develop days after exposure to low temperatures, rather than appearing immediately. Initially, leaves may appear water-soaked, translucent, or wilted, followed by browning or blackening of the affected tissue. In severe freeze cases, the tissue becomes mushy as the internal structure collapses.
Resist the urge to immediately prune off damaged tissue. Waiting a few days allows a clear line of demarcation to form between the dead, damaged tissue and the healthy parts of the plant. Once this boundary is clear, all affected areas should be removed using a sterilized cutting tool to prevent the spread of decay or disease.
Pruning should be followed by applying a protectant to the open wounds, as damaged tissue is susceptible to secondary bacterial and fungal infections. A broad-spectrum fungicide or a dusting of ground cinnamon can be applied to the cut surfaces to act as a mild antiseptic and drying agent. After treatment, the orchid should be placed in a protected, humid environment out of direct sun, and watering should be minimized until new growth is visible.