Impatiens are popular annual bedding plants, prized for their vibrant color in shady garden spaces. These flowers, including common Impatiens walleriana and New Guinea varieties, are native to tropical and subtropical regions. This origin makes them highly susceptible to cold temperatures. Their succulent stems and thin leaves hold high water content, which leaves them vulnerable to temperature drops. Understanding the precise temperature thresholds is necessary for gardeners to protect them from unexpected cold snaps.
The Critical Killing Temperature
The definitive killing temperature for impatiens is 32°F (0°C), the freezing point of water. As tropical plants, they cannot tolerate ice crystals forming within their tissues. Exposure to freezing temperatures, even briefly, results in the total collapse and death of the plant.
The danger threshold is slightly higher than the absolute killing temperature. Temperatures dipping below 35°F (2°C) can cause significant damage and often lead to death, especially if the cold lasts for several hours. Prolonged cold stress overwhelms the plant’s defenses. Any forecast predicting temperatures below 40°F (4.4°C) should be considered a serious threat requiring protective action.
Understanding Chilling and Frost Damage
Impatiens suffer from two distinct types of cold injury: chilling damage and freezing damage. Chilling injury occurs when temperatures are low but remain above freezing, typically in the range of 40°F to 50°F (4.4°C to 10°C). This non-freezing cold disrupts the plant’s cellular membranes, impairing essential metabolic processes like photosynthesis.
Visual symptoms of chilling injury often appear only after the plant warms up. These symptoms include water-soaked spots on the leaves or a general wilting and yellowing. Prolonged exposure to this cold range prevents the plant from absorbing water and nutrients, leading to slow decline.
In contrast, freezing damage, or frost, occurs when the temperature drops to 32°F (0°C) or below. Ice crystals form outside the plant cells, drawing water out and causing severe dehydration. If the temperature drops quickly, ice can form inside the cells, physically rupturing the cell structure and leading to immediate death. The result is rapid tissue blackening, a mushy texture, and complete collapse of the stems and leaves.
Factors Influencing Cold Survival
The air temperature reported by a weather service does not always reflect the actual temperature experienced by the plant. This temperature is influenced by several external variables.
The duration of the cold event is a significant factor affecting survival. A brief dip to 32°F is less damaging than several hours hovering near 35°F, as prolonged duration allows the plant’s core temperature to drop fully.
Wind speed contributes to cooling by accelerating the loss of heat from the plant’s surfaces, similar to wind chill on human skin. This convective heat loss means a windy night at 38°F can be more damaging than a still night at the same temperature.
Conversely, the proximity of plants to structures like a house foundation or a brick wall can create a microclimate. Radiated heat from these structures slightly raises the local temperature.
Moisture also plays a role in cold susceptibility, particularly the moisture level of the soil. Wet soil transfers and retains cold more effectively than dry soil, potentially cooling the root zone faster than the surrounding air. Protecting impatiens requires recognizing that the ground level may be several degrees colder than the official forecast.