Impatiens (Impatiens walleriana and New Guinea Impatiens) are popular annuals grown for vibrant, continuous color in shady garden spaces. These tropical plants, native to warm regions of Africa and Asia, are sensitive to temperature fluctuations, particularly cold. Understanding their thermal limits is necessary for successful cultivation. Gauging the precise moment to plant outdoors and managing mid-season temperature spikes are crucial steps for ensuring these plants thrive until the first heavy frost.
The Critical Cold Threshold
Impatiens are highly vulnerable to cold, and their survival is threatened by temperatures approaching the freezing point. The absolute minimum temperature these plants can tolerate before suffering fatal damage is 32°F. Gardeners must wait until all danger of frost has passed before planting them outdoors for the season.
Even temperatures above freezing can cause significant damage to sensitive tissue, a condition known as chilling injury. Sustained exposure to temperatures below 40°F to 45°F slows growth and causes physiological stress. Chilling injury manifests visibly as wilting, darkening foliage, or blackening leaves and stems, indicating cellular damage.
To prevent this early-season stress, planting should be delayed until nighttime temperatures are consistently above 50°F. Plants struggle when temperatures drop below this level overnight, and waiting ensures the soil has also warmed sufficiently. This practice avoids the risk of stunting or losing plants to unexpected cold snaps.
Managing Heat Stress and Upper Limits
While impatiens are tropical in origin, they are susceptible to intense summer heat, especially when combined with direct sun exposure. Standard Impatiens walleriana suffers severe stress when temperatures consistently climb above 85°F. High ambient temperatures dramatically increase the rate of transpiration, the plant’s natural process of moisture loss through its leaves.
This increased moisture demand means high temperatures quickly exacerbate inadequate watering, leading to rapid dehydration. Visual indicators of heat stress include chronic wilting, reduced flowering, or the dropping of flower buds. Extended exposure to intense midday sun combined with high heat can also cause leaf scorch, where the foliage develops dry, brown, or white patches.
The need for consistent soil moisture is magnified during heat waves, often requiring watering twice daily, particularly for container plants. If the soil dries out completely, the plant quickly shrivels. While impatiens often recover after rehydration, chronic wilting weakens the plant’s overall health. Maintaining adequate soil moisture is the primary defense against thermal stress once the temperature exceeds 85°F.
Optimal Temperature Range and Varietal Differences
Impatiens perform best within a narrow thermal window that maximizes their growth and blooming potential. The ideal daytime temperature range for most Impatiens, including Impatiens walleriana, is between 68°F and 85°F, with night temperatures above 60°F. Within this range, the plant efficiently photosynthesizes and directs energy toward producing colorful blooms.
Differences exist in temperature tolerance between the two main types of garden impatiens. The common Impatiens walleriana is the more delicate, highly sensitive to heat, and requires deep shade, especially in the afternoon. New Guinea Impatiens (Impatiens hawkeri) show greater resilience to both heat and sunlight.
New Guinea Impatiens tolerate a slightly wider optimal range, performing well with day temperatures up to 85°F and tolerating higher heat, though they may struggle if temperatures exceed 90°F for long periods. This varietal difference allows them to be successfully grown in partial sun, as their physiology is better adapted to handling warmer, brighter conditions than walleriana.
Protecting Impatiens During Temperature Extremes
Safeguarding impatiens requires proactive measures against sudden temperature swings. When an unexpected cold dip is forecast, especially in early spring or late fall, container-grown impatiens should be temporarily moved into a sheltered location, like a garage or porch. Ground plants can be shielded by loosely covering them with lightweight fabric, such as an old sheet or a commercial frost blanket, which traps radiant heat rising from the soil.
To mitigate the effects of extreme heat, the focus shifts to moisture management and placement. Positioning impatiens in deep or partial shade is the most effective heat-protection strategy, shielding them from the intense afternoon sun. During heat waves, providing deep, consistent watering is crucial; moist soil helps stabilize root temperatures.
Ensuring good air circulation around the plants helps prevent the high humidity and stagnant air that can invite fungal issues. Applying a layer of mulch around the base of the plants helps to insulate the soil, keeping the root zone cooler and significantly reducing the rate of water evaporation. These actions ensure the plants remain hydrated and physiologically stable through the summer.