The common garden geranium, botanically known as Pelargonium, is a popular flowering plant prized for its vibrant, long-lasting blooms. Native to the subtropical regions of Southern Africa, these plants are not naturally equipped to handle extreme temperature fluctuations. Temperature is the biggest factor influencing the plant’s health and prolific flower production. Understanding the thermal limits of Pelargonium is important for ensuring its survival and maximizing performance.
The Ideal Temperature Range for Growth
Pelargoniums are tender perennials, typically cultivated as annuals outside of warm climates. For robust growth and abundant flowering, these plants thrive in a narrow, consistent temperature range. Optimal daytime temperatures should hover between 70°F and 85°F (21°C to 29°C), which encourages active photosynthesis.
The nighttime temperature is equally important for metabolic processes. When temperatures remain consistently above 55°F (13°C) overnight, the plant rests and consolidates energy gathered during the day. If nighttime temperatures drop below 50°F (10°C), growth rate will slow significantly, and blooming may be delayed or halted.
Strategies for Protecting Against Cold Damage
Pelargoniums have a low tolerance for cold and no tolerance for hard frost. Freezing temperatures cause permanent damage, with most varieties suffering significant harm when temperatures drop below 28°F (-2°C) for even a short period. The minimum survival temperature is just above freezing, around 32°F (0°C), though exposure to such cold will severely stress the plant.
Lower temperatures trigger visible stress responses, such as a reddening of the leaves, which can occur when the air temperature dips to about 40°F (4°C). If the plant is exposed to actual freezing, the leaves will quickly turn black, wilt, or become necrotic, indicating tissue death. To protect container plants from a sudden, short-lived cold snap, gardeners can move them against a warm house wall, which acts as a heat accumulator and moderates temperature differences. Elevating the pots off the ground is also beneficial, as cold air collects at ground level where it can cause damaging ground frost.
For long-term cold protection, the most reliable strategy is overwintering indoors. Before the first expected frost, plants should be brought inside and cut back by about two-thirds to reduce their size. The dormant plants should be kept in a cool, bright location, ideally with temperatures maintained between 45°F and 50°F (7°C to 10°C), and kept relatively dry to prevent root rot. A popular alternative is to take stem cuttings in late summer, propagating new, smaller plants that are easier to keep indoors over the winter.
Managing Geraniums in High Heat Conditions
While Pelargoniums prefer warmth, their tolerance is typically exceeded when temperatures rise above 85°F to 90°F (29°C to 32°C). Heat stress often begins with the foliage turning chlorotic (yellow), and then becoming bleached white or necrotic. This damage reduces the plant’s ability to photosynthesize and leads to stunted growth.
The first sign of heat stress is wilting, which is the plant’s attempt to conserve water and requires prompt attention. Container plants should be relocated from direct full sun to a spot that receives partial or afternoon shade, which lowers the ambient temperature. Increasing the frequency of watering is necessary during hot spells, but ensure the soil has good drainage to avoid soggy roots. Good air circulation is also beneficial, especially when plants are clustered together or placed near a direct heat source.