From a plant's perspective, temperature is not merely a number on a thermometer; it is a fundamental regulator of physiological processes. For geraniums (Pelargonium spp.), every cellular function, from enzyme activity to membrane fluidity, is optimized within a specific thermal range. Straying outside this range forces the plant into a state of stress, where it must expend precious energy on survival rather than growth and reproduction. Enzymes, the catalysts for photosynthesis and respiration, operate with peak efficiency within this ideal window. Temperatures that are too cold slow these processes to a crawl, while excessive heat can cause them to denature, effectively halting growth.
The ideal temperature range for robust geranium growth, flowering, and overall health is between 65°F and 75°F (18°C and 24°C) during the daytime. Within this band, the biochemical machinery of photosynthesis operates at near-maximum capacity. The plant can efficiently convert light, water, and carbon dioxide into the sugars and carbohydrates needed for producing strong stems, lush foliage, and prolific blooms. Respiration, the process of burning those sugars for energy, is also balanced, allowing for sustained growth without the plant depleting its own resources.
A critical aspect often overlooked is the nighttime temperature. Geraniums benefit from and indeed require a period of cooler temperatures after sunset. An ideal nighttime range is between 50°F and 60°F (10°C and 15°C). This drop is crucial for several reasons. It significantly reduces the rate of respiration. With less energy being burned through respiration, more of the sugars produced during the day's photosynthesis can be diverted to root development, flower bud formation, and overall biomass accumulation. This daily cycle of warm days and cool nights mimics their native habitat and promotes harder, more compact, and floriferous plants.
When temperatures fall below 50°F (10°C), geraniums enter a state of cold stress. Growth slows dramatically as enzymatic activity declines. Prolonged exposure to temperatures near or below freezing causes the water within plant cells to form ice crystals, rupturing cell membranes and leading to black, wilted, and necrotic foliage—a condition often fatal. Conversely, sustained heat above 85°F (29°C) induces heat stress. The plant's rate of respiration can exceed its rate of photosynthesis, leading to a net energy loss. To conserve water, stomata (pores on the leaf surface) close, which also traps oxygen and leads to the production of harmful reactive oxygen species. This manifests as wilting, yellowing leaves, scorched leaf margins, and a cessation of flowering as the plant enters survival mode.
Temperature directly influences the geranium's reproductive cycle. Consistently warm temperatures within the ideal range promote continuous flower bud initiation. However, extreme heat can prevent bud formation altogether. Furthermore, temperature affects anthocyanin pigments responsible for the vibrant red, pink, and purple hues in geranium flowers. Cooler conditions, especially during the night, often intensify and deepen these colors, resulting in more vivid blooms, while excessive heat can lead to faded flower color.
