Geraniums (Pelargonium spp.), from a plant's physiological perspective, are sun-loving, warm-weather perennials whose cellular processes, growth patterns, and overall vitality are exquisitely tuned to specific temperature ranges. Understanding this internal relationship is key to cultivating truly healthy plants.
Within this band, a geranium's metabolic machinery operates at peak efficiency. Enzymatic activity, crucial for photosynthesis, respiration, and nutrient conversion, is optimal. The stomata on the leaf undersides open readily to facilitate robust gas exchange, taking in ample carbon dioxide without excessive water loss. This results in vigorous vegetative growth: sturdy stems, lush and deeply colored foliage, and the abundant production of carbohydrates that fuel the development of flower buds. Root cells in the soil also divide and expand effectively in this range, establishing a strong foundation for the plant.
Geraniums can persist outside the ideal range, but they begin to exhibit signs of metabolic stress. At the cooler end, around 55°F (13°C), cellular processes slow down significantly. Growth becomes sluggish, leaf color may pale slightly (chlorosis), and flower production halts as the plant conserves energy. Conversely, as temperatures approach 85°F (29°C), the plant enters a state of heat stress. To prevent desiccation, stomata begin to close, severely limiting carbon dioxide intake and effectively putting the brakes on photosynthesis. Respiration rates may exceed photosynthesis, burning through energy reserves. This leads to wilting, potential bud blast (dropping unopened buds), and reduced flowering.
These temperatures push the plant beyond stress into physiological danger. Prolonged exposure to temperatures below 50°F (10°C) can cause chilling injury. Cell membranes lose their fluidity and become damaged, leading to leaky cells and the appearance of unsightly, reddish-purple discoloration on the foliage. A hard frost will cause intracellular ice crystals to form, rupturing cell walls and resulting in blackened, mushy, and fatal damage. On the opposite extreme, sustained heat above 90°F (32°C) forces the plant into survival mode. Photosynthesis effectively shuts down, resources are depleted, leaves may scorch and crisp at the edges, and the plant becomes highly susceptible to pests and diseases due to its weakened state.
The temperature during the dark period is equally critical. Geraniums, like many plants, undergo vital recovery and building processes at night. An ideal nighttime range is between 55°F and 60°F (13°C to 16°C). This cooler period allows for reduced respiration rates, meaning the energy produced during the day is conserved for growth and flowering rather than being burned off. It also helps the plant manage water pressure (turgor) within its cells more effectively. Consistently warm nights prevent this necessary rest period, leading to spindly, etiolated growth and significantly fewer flowers.
