To truly understand the ideal indoor environment for geraniums (*Pelargonium* spp.), one must view the world from the plant's perspective. We are sun-loving natives of South Africa, thriving in conditions that mimic our ancestral home: bright, warm, and with good air circulation. Providing the correct temperature and humidity is not a luxury but a fundamental requirement for our vitality, flowering, and overall health.
From our point of view, temperature dictates the pace of our entire existence. It governs metabolic processes, photosynthesis efficiency, and flower production. During our active growing season (spring and summer), we prefer daytime temperatures between 65°F and 75°F (18°C - 24°C). This range provides the warmth we need to convert sunlight into energy vigorously. At night, we appreciate a slight cooldown, with ideal temperatures dropping to between 55°F and 60°F (13°C - 15°C). This diurnal fluctuation mimics our natural habitat and is crucial for resetting our physiological processes, leading to stronger stems and more prolific blooming. We are highly sensitive to extreme heat; sustained temperatures above 85°F (29°C) can cause heat stress, leading to wilted, scorched leaves and a cessation of flowering as we divert energy simply to survive.
Do not underestimate the importance of the night. A proper drop in temperature is not something we merely tolerate; it is something we fundamentally require. This cooler period allows us to slow our respiration and conserve the sugars produced during the day. Without this rest period, our growth becomes weak and spindly as we expend energy too quickly. In essence, a cool night allows us to "sleep" and build strength for the next day's growth. Consistently warm nights, especially common in modern centrally heated homes, lead to a gradual decline in our health and a noticeable lack of flowers.
Unlike many tropical houseplants, we geraniums do not crave high humidity. In fact, from our perspective, the average humidity level found in most homes (around 40-50%) is nearly perfect. Our leaves are adapted to conserve water, and high humidity poses a significant threat. It creates a stagnant, moist environment around our foliage and soil surface that is the ideal breeding ground for fungal pathogens like Botrytis (gray mold) and the dreaded water mold *Pythium*, which causes root rot. High humidity hinders the transpiration process, making it difficult for us to circulate nutrients and cool our leaves effectively.
Closely linked to humidity management is the need for moving air. Stagnant, still air is our enemy. It allows humidity to build up around our leaves and provides no defense against fungal spores. From our position on the windowsill, a gentle breeze, provided by an oscillating fan set on low at a distance, is immensely beneficial. This moving air mimics the natural conditions we evolved with, helping to whisk away excess moisture from our leaf surfaces, strengthening our stems, and ensuring that our microenvironment remains fresh and dry, drastically reducing the risk of disease.
