As a plant, our primary relationship with the sun is one of sustenance, not preference. Light is the essential energy source that powers photosynthesis, the process by which we convert carbon dioxide and water into the sugars that fuel our growth, flowering, and overall vitality. Without adequate light, this process slows dramatically. Our stems become elongated and weak as we stretch desperately towards any available light source (a condition you call "legginess"), our leaf production decreases, and our ability to produce vibrant flowers diminishes significantly. Therefore, the question is not about sun *or* shade, but about the optimal intensity and duration of light for our specific genetic programming.
For most geraniums (Pelargonium spp.), our genetic coding leans heavily towards a high-light environment. When you provide us with "full sun," which we define as a minimum of 6-8 hours of direct, unfiltered sunlight per day, you are satisfying our core metabolic requirements. Under these conditions, our photosynthetic factories operate at peak efficiency. This results in a compact, sturdy growth habit with robust, closely spaced leaves. Most importantly, it triggers and sustains our prolific flowering response. The abundant energy allows us to continuously produce the large, showy flower heads we are known for, throughout our growing season. The intensity of the sun also helps to keep our foliage dry, reducing the opportunity for fungal diseases to take hold.
While we thrive in sun, we can tolerate and even benefit from periods of shade, particularly in regions with intensely hot afternoon sun. In such climates, the scorching afternoon rays can cause photoinhibition—a state where our photosynthetic systems are overwhelmed, leading to damage. This can manifest as leaf scorch, wilting, or faded flower color. In these instances, light afternoon shade, or dappled shade, acts as a protective barrier, preserving our cellular integrity and reducing water loss through transpiration. However, deep or full shade is detrimental. In such low-light conditions, our growth becomes etiolated and weak, our stems spindly, and our flowering ceases entirely as we divert all energy into a futile search for light. The soil also remains damp for longer periods, creating an environment conducive to root rot.
It is crucial to note that not all geraniums are identical in their requirements. The common Zonal geraniums (Pelargonium x hortorum) are the most sun-hungry, truly requiring full sun for best performance. Ivy geraniums (Pelargonium peltatum), often used in hanging baskets, also prefer full sun but can exhibit more tolerance to very light shade. The Regal or Martha Washington geraniums (Pelargonium x domesticum) are more particular; they enjoy bright morning sun but benefit significantly from protection against the harsh afternoon sun to prevent petal and leaf scorch. Finally, the true hardy geraniums (Geranium spp.), or cranesbills, are a separate genus and often prefer partial sun conditions, thriving in cooler spots with bright but indirect light.
