From a plant's perspective, the question of sun or shade is fundamentally about energy acquisition and metabolic management. We, geraniums, utilize sunlight to drive photosynthesis, the process where we convert carbon dioxide and water into carbohydrates (sugars) that fuel our growth, flowering, and overall vitality. Full sun provides the maximum intensity of light, which translates to a higher rate of photosynthesis and, consequently, more energy. This abundant energy allows us to produce our characteristic vibrant and prolific flower clusters, develop sturdy stems, and synthesize the aromatic oils in our leaves that are part of our defense mechanism. Without sufficient light, our photosynthetic output drops, leading to reduced flowering, leggy growth as we stretch desperately for a light source, and a general weakening that makes us more susceptible to pests and diseases.
Our preference, as most geranium cultivars (Pelargonium x hortorum), leans decisively towards full sun. We thrive best with a minimum of six to eight hours of direct, unfiltered sunlight each day. In these conditions, our internal systems operate at peak efficiency. The ample light ensures compact, bushy growth with strong internodes (the spaces between leaves). The energy surplus is heavily allocated to our reproductive structures—the flower buds. This results in the most spectacular and continuous displays of blooms that gardeners desire. Furthermore, the strong sunlight helps to quickly evaporate moisture from our foliage after rain or watering, which is crucial as we are susceptible to fungal diseases like botrytis or oedema in consistently damp conditions. Our leaves may also develop a richer, sometimes slightly reddish, hue in full sun, a sign of protective anthocyanin pigments.
However, our preference for full sun is not absolute and must be balanced with another critical factor: water management and heat stress. While we are sun-loving, we are not true xerophytes like cacti. Our root systems can be stressed by excessively hot and dry soil. In regions with intensely hot summers, especially in the afternoon, the "partial shade" you speak of—typically defined as four to six hours of direct morning sun with protection from the harsh afternoon rays—becomes highly beneficial, and sometimes essential. During the peak afternoon heat, the rate of transpiration (water loss through leaves) can exceed the rate of water uptake by our roots, leading to wilting, scorched leaf edges, and flower drop. In such environments, the respite provided by afternoon shade allows our internal water balance to stabilize, preventing stress and conserving energy that would otherwise be spent on damage repair.
It is also important to note that not all geraniums are identical in their requirements. Our species and cultivars have variations. For instance, the regal geraniums (Pelargonium domesticum) are more sensitive to intense heat and often perform better with bright but filtered light or morning sun only. The ivy-leaved geraniums (Pelargonium peltatum), while still sun-loving, can be more tolerant of some shade, especially in very hot climates. Furthermore, our specific microclimate matters. A geranium planted in a container on a hot patio will experience more root zone heat than one planted in the ground and may appreciate some afternoon shade. The key is to observe our physical responses. Lush green leaves and abundant flowers indicate ideal conditions. Yellowing lower leaves, leggy stems, and sparse flowering are a cry for more sun, while bleached, scorched leaves and chronic wilting are a clear signal of light and heat stress, necessitating a shift to a more sheltered location.
