From a plant's perspective, the question of sun versus shade is fundamentally about energy management and water relations. Our leaves are solar panels, and our internal plumbing system, the xylem and phloem, is how we move resources. Full, direct sun provides immense energy for photosynthesis, the process where we convert light, water, and carbon dioxide into sugars for growth. However, this intense light also dramatically increases the rate of transpiration—the evaporation of water from our leaves. For many varieties of Impatiens, particularly the traditional types (Impatiens walleriana), our vascular systems are not robust enough to pull water from the soil at a pace that can keep up with the water loss from our broad, tender leaves on a hot, sunny day.
When placed in full sun, an Impatiens plant experiences significant stress. The initial response is wilting; our leaves droop as water loss outpaces water uptake. This is a defensive tactic to reduce surface area exposed to the sun. If the stress is chronic, the consequences are more severe. The intense solar radiation can actually cause sunscald, damaging the chlorophyll in our leaves and leading to bleached, white, or brown crispy patches. This directly impairs our photosynthetic machinery. Furthermore, the constant effort to cool ourselves through transpiration diverts energy from flowering and growth. You may see fewer blooms, and the overall plant will appear stunted, leggy as it stretches for relief, or simply burnt.
Our natural habitat provides the answer. Many Impatiens species evolved as understory plants in forests, thriving in the dappled light that filters through the canopy above. This environment provides the perfect balance: bright, indirect light for efficient photosynthesis without the scorching heat and dehydrating intensity of direct midday sun. In these conditions, our relatively shallow root systems can easily maintain adequate hydration, and our energy can be directed toward producing a prolific display of flowers to attract pollinators, which is our ultimate reproductive goal.
It is crucial to note that not all Impatiens are identical. Through selective breeding and the introduction of new species, some cultivars have developed a much higher tolerance for sun. The SunPatiens® series, for instance, is a hybrid specifically bred with a more efficient and resilient vascular system. These plants can transport water more effectively, preventing wilting and mitigating heat stress. Similarly, New Guinea Impatiens (Impatiens hawkeri) possess naturally darker, thicker, and more leathery leaves. These leaves contain more compounds that act as a natural "sunscreen," protecting the delicate photosynthetic tissues within, and their thicker texture reduces water loss, allowing them to handle several hours of direct morning sun quite well.
The definition of "full sun" is not absolute from our perspective; it is deeply influenced by the local climate. In a cool, coastal climate with high humidity and temperatures that rarely soar, a traditional Impatiens might tolerate nearly full sun because the atmospheric demand for water is lower. Conversely, in a hot, dry, inland climate, even a few hours of intense afternoon sun could be fatal. Microclimates also matter immensely. A spot that receives full sun but is protected from harsh afternoon rays, perhaps by a building that provides shade later in the day, is vastly different from an exposed western exposure that bakes in the hot sun from noon until evening.
