From a botanical perspective, most common Impatiens walleriana (busy Lizzies) are naturally adapted to the dappled light of forest understories. Their succulent stems and broad, thin leaves are highly efficient at photosynthesis in low-light conditions but are poorly equipped for full sun. Intense, direct sunlight accelerates transpiration (water loss), leading to rapid wilting as the plant cannot uptake water from the soil quickly enough to replace what is lost. Furthermore, the high photosynthetic photon flux can cause photoinhibition, a state where the photosynthetic apparatus is damaged by excess light energy, resulting in sunscald (bleached or brown patches on leaves) and overall stunted growth.
However, through selective breeding and the introduction of new species into cultivation, several Impatiens varieties have developed physiological and morphological adaptations that allow them to tolerate, and even thrive in, full sun. The primary adaptation is a more robust and efficient root system that facilitates greater water uptake to keep pace with transpiration demands. Secondly, many sun-tolerant varieties feature leaves with a thicker cuticle (the waxy outer layer) or smaller, waxier leaves that reduce water loss. Some may also have more advanced photoprotective mechanisms, such as increased production of anthocyanin pigments (which can give foliage a reddish tint), which act as a "sunscreen" by dissipating excess light energy as heat, thereby protecting the photosynthetic tissues.
When selecting Impatiens for full sun, it is crucial to choose varieties that have been specifically bred for this purpose. The most prominent group is the SunPatiens® series. These are vigorous interspecific hybrids that exhibit exceptional heat and sun tolerance, strong root systems, and the ability to quickly recover from any wilting. They are available in a wide range of colors and sizes, from compact to spreading. Another excellent option is the Beacon® series, which is specifically bred for resistance to downy mildew and performs well in both sun and part shade. For a more native and drought-tolerant option, consider Impatiens capensis (Jewelweed), though it is often grown as a wildflower rather than a bedding plant.
Even with these adapted varieties, their tolerance is not absolute and is contingent upon proper horticultural practices. The most critical factor is consistent and ample moisture. While their roots are more efficient, they are not drought-tolerant and will require frequent, deep watering to maintain turgor pressure and cool root zones. Secondly, the soil must be exceptionally well-draining yet moisture-retentive, ideally amended with organic matter like compost. A layer of mulch is highly beneficial to help conserve soil moisture and keep roots cool. Finally, in regions with extremely intense afternoon sun, providing light afternoon shade can still be advantageous to reduce plant stress and conserve water.
