Optimal light exposure for indoor plants
Finding the right level and kind of light exposure for indoor plants can be a bit of an exercise in trial, error, and long-term observation. Some guides will talk about North or South exposure in windows, but these distinctions matter less than observation of light conditions in urban environments with unpredictable vistas.
While every space and species is different, there are some general principles that can guide the placement and lighting of your chlorophyllic companions.
Find a space that feels like home
Each and every plant that grows indoors either grows somewhere in the wild, or has been bred as a cultivar from a wild species. Thus each and every houseplant has a set of environmental conditions for which it is optimally evolved. The key variables in the evolution of these ecosystem niches are soil, water, nutrients, and—importantly—light.
Looking at the morphology (shape) of a plant can give some important clues as to what kind of light conditions it has adapted.
Cacti, for example, have evolved large photosynthesizing stems that absorb light from all angles in their relatively bare environs, and their non-photosynthetic leaves have evolved into prickly spines to keep grazers away from the water inside. Therefore, a cactus needs bright, 360º light exposure that is best provided by a solarium or under a strong artificial grow light. Too much one-sided light (like a small window), can cause a cactus to become etiolated (elongated, reaching for light), or cause it to grow unevenly (due to a growth hormone called auxin that curves plants in response to light).
With leafier plants than cacti, light needs can be predicted (roughly, and with many exceptions), by the surface area of the leaves: although this is truer for shade-lovers than the other way around. Consider leaves as the sort of biological solar panels they are: the less light available, the bigger they need to be to generate the same amount of energy. Thus large, broad-leafed plants with deep green leaves that would be at home on a forest floor—like peace lilies (Spathiphyllum spp.), or Devil’s ivy (Epipremnum aureum)—are well-adapted to growing in low or normal indoor light. In response to too much light, their leaves may burn, or grow much smaller than usual. Indeed, abnormalities in leaf size can be a diagnostic tool for assessing lighting conditions.
Turn, turn, turn
Auxin, mentioned above, is a plant growth hormone that suffuses the growing tip on the “dark side” of a plant. It causes the cells on the dark side to grow longer, which in turn causes the plant to curve towards the light. In this way, relatively immobile entities like plants can chase the light, within limited dimensions.
For windowsill herb and vegetable gardens, or window-adjacent houseplants, turning them 180˚ on a regular schedule is key to provoking even and attractive growth. This allows the auxin in the plant to spiral around the growing tip, avoiding desperate, uneven and tilted growth towards the one-sided light source.
Windows are all well and good, but for many plants beyond the usual low-light loving houseplant suspects, some supplementation of indoor lighting may be in order. Thankfully, with the continued development of LED technology, indoor grow lighting is becoming cheaper, longer-lasting, and much less energy consuming than ever.
Full-spectrum solar bulbs are the sort of multivitamin for plant growth. They should be adjustable, so that the light source can be placed close to seedlings or small plants, and be ratcheted upwards at the plant grows larger. Without the ability to adjust the proximity of the bulb, young plants run the risk of growing weak and thin stems striving for a far-away light source, and established plants run the risk of being burned or singed on a too-close bulb.
For the more advanced grower, warm (red) and cool (blue) bulbs can be used to provoke different kinds of growth, or in concert with one another for plants in multiple stages. Warm lighting will generally provoke flowering, while cool lighting will promote the growth of leaves and shoots (called vegetative growth).
Light exposure can go wrong for a variety of reasons: maybe a window isn’t as sunny as previously thought, or a blue grow light is situated too far away for a plant to make any use of it whatsoever. In any case, learning more about the plant you are trying to grow and how it grows in nature is the first step in providing the right conditions in your home.