The Plectranthus scutellarioides, commonly known as Coleus, is a popular ornamental plant celebrated for its foliage, which displays an incredible spectrum of colors and patterns. The appeal of the Coleus rests on the vibrancy of its leaves. Maintaining this vivid coloration depends entirely on providing the correct amount of light, which is the most important factor in its care. Light requirements are not uniform across the species but change based on the specific cultivar and the pigments present in its leaves.
Understanding How Color Pigmentation Affects Light Needs
The striking colors of Coleus leaves are determined by specific pigments: chlorophyll for green, and anthocyanins for shades of red, purple, and maroon. Anthocyanins, responsible for darker colors, function as a natural sunscreen for plant cells, protecting chlorophyll from damage in high light conditions. When exposed to higher light levels, many Coleus varieties increase their production of these anthocyanins, which intensifies the red and purple hues.
This relationship divides Coleus into two major categories based on light tolerance. Varieties with deep red, burgundy, or dark purple leaves, often labeled “Sun Coleus,” have a higher concentration of anthocyanins. These darker varieties typically require four to six hours of direct morning sun to fully express their color without fading. They are better equipped to handle higher light intensity, which prevents their colors from becoming dull or washed out.
Conversely, varieties that are highly variegated, neon, or predominantly chartreuse and light green contain less anthocyanin pigment. These “Shade Coleus” varieties require bright, indirect light and are susceptible to leaf scorch from direct sun. If these lighter varieties receive too much sun, the cells can be damaged, leading to bleached or crispy foliage. Leaf color is the most reliable guide for determining sun tolerance, as darker colors indicate a greater need for light to maintain saturation.
Symptoms of Too Much or Too Little Sun Exposure
A Coleus provides clear visual signals when its light exposure is incorrect, allowing for timely adjustments. When receiving too little light, the plant exhibits etiolation, where stems become stretched and thin as it reaches for a light source. Under-exposure also causes leaves to grow smaller than normal. The vibrant colors will often recede or “revert” to a plain green hue as the plant prioritizes chlorophyll production over accessory pigments.
Signs of excessive sunlight are often immediate and damaging, appearing as leaf scorch. This is identifiable by crispy, brown margins or bleached white/yellow spots on the leaf surface. Lighter-colored varieties are vulnerable to this damage, especially from afternoon sun. Too much light can also cause the leaf coloration to become dull, washed-out, or faded as the pigments are overwhelmed. The plant may also wilt, even if the soil is moist, because the leaves transpire water faster than the roots can supply it under high heat and light.
Optimizing Placement Indoors and Outdoors
When growing Coleus indoors, placement near a window should be selected based on the plant’s light needs. An east-facing window is often ideal, providing several hours of gentle morning sun that is unlikely to scorch the leaves. South-facing windows deliver the most intense light, making them suitable only for the darkest, sun-tolerant varieties or if the light is filtered. For “bright indirect light,” the plant should be positioned where it sees a large portion of the sky but where the sun’s direct rays do not fall upon it for extended periods.
For outdoor placement, the difference between morning sun and afternoon sun is paramount. Morning sun, generally before noon, is less intense and provides necessary light energy without damaging heat. Intense afternoon sun, particularly from 3 PM onward, should be avoided for all but the most sun-hardy cultivars. These cultivars should still be monitored for signs of scorching. If moving a Coleus from a low-light indoor environment to a brighter spot outside, acclimate the plant gradually over about two weeks to prevent light shock and leaf burn.