How Much Light do Plants Really Need?

Light is the most important part of plant care as they need it to photosynthesize. Lighting affects the growth of a plant and how long the growing period is. When considering light levels for indoor plants look at the intensity, duration, and quality of light.

Light intensity influences how a plant makes food, stem length, leaf color, and flowering. A plant grown in low light will generally have very long stretched out stems, called etiolation. Certain plants will have different colors of leaves depending on how much light they receive. The intensity of light a plant gets depends on the nearness of the light source, with intensity getting lower as you move away from the light source whether it’s from a window or artificial lighting.

Southern facing windows have the most intense light, east and west windows get about 60% of the light a southern window gets, and a northern facing window gets about 20% of light a southern window gets. Other things can influence how much light comes through a window, like curtains, trees, weather season, other buildings, and the cleanliness of the window. Light colored surfaces will increase the intensity of light available and dark colored surfaces will decrease light intensity.

Duration of light is of some importance but usually only to plants that are photosensitive, like the Poinsettia, Kalanchoe and Christmas Cactus that only flower when the day length is short. Most flowering indoor plants are indifferent to day length.

Low intensity light can usually be compensated for by increasing the time plants are exposed to light, but no more than 16 hours per day. Plants need some period of darkness to develop properly. Too much light is just as harmful as not enough light. When a plant gets too much direct light the leaves can become pale, sunburn, turn brown and die.

There are a few plants that can tolerate bright direct sun, but may need to be acclimated slowly to increasing light if the plant is currently in low light. Putting a plant directly from low light to direct sunlight, or vice versa, is likely to kill the plant as plants produce different leaves for bright and low light. Sun leaves grown in bright direct light will have less chloroplasts and chlorophyll, while shade leaves grown in low light will have lots of chloroplasts and chlorophyll. If a plant grown in bright direct light is placed in low light, it will have to either alter the existing leaves or drop all the sun leaves and grow back shade leaves that are more efficient at photosynthesis. To reduce shock when moving a plant from bright to low light, or the reverse, you need to acclimatize it by slowly reducing, or increasing, the amount of light it receives over a 4-8 week period.

Plants that can tolerate direct light are Asparagus Fern, Foxtail Fern, Donkey’s Tail, Poor Man’s Croton, Dumb Cane, Devil’s Ivy, Jade Plant, Ponytail Palm, Rubber Plant, Schefflera, Snake Plant, Swedish Ivy, Velvet Plant, and Wandering Dude.

Plants that can tolerate low light are Areca Palm, Chinese Evergreen, Corn Palm, Heart-leaf Philodendron, Maidenhair Fern, Parlor Palm, and Snake Plant.

Most other plants do just fine in average to bright light.

This post was written using information from Michigan State University's Master Gardener Extension textbook.