Light is food for plants. Through photosynthesis, leaves turn light, water and carbon dioxide into the sugars that fuel growth. Without enough light, a plant slowly uses up its reserves, grows weak and becomes more vulnerable to overwatering and pests. Most houseplant problems begin with a plant in the wrong light.
Indoor light is far weaker than it seems to our eyes. A spot two or three metres from a window may look bright to you but receive only a small fraction of the light found next to the glass. Human eyes adjust automatically, so we underestimate how quickly light falls away from windows.
Plant labels often say bright indirect light, medium light or low light. Bright indirect light usually means near a window with lots of sky visible but little direct sun on the leaves for most of the day. Medium light is a little further back or near a smaller window. Low light means plants can survive, but rarely thrive, far from windows.
A simple way to judge light is the shadow test. Hold your hand above where the plant sits at midday. A crisp, defined shadow means strong light; a soft, fuzzy shadow means medium light; a faint or absent shadow means low light.
Common mistake to avoid
The most common mistake is buying a plant because it looks good in a dim corner. Even low-light plants such as snake plants and ZZ plants grow better within a few metres of a window. Choose the spot first and the plant second.
Try this
Use the shadow test at midday in three spots where you have or would like plants, and note whether each is strong, medium or low light.
Key takeaways
- Light is the main driver of plant health
- Indoor light falls away quickly from windows
- Use the shadow test to judge light levels