Understanding the specific quality and intensity of light entering a room is fundamental to plant health, as light drives photosynthesis. Terms used in horticulture, such as “bright indirect light,” are highly subjective and often lead to misplaced specimens that fail to thrive. This article clarifies the common assumption that a north-facing window automatically provides bright indirect light.
Decoding Plant Light Terminology
Indoor light is generally categorized by intensity, and these terms relate directly to how much energy is available for a plant’s growth. Direct Light refers to unfiltered sunlight that creates sharp, well-defined shadows and typically measures over 2,000 foot-candles (ftc) indoors. This high intensity light can easily scorch the leaves of tropical houseplants that are not adapted to it.
Indirect Light is light that has been scattered, filtered through a sheer curtain, or reflected off a wall or ceiling before reaching the plant. This diffused light prevents leaf burn but still provides the necessary energy for growth. The term Bright Indirect Light is used for the highest concentration of this diffused light, generally ranging from 1,000 to 2,000 ftc, and is preferred by many common houseplants.
A simple way to determine the light quality is by observing the shadow cast by your hand at midday. A sharp, distinct shadow indicates direct sun, while a soft, fuzzy shadow suggests indirect light. Low Light conditions, where shadows are barely visible or non-existent, fall below 250 ftc and are only suitable for a limited number of highly tolerant plant species.
How Window Orientation Affects Light Quality
The intensity and duration of light a window receives are entirely dependent on its orientation relative to the sun’s path across the sky in the Northern Hemisphere. South-facing windows receive the longest duration and the highest intensity of sunlight throughout the day, particularly during winter. This exposure provides the most powerful direct light, which must often be filtered or moved back several feet to achieve indirect light conditions.
East-facing windows are exposed to the gentle, cooler sun of the morning hours, making them excellent locations for plants that prefer bright, unfiltered morning light. Since the morning sun is less intense than the midday or afternoon sun, it is less likely to cause leaf burn, often naturally providing a form of bright indirect illumination.
West-facing windows receive the hot, intense afternoon sun, which can quickly overheat and damage foliage. While the light is direct for several hours, it is often considered less powerful overall than a full day of south-facing exposure.
Conversely, north-facing windows never receive direct sunlight in the Northern Hemisphere. This fundamental difference dictates the light quality they offer compared to the other three directions.
North-Facing Windows: Classification and Plant Suitability
The light from a north-facing window is consistently soft and uniform throughout the day, which makes it reliably indirect, but it rarely qualifies as “bright.” In terms of measured intensity, a north-facing windowsill often registers in the range of 100 to 400 foot-candles (ftc), which places it firmly in the Medium to Low Light category. For comparison, a south-facing window can receive approximately five times that intensity.
This low intensity is due to the window only receiving light that has been reflected or diffused from the atmosphere and surrounding landscape. For a north-facing window to approach true “bright indirect light” levels, it would need to be exceptionally large, entirely unobstructed, and have highly reflective surfaces nearby, such as a bright white wall or a light-colored building. Without these factors, placing a plant that requires 1,000 ftc next to a north-facing window will likely result in slow, leggy growth over time.
Plants that naturally grow on the forest floor or in shaded understories are the best candidates for this soft illumination. These species are tolerant of lower light levels and will thrive in the consistent, gentle light provided by a north-facing exposure. To maximize the available light, ensure the windows are kept clean and position plants directly on the sill to benefit from every available lumen.
Suitable varieties include:
- The ZZ Plant (Zamioculcas zamiifolia)
- The Snake Plant (Sansevieria)
- Pothos (Epipremnum aureum)
- The Cast Iron Plant (Aspidistra elatior)