A Guide to Giving Your Houseplants the Perfect Sunlight

Monitoring the lighting needs of houseplants can be a challenge. Here’s a guide to getting it right, no matter the time of year. Read on to learn how to gauge indoor light intensity and make changes for healthier plants.

Every plant grower knows that light is critical for healthy houseplants. Without it, your plants will wither and never reach their full potential. However, determining precisely how much light each plant needs can be challenging, and you may feel limited by what’s naturally available in your space.

Light, whether natural or artificial, provides plants with the energy they need to photosynthesize. While lighting requirements vary by variety, all species need a minimum light exposure to continue growing.

The key to healthy indoor plants is determining how much light they need and modifying your space to accommodate those requirements. Unsure where to start? Here’s what you need to know.

More or Less Light?

It can be challenging to determine whether your plants require more or less light exposure. Some signs that your place has too much light exposure include:

In contrast, slow, leggy growth is a sign that plants are struggling to get enough light and need more direct exposure.

Understanding Indoor Lighting Needs

Growing plants indoors is a far different process than outside. Even the brightest window lets in only a fraction of light compared to being outside.

As a general rule, expect to lose up to 50% of the light intensity even just one meter away from the window. But even so, the window you choose will make a big difference in how much light exposure your houseplant gets.

The Importance of Window Orientation

Where your windows are located in the home has a direct effect on the light exposure for your houseplants. Here are the general rules for everyone in the Northern Hemisphere.

North-Facing Windows: These windows tend to provide the weakest light, and often spend most of the day shaded out. They will work for shade-loving plants but might be too dark to support plant growth in the winter.
South-Facing Windows: Known for providing the strongest light intensity, these windows are ideal for sun-loving plants that can tolerate direct light. Any outdoor plants brought inside should be put near south-facing windows to maximize sunlight exposure.
East-Facing Window: These windows tend to get lots of early morning light exposure, but this light isn’t as strong as it would be midday. Keep plants that need moderate sunlight in these spaces.
West-Facing Window: These windows get lots of afternoon and evening sun. In fact, the light exposure can be as strong as south-facing windows in the summer months. Keep your most sun-loving plants in this space.

How to Measure Light Intensity

The intensity of light is measured in units known as foot candles (ftc). As a way of explanation, one foot candle is the approximate brightness of a single candle placed one foot away.

Outdoor light is exceptionally bright compared to indoors. Even in the middle of winter, outdoor brightness easily reaches over 400 ftc. In contrast, north-facing windows might not ever get over 40 ftc.

What do these terms mean for light intensity? Here’s a primer on different indoor light intensities:

20-50 ftc: Low light intensity, similar to deep shade. This is too dark for most plants.
50-100 ftc: Low light intensity, but considered bright enough to read. This is suitable for shade-loving plants.
100-200 ftc: Bright but indirect sunlight that will produce a shadow. This works best for plants needing indirect light.

200-400 ftc: Considered to be about 40% of the light intensity of the midday sun, this bright indoor light is good for plants that require lots of light exposure.
400-500+ ftc: Measuring about 50% the intensity of noon light, this strong, direct light brightens a room and is ideal for plants that require lots of sunlight.
1,000-1,200 ftc: Full sun outdoors.

Note that it’s hard to gauge light intensity objectively, as our eyes acclimate to different lighting conditions and will perceive them as more similar than they are.

One way to gauge the lighting is to assess the shadows cast. Sharp, well-defined shadows are signs of bright light, while fuzzy, indistinct shadows indicate moderate light. If shadows are faint or non-existent, then you’re dealing with low light intensity.

A better option is to measure light intensity with professional tools, such as a light meter. Look for the type that will give you a reading in foot-candles or lux (one foot candle is equivalent to about 10 lux). You can also consider using phone apps for a measurement, though these tend only to give estimations.

How to Manage Sunlight Intensity

The brightest indoor spaces rarely approach even half the brightness of being outdoors. Even so, it’s possible to burn plants by placing them too close to windows. That’s due to the magnifying effects of the glass, which can concentrate the heat in one direction. For this reason, it’s important to keep plants a few feet away from glass on the brightest days.

You can also dim down the reflective power of windows by covering them with sheer curtains that filter the light.

If you want to enhance the light within a room naturally, ensure that walls are painted light colors and place mirrors around the room. Both will reflect light around and boost the ambient brightness within your space. However, the easiest way to increase light intensity is by adding grow lights to the planting space.

Note that light intensity within your home will change throughout the year. Houseplants that thrive in sunny south-facing windows in the summer might need supplemental light to get through the winter months when sunbeams are weaker and angled lower in the sky. Likewise, you may need to move sensitive plants to a more protected space once the summer sun starts to heat your windows.

For this reason, it’s important to be flexible with your houseplant placements. For certain varieties to be at their healthiest, they may need to rotate through your home to optimize their light exposure for the time of year.