Light for Houseplants
How much light do my houseplants need?
Lighting is the most important factor in deciding which plants belong in your indoor landscape. Unfortunately, in this case, our complex and sophisticated eyes work against us.
Human eyes are very adaptable. This makes us poor judges of light intensity. As we move from room to room, our eyes adjust extremely well to different light levels. Too well! The desk space by the window may be ten times brighter than the “slightly” darker workspace across the hall. Our eyes adjust even as we turn our gaze. And, after a couple of minutes, both light levels may look like “bright, indirect light” to us.
How do we measure light?
Light is measured in specific units, the same way distance is measured in units. Common units of distance are inches, meters, and miles. Likewise, there are many units for measuring light based on human eye sensitivity: foot-candles (ftc), lux, candela, and lumens¹. Other units are commonly used to measure light for plant growth. In this article we will use foot-candles, abbreviated ftc. One foot-candle is approximately the brightness of one candle, one foot away.
Even in the winter, the light outside, at noon, can reach over 4000 ftc. In a north facing window, however, it may never reach even 400 ftc. In an office lit only by fluorescents bulbs, the light levels are often 40 ftc or fewer. Incredibly, we can see extremely well in all three environments, and after only a few minutes of acclimating. Our eyes are extraordinarily adaptable.
For this reason, terms like “low light” or “bright indirect light” can be vague and even misleading. Our eye’s adaptability can make it very hard to compare light levels. This is especially true indoors, where the differences in light levels are less obvious.
Measuring light in your indoor landscape
The best solution to measure light levels is to use the appropriate equipment. A hand-held, digital light meter is often the best tool for the job. Purchasing expensive models, like those designed for precise research or those used by photographers, is not necessary. If your unit displays readings in foot-candles or lux, and is accurate to one foot-candle, it will do for your houseplants.
There are also many phone applications that help your device measure light levels. The accuracy of these measurements varies based on the specific device and calculation method. They are usually accurate enough to determine a change in light levels but should only be used for approximations.
Below are guidelines for determining the light levels in your home. The terms “low,” “medium,” and “high,” like the term “bright, indirect light,” are generalizations, not scientific terms.
| Foot-candles (mid-day) | Light Level | Description of area’s light intensity at mid-day |
|---|---|---|
| 25-100 ftc | Low | Areas with light intensity this low usually receive very little natural light. Many are lit by overhead lights only. They are often far from windows or found near a heavily shaded area. |
| 100-500 ftc | Medium | Areas with more moderate light intensity are usually near windows but receive no direct sunlight. They are often found in unshaded, north facing windows or in shaded east or west facing windows. |
| 500-1000 ftc | High | Areas with higher light intensity are usually near windows and may receive some direct light. If there is direct light, however, it is softened by shade from window treatments or mottled by outdoor foliage. They are often found near unshaded east or west facing windows. It may also be found near shaded south facing windows. |
| Over 1000 ftc | Direct sunlight | These areas are directly in front of windows with nothing between the plants and the sun but clear glass. Four or more hours of exposure to the sun’s rays is best for plants requiring “direct light.” This is usually found in unshaded south or southwest facing windows. |
Which plants can I grow?
What plants can be grown in your home depends on several factors. Temperature, humidity, watering, and fertilization are all under a homeowner’s control. Light intensity, on the other hand, can be very difficult to adjust. The direction a window faces, the weather and landscape outside, the time of year; all these affect how much light reaches your houseplant.
Most plants have a very narrow range of light intensities they can tolerate. For this reason, light levels should be the first thing you consider when choosing a houseplant. Even indoors, remember, “right plant, right place.”
In the home, where sunlight is scarce, houseplant enthusiasts have two choices. They may choose plants adapted to the light they have available or they can install additional “grow” lights.
Below is an infographic estimating light and water requirements for popular houseplants. Read on for answers to common questions about artificial lighting and grow lights.
Questions about artificial lighting
Q: Can plants be grown entirely under artificial light?
A: Yes. Plants need light to convert carbon dioxide and water into the sugars that fuel their growth. If the light source is appropriate, many plants will not be able to tell the difference. See the questions below to understand which light sources are appropriate for plants.
For some plants the amount of time they spend under the light source will encourage or discourage flowering. Be sure to research your plant’s specific light needs.
Q: Does the type of bulb matter?
A: Yes and no. Plants can be grown under a number of different light sources, but some are better than others. Below are the three most common.
- Incandescent bulbs can be used but they are very inefficient at converting energy to light. Because of this, they are expensive to keep on for long periods of time. They may also produce too much heat and far-red colored light for some plants.
- Fluorescent lights last longer than incandescent bulbs. They also more energy efficient.
- LED lights are currently the most expensive to buy, but are becoming less so. Long-term, however, there are significant savings in LEDs. These lights last much longer than incandescent and fluorescent lamps. They also use less electricity to produce the same amount of light.
All three of these options will work as grow lights, but the best options are fluorescents and LEDs. Incandescent bulbs are hot, short-lived, and expensive to keep on for the eight or more hours most plants require.
Q: Does the color of the light matter?
A: Yes. The light we receive from the sun, and light bulbs, is actually many colors mixed together. A prism can separate the light and show you the full rainbow of colors in sunlight. Although plants do not need all of these colors of light to survive, they will grow better under a broad spectrum of light. Sunlight provides a broad spectrum of colors, but bulbs labeled “broad spectrum” are also available on the market.
Photosynthesis is the process by which plants absorb light. From this energy they produce the sugars they need to grow and thrive. A major light-absorbing pigment in this process is a protein called “chlorophyll”. Chlorophyll is green. This is why plants are green; they are full of chlorophyll.
For grow lights, lights labeled “cool white” work well. “Warm white” and “daylight” may also work. Lamps tailored to a plant’s needs are also available.
For more information about houseplants or grow lights, contact your county Extension office.
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Notes
1. Lux is one lumen per square meter, which differs from foot-candles by about a factor of 10.