When it comes to lighting a greenhouse, nothing works better than natural light. If you use your greenhouse to grow plants in winter, or if the structure or placement of your greenhouse limits the light that enters it, you may need supplemental light. Be aware that the wrong kind of light can stunt plant growth.
Not Enough Light
A common problem with greenhouse lighting is not enough light. If plants don't get enough light, they tend to stretch, growing ever higher, seeking more light. Light-starved plants become spindly and top-heavy. Energy that could go into leaves, flowers and fruit goes into stems, and the plant weakens as a result. Getting plants enough light involves making sure the artificial bulbs have a high enough wattage. It also involves making sure the light is close enough to the plant. For seedlings especially, the light should be only 1 to 2 inches from the bulb.
Too Much Light
Plants can get too much light. During the day, plants use light and water to make starches and oxygen. At night, the plant converts those starches to sugars and stores them. One of the problems with greenhouse lighting is that it can be left on around the clock to spur fast growth, but doing so compromises the health of plants. Plants given too much light become pale, sometimes sunburned. A period of roughly eight hours of darkness each night helps plants maintain their health.
The Wrong Kind
Plants use mostly red and blue light for photosynthesis. High-pressure sodium lights put out most of their light in the yellow range, which is virtually unusable by plants. Incandescent lights put out wider range of light, but they put out heat, something that can harm small, tender plants. Metal halide and fluorescent tubes are better choices. They are cool and efficient and they put out light the plants can use.
Uneven light means some plants will grow well while others languish. Though different plants have different light requirements, in general, greenhouse lighting should provide 20 to 40 watts of light per square foot, spread evenly across the growing surface. That light must also be able to reach all the leaves. Plants that are spaced too closely will have some leaves that are shaded all the time.
- North Carolina State University: Height Control of Commercial Greenhouse Flowers
- National Gardening Association: Grow Lights on Water Hyacinths
- Mother Earth News: Use the Right Light for Seed-Starting Success
- University of Illinois Extension: Growing Transplants
- Aggie Horticulture: House Plants
- The Growing Edge: Light
- Crop King: Light in the Greenhouse: How Much Is Enough?
- University of New Hampshire Cooperative Extension: Evaluating Supplemental Light for Your Greenhouse
- Chemistry World: What Do Plants Do at Night?
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