How to select the best Grow Light for your greenhouse
Why use a Grow Light?
Most plants need light to thrive because light is essential for photosynthesis. Without it, plants could not make food. But light can also be too intense, too hot, or last too long for growing healthy plants. In general, more light seems to be better. Plant growth accelerates with abundant light because more of the plant’s leaves have exposure; which means more photosynthesis. Two years ago I left two identical planters in the greenhouse for the winter. One was placed under a grow light and one was not. By spring, the difference was astounding. The plants in the container under the light were nearly 30% larger than those not receiving the additional light. Other than for those few months, the two containers have always been side by side. Years later it’s still evident which container was under the light. The container that did not get the added light is perfectly healthy, just smaller. With many plants, however, winter days are just not long enough. Many plants need 12 hours or more of light per day, some need as many as 18.
Adding grow lights to your greenhouse is an excellent option if you live in the North and don’t get many hours of winter daylight. Grow lights are an excellent option to replace some of the missing rays. Maybe you do not have an ideal southern location on your property for a greenhouse. Use grow lights to supplement the day’s length as well as the quality and intensity of light. If your greenhouse covering does not diffuse sunlight well, you can add lights to fill in shadows for more even growth.
Types of Grow Lights
Not all light is the same. Plants respond differently to different colors of light. Light on either end of the spectrum, blue light or red light, have the greatest impact on photosynthesis. Blue light, referred to as cool light, encourages compact bushy growth. Red light, on the opposite end of the spectrum, triggers a hormone response which creates blooms. Grow lights producing the orange and reddish lights typically produce substantial heat, however, some lights are able to produce full spectrum light with out the heat.
Grow Lights come in all shapes, sizes and price ranges. As a general rule, inexpensive lights to purchase tend to be the most expensive to operate and the least effective. While price is not necessarily an indicator of performance, many of the efficient grow lights require ballasts as well as specialized fixtures. There are a few basic types of grow lights:
Incandescent Lights, Fluorescent, T-5 Fluorescent, High Intensity Discharge (HID) and LED. These lights run the gambit of performance and price range.
The least expensive lights to purchase cost around $30. These incandescent lights work well for specific plants where the light is placed a minimum of 24” from the plant. These lights get extremely hot so they must be used with care. Spot grow bulbs, color corrected incandescent lights, install easily and are good for use with a specific plant or a small grouping of plants. Most spot incandescent bulbs last less than 1,000 hours. Some light fixtures come with a clip handle so you can put them exactly where they’re needed.
Fluorescent grow lights are a common choice for homeowners. Fluorescent lights are reasonably energy efficient and relatively easy to install. A typical fluorescent bulb will last approximately 20,000 hours. Fluorescent light is typically on the blue end of the spectrum. Blue light encourages bushy compact growth which makes them perfect for seed starting. Blue light is also cool to the touch making it possible to place lights within just a few inches of the seedlings.
New full-spectrum fluorescent lights provide the red spectrum as well to encourage blooming. Combining the lights in a fixture makes for even, all around growth. The next generation in fluorescent lighting includes the new T-5 lights. These new lights have extremely high output but are energy efficient and long lasting. The T-5 lights triple the light output of normal fluorescent lights without increasing the wattage. Plants absorb a high percentage of T-5 lighting because the fixtures function well very close to plants. High output bulbs require a high output fixture to operate, so the bulbs and normal fluorescent fixtures will not work together.
Many commercial growers use High Intensity Discharge lights because they have extremely high output and cover a wide area. HID lights hang high above plants (hanging height determined by the wattage) so the light works well for large growing areas. Operating HID lights require a ballast to deliver power to the lamp and fixture for the light. Most fixtures have a reflective hood which directs light back towards plants. HID lights emit high heat, so they must be placed away from plants to avoid burning the leaves. There are two types of HID lights, Metal Halide and High Pressure Sodium.
Metal Halide produces blue light that most closely resembles natural light. This would be the preferable light if used as the primary lighting because plant growth most closely resembles plants grown outdoors. This type of lighting is excellent for areas that do not get natural sun.
The second type of HID lighting, High Pressure Sodium, is excellent for supplemental lighting in a greenhouse because the red light enhances flowering. The bulbs also last substantially longer than Metal Halides. Used as supplemental lighting, the orange-red light does not create leggy growth. It is interesting to note that plants do not appear as healthy under the reddish light as the blue because our eyes do not adjust well to the color. High Pressure Sodium bulbs have a long life; however, they should be replaced according to manufacturer’s instructions. Bulbs continue to light beyond their useful life, however, the energy draw increases dramatically and the output lowers significantly.
Metal Halide and High Pressure Sodium lights generally do not use the same fixtures, however, some fixtures are convertible. With convertible fixtures, bulbs swap easily. A simple flip of a switch converts the fixture after changing the bulb. This is a good option for serious growers who want the benefits of compact growth, blooms and energy efficiency. The start up cost is not insignificant.
However, the combination offers a tremendous amount of control in the greenhouse.
The newest type of grow lights use LED technology. One major advantage to the LED lights is the small size. LED lights are only a few inches in diameter and are easy to mount. In some greenhouses, LED lights may be the only practical light option. Hanging most grow lights requires a strong greenhouse structure and a place to hang the lights. LED lights weigh a fraction of other lights and are easy to configure where needed. According to LED manufacturers, LED grow lights maximize blue and red light to provide and excellent balance for plants. They do not have much green-yellow light. Since humans see green-yellow light best LED grow lights appear dim to our eyes. This is an exciting new technology that will be interesting to watch as it develops.
Now that I’ve given you a good rundown on greenhouse lighting options, it’s also important to mention darkness. Almost all plants benefit from a period of six hours or more of darkness. It’s a good idea to know how much light your plants need, but unlike commercial growers, hobbyists often have a wide variety of pants so they need to take a broad approach to lighting. Fluorescent lights offer excellent overall lighting options.
The amount of daylight varies across the country. In some regions, the length of daylight changes quickly. Here in Oregon, the day will be three minutes shorter tomorrow than it was today. That means supplemental lighting requirements will change fairly rapidly from month to month. One of the best ways to manage those changes is with an automatic light timer. A timer will help you easily adjust to changing daylight lengths as well as save energy and bulb life.
If you chose to use any type of florescent lighting, you will need to account for plant growth. Fluorescent lights perform best when positioned very close to plants. As plants grow into the light, it is important to raise the fixture. Generally only the plants touching the lights will burn, but be prepared because they grow quickly. Adjustable hangers are a good solution. These hangers move easily allowing you to make quick adjustments.
Whether you just want to give a special plant a boost, or you plan to grow right through the winter, grow lights are a great option in a greenhouse. For supplemental light, grow lights operate for only a few hours a day. Once you have the fixtures and the bulbs there is little cost, but it is an investment to get started. When purchasing a system it’s a good idea to think about your goals and how you may want to change in the future. Do you want to start seeds and get a jump on the season, or do you want to create a year-round oasis? Keep in mind as you’re planning; as plants grow, so do your ambitions. Nothing feeds the soul in the dead of winter quite like walking into your greenhouse and being greeted with beautiful, healthy, thriving plants!
Michelle Moore is a member of Garden Writers Association. Michelle studied business and communications at Pacific Lutheran University in Tacoma, Washington. After graduation, she completed a Fulbright Scholarship then earned an
International MBA from Thunderbird, The Gavin School of International Management. With nearly 20 years of experience working with greenhouses, Michelle recently became an Oregon State University Master Gardener. Michelle and her husband find that even under gray skies you can find hints of summer all year when you have a greenhouse! She may be contacted at firstname.lastname@example.org or visit her website at www.greenhousecatalog.com
Published in Garden and Greenhouse Magazine, www.gardenandgreenhouse.net