Artificial Light for the Greenhouse
by John Berends
Free standing greenhouses placed away from buildings, trees and other obstructions may not require artificial light most of the time if they are in areas of the country with optimal light conditions. For the most of us, and particularly the home greenhouse enthusiast who generally has a greenhouse connected to the house on one or more sides, and thus partly shaded by it, or in a part of the country not blessed with optimal growing conditions, artificial light may not only be beneficial, but required for year round productive growing.
Artificial light is generally used in one of the following two ways or both in the greenhouse:
To provide high intensity light when the natural sunlight available is not sufficient to provide optimal plant growth.
To extend the hours of natural daylight or to provide a night interruption to maintain the plants on long-day conditions.
When light is provided at optimal levels, where it was lacking before, it can significantly increase the health, strength, growth rate and yield of your plants. Supplementing natural sunlight in a backyard greenhouse allows for the virtual elimination of seasonal and geographical restraints. In addition, by extending the day length with supplemental lighting, you can greatly enhance your growing success. Container plants that are kept outdoors on decks and patios during the summer can be moved indoors during the winter under artificial lighting, allowing them to thrive year round.
There are four basic types of lighting available for greenhouse use: incandescent, high intensity discharge (HID) fluorescent, high intensity fluorescent and light emitting diode (LED) lights.
Incandescent lights do not require ballasts like other more expensive plant lights, but they generally do not provide sufficient light for most greenhouse applications. Small lamps with incandescent light bulbs can offer intrigue to the greenhouse and provide light for reading or nighttime accent. Incandescent bulbs also work when used in wall fixtures to light greenhouse doorways. Lights known as “Spot Grow” are also available in 75 and 150 watt and can be used to provide supplemental light to single plants. They do, however, produce substantial heat, so care must be taken not to place them too close to foliage.
High Intensity Discharge (HID)
There are two types of (HID) lamps: high pressure sodium lamps and metal halide lamps. These lights resemble large incandescent lamps. They are very high wattage and produce light in the red and blue spectrums that are beneficial to various stages of plant growth. High Pressure Sodium Lamps are considered by many growers to be the best lights available for providing supplemental greenhouse light. The light produced is in the red/orange spectrum, which is especially beneficial in promoting flowering/budding in plants. Metal halide lamps are better for use as the primary light source for plants in settings without much natural sunlight. The light produced is weighted in the blue spectrum, which is especially beneficial in promoting plant growth. The fluorescent lights produce very small amounts of infrared heat.
Conversion lamps are available which have ballasts and fixtures that can be used with either high pressure sodium or metal halide bulbs. This allows for the use of metal halide bulbs for advanced growth and high pressure sodium bulbs during the flowering and fruiting stages. There are two disadvantages that these lights have for greenhouse use. They require large heavy ballasts that are generally separate from the reflectors (light fixtures). These ballasts take up considerable space and generally detract from greenhouse esthetics. These lights also produce considerable heat which may be detrimental to the greenhouse during warmer seasons. The heat requires that the lights be placed a substantial distance (30-36 inches) from plants, which diminishes the light available to the plants. These lights are also more expensive to operate than other light options.
Traditional T12 and T8 fluorescent fixtures are simply not powerful enough to light an area more than 8" - 10" below the bulb. This type of light can work for starts and seedlings, or overhead working light, but is a poor light source for growth and budding, primarily because of low lumen output. With the advent of T5 lights which put out over three times the light of traditional florescent fixtures and in warm and cool spectrums, fluorescents can now provide a worthy alternative for greenhouse lights.
These new fluorescent lights have the advantage of higher light efficiency with low heat. A common strategy is to mix cool with warm bulbs to provide a full spectrum for plant growth and flowering/fruiting. Although these lights do not match the intensity of HID lamps, the difference in light intensity at the leaf surface, where it matters, is similar due to the fact that the cooler florescent bulbs can be placed much closer (6-18 inches) to plants. Remember that the intensity at plant level decreases exponentially for each inch that lights are raised above the leaf surface. Fixtures for these florescent lights have a much lower profile than HID fixtures, as well as light weight self contained ballasts.
Light Emitting Diode (LED)
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. Additional advantages include the production of very little heat and long life.
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 and generally there is no light produced except that which promotes photosynthesis. Since humans see green-yellow light best LED grow lights appear dim to our eyes. Manufacturers also state that LED lights consume up to 80% less energy than HID lightsand up to 30% less than fluorescents. LED grow lights can also last up to 11 years before needing changes so this is an exciting new technology that will be interesting to watch as it develops.
Guidelines & Sources
A general guideline for greenhouse lighting is 25 watts per square foot of garden space if the artificial light is used as a primary light source. One fourth the wattage is needed when using LED lights. Less is needed if artificial lights are used to supplement natural sunlight or if you are growing plants that do not require as much light (i.e.: lettuce). However, many gardeners prefer to double or even triple the recommended wattage to achieve faster growth rates with high light plants. Keep in mind that plants need periods of darkness too. Most indoor gardeners use lighting from 10 to 16 hours per day.
Light Hangers and Movers
Regardless of which type of light is installed it is important to have a good suspension system that will allow for the height of the light to be adjusted easily as plants grow. Also, make sure to check to see that the system you buy is rated to hold the weight of your lights.
Light movers are used to move lights back and forth, on a track or cable, across the growing area. Using these devices can allow you to increase the coverage of your lights and with lights that produce excess heat, they offer some cooling effect that can allow lights to be a little closer to the plants than with static lights. Movers also simulate the movement of the sun which gives plants some light penetration from the sides rather than just intense light from above. It may take some experimentation; however, to determine how much light each plant under the track is getting and to what extent moving light may impact plant growth.
John Berends is a greenhouse grower and Garden & Greenhouse contributing editor.