Light system for Greenhouse

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. Artificial light is generally used in one of the following two ways or both in the greenhouse:

 1. To provide high intensity light when the natural sunlight available is not sufficient to provide optimal plant growth. 

 2. 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 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, can greatly enhance our growing success. 

There are four basic types of lighting available for greenhouse use: 

1. Incandescent 

2. High intensity discharge (HID) fluorescent

 3. Compact Fluorescent Lights 

4. Light emitting diode (LED) lights. 

Incandescent 

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 watts 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: 1. High pressure sodium lamps 2. Metal halide lamps. These lights resemble large incandescent lamps. They are very high wattage and produce light in the red and blue spectrum that are beneficial to various stages of plant growth.

  (a) High Pressure Sodium Lamps are considered to be the best lights available for providing supplemental greenhouse light. The light produced is in the yellow/ red spectrum, which is especially beneficial in promoting flowering/budding in plants. 

    (b) 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 vegetative plant growth. They are less popular than HPS lamps for flowering and fruiting, if metal halide lamps are used in flowering stage, they are often of a higher rated power, such as 1000 W, to enhance more red light output.

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.

Compact Fluorescent Lights 

The fluorescent lights produce very small amounts of infrared heat. 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) 

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. 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. LED lights consume up to 80% less energy than HID lights and up to 30% less than fluorescents.