Whether it’s to provide supplementary lighting or to extend the length of the photoperiod, some greenhouse growers are equipping their greenhouses with artificial light sources. Before purchasing a lighting system for a greenhouse, a grower should first determine which type of lighting system will best meet his or her needs.
T-12 fluorescent lights are the standard four foot fluorescent lights commonly used in garages and shops. In fact, many people refer to T-12 fluorescents as “shop lights”. Although T-12 fluorescents are just fine for starting seeds indoors or illuminating indoor houseplants, they usually lack the intensity and efficiency to provide light to plants in a greenhouse.
T-5 fluorescents are a more powerful and efficient version of fluorescent lighting. Still available in four foot lengths, the skinnier T-5 bulbs are 54 watts each. Specialty horticultural enhanced bulbs are available for T-5 fixtures in both the warm (red) and cool (blue) spectrums. The warm spectrum bulbs are rated around 3000 K and the cool spectrum bulbs are rated around 6400 K. During the fruiting or flowering stage of growth most plants require a higher percentage of red spectrum light. During the vegetative stage of growth plants require more blue spectrum light. Many T-5 fixtures hold multiple bulbs and allow “banks” of bulbs to be turned on or off. In other words, it is possible to set up a T-5 fixture to customize the light spectrum in your greenhouse. During the vegetative stage, the bank with the blue bulbs should be on. When the transition or flowering stage begins the banks with the red spectrum bulbs can be activated in order to give the plants the more appropriate spectrum for that stage of growth. Generally speaking, T-5 fluorescents are the most cost effective option for the hobbyist greenhouse grower.
Metal halide is a type of high intensity discharge (HID) lighting that can be used in greenhouses. Metal halide lighting systems are more intense than fluorescent lighting fixtures and are a good choice when growing roses, tropical palm, or other plants that will require more intensity than fluorescents can provide. Metal halides generally produce a white or blue spectral output which makes them a good fit for vegetative growth. Metal halides are commercial quality fixtures that come in 400, 600, or 1000 watts. Metal halide lighting systems require three components: the bulb, the ballast, and the reflector or socket base. These lighting systems create a good amount of heat and can be quite expensive to operate. However, in some cases, metal halide is the only way to provide plants with the intense blue light spectrum they require for vegetative growth.
High Pressure Sodium
Like metal halide, high pressure sodium is a type of HID lighting. These lighting systems are also available in 400, 600, and 1000 watts. The biggest difference between high pressure sodium and metal halide is the spectral output. High pressure sodiums provide a good amount of red spectrum. This makes these lighting systems a great fit for fruiting or flowering plants. Since high pressure sodiums are also a type of HID lighting system, they will also require a ballast and reflector or socket.
If you have determined that your greenhouse will require a HID lighting system and you plan to provide artificial light during the plant’s vegetative and flowering stages, you should look into a convertible ballast. Convertible ballasts allow the grower to use both metal halide and high pressure sodium bulbs in the same fixture (but not simultaneously). A grower can use his or her metal halide bulb during the vegetative stage and then switch to a high pressure sodium bulb during the flowering stage.
There is certainly an emerging opportunity for LED lighting systems to be incorporated into the greenhouse. Consumers should be aware that LEDs are very spectral specific and they need to make sure the LED light system they purchase provides the spectral output required by their plants. Unfortunately, there are a lot of variables to consider when comparing LED lighting fixtures. The light spectrum, actual wattage, and type of lens used are all things to consider when purchasing a LED system. There is also a wide variance in overall quality. Many of the cheap LED fixtures will not stand up to the conditions found within a greenhouse. There is no doubt that LED lighting systems will continue to shape the future of horticulture but it is important for consumers to closely examine what they are purchasing.
Maximum Photosynthetic Activity
400-500 and 600-700 nm