Mar 18, 2011
Supplemental lighting guidelines for greenhouses

High-intensity supplemental lighting can increase photosynthesis and plant quality of crops during periods of dark weather. However, until recently, the definition of “dark” weather was ambiguous and subjective. Research at several U.S. universities has generated specific information on how ornamental plants respond to the daily light integral (DLI). With this information, the benefit of supplemental lighting can be ascertained based on the ambient DLI and the plant response to DLI.

DLI refers to the number of light particles, or photons, received during one day in a particular location and area. Photons that have a wavelength between 400 and 700 nanometers (nm) provide the energy for photosynthesis, which is the process of converting water and carbon dioxide into sugars and oxygen. These sugars are used for plant growth. The DLI specifically refers to the amount of light received in one day in one square meter of area, which equals 10.8 square feet. Conceptually, DLI is similar to a rain gauge, where photons of light accumulate to a value at the end of the day. Thus, the DLI cannot be measured instantaneously; a light meter must be used at regular, frequent intervals throughout the day (for example, once every 10 minutes) to measure how much light is received. From those values, the DLI can be mathematically determined. Alternatively, a DLI meter, such as the Greenhouse Weather Tracker model 305 from Spectrum Technologies, can be used to automatically measure the DLI each day.

DLI is measured in mol·m-2·d-1, which means the number of particles of light (in moles) per square meter (m-2) per day (d-1). The maximum DLI we receive outdoors is about 60 mol·m-2·d-1 on a cloudless day in the summer. On a dark winter day in the northern part of the United States or Canada, the outdoor DLI could be less than 5 mol·m-2·d-1. Inside a greenhouse, the structure and glazing materials commonly reduce light transmission by 35 to 50 percent. Other obstructions, such as hanging baskets and heat pipes, further reduce the DLI. Therefore, the average DLI inside a greenhouse in the United States usually ranges from 5 to 30 mol·m-2·d-1.

What DLI is needed to grow high-quality transplants and finish plants? The answer depends on the crop, but a common target minimum DLI inside a greenhouse is 10 to 12 mol·m-2·d-1. In the northern half of the United States, the typical greenhouse DLI is below 12 mol·m-2·d-1 for about three months of the year. Therefore, the benefit of supplemental lighting to increase the DLI is greatest during these three months. In addition, growers are wise to maximize the amount of natural light that can reach their crops. For example, shading should be removed, the glazing should be clean and overhead hanging baskets should be kept to a minimum.

Except for shade plants, plant quality generally increases as the average DLI increases, even beyond 12 mol·m-2·d-1. More specifically, as the DLI increases, branching, rooting, stem thickness and flower number often increase and sometimes plant height decreases. When light is limiting, some crops also flower earlier when grown under a higher DLI from supplemental lighting.

Supplemental lighting is most efficient when delivered by stationary high-pressure sodium (HPS) lamps, because they are the most efficient at converting electricity into photosynthetic light. They also emit a lot of radiation (heat) that can increase media and plant temperature, which is an added benefit for growers in cold climates. For example, in a lighting study we performed in late January and early February in Michigan, HPS lamps at an intensity of 680 footcandles (90 μmol∙m-2∙s-1) increased the average plug media temperature by 4.3° F compared to the media of nonlighted plugs. The most common fixture consumes 400 watts, but 600- and 1,000-watt fixtures are also used and are slightly more electrically efficient. You can work with a greenhouse lighting company to develop a lighting plan (number and type of fixture, mounting height, type of reflector, etc.) for your needs.

HPS lamps with a rotating reflector are used to deliver photoperiodic lighting and are not effective at increasing the DLI. Metal halide lamps emit more of a blue-white light and are sometimes used by commercial greenhouse growers, but their bulb longevity is shorter and they aren’t quite as electrically efficient as HPS lamps. Numerous companies are developing light-emitting diodes (LEDs) for greenhouse applications. The efficiency and intensity of LEDs continues to rapidly advance. However, in the vast majority of situations, LEDs are currently not an economically viable option for growers of crops given their much higher purchase cost, lower light intensity, or both.

The amount that the DLI is increased with supplemental lighting depends on the intensity delivered at plant canopy level and the number of hours of operation per day. The most common supplemental lighting installation for greenhouses used to produce crops is 350 to 500 footcandles (equivalent to 46 to 66 μmol∙m-2∙s-1 from HPS lamps). Higher light intensities are often used in the production of greenhouse vegetables. The higher the intensity and the more hours that the lamps are used, the greater the increase in DLI. For example, operating HPS lamps that provide 500 footcandles of light for 16 hours per day increases the DLI by 3.8 mol·m-2·d-1. This may seem like a low value, but this could double the DLI on a cloudy, short day during the winter.

Supplemental lighting has the greatest effect on photosynthesis when lamps are used during darkness or when the ambient light levels are low. The benefit of supplemental lighting is low when natural light levels are moderate, and benefits are essentially nonexistent during sunny conditions. Therefore, to maximize use of supplemental lighting, operate lamps during a portion of the night, on cloudy days in the winter, and in the early morning and late afternoon on sunny winter days. For many crops, the value of supplemental lighting is relatively small once we naturally receive at least 12 mol·m-2·d-1. Therefore, supplemental lighting is typically not used on ornamentals grown in greenhouses in northern latitudes from April through September.

Purchasing, installing and operating a supplemental lighting system is expensive. If the greenhouse is located in the northern half of the United States and crops are produced during the winter and early spring (October through March), and/or young plants (liners or plugs) are produced, and you can receive a higher price for a higher quality plant, supplemental lighting can be economically viable.

For more information on greenhouse lighting, visit
This information was first presented at the 2011 Mid-Atlantic Fruit & Vegetable Convention.

By Erik S. Runkle

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