High-value specialty crops, such as fruits, vegetables and ornamentals, account for a tiny percentage of U.S. farmland but yield about 40 percent of farmland revenues. In many regions, these crops are grown in shielded environments, such as greenhouses.

Green plants need light, which they use to produce biomass from water and carbon dioxide in a process called photosynthesis. Light exposure varies by time of year and location. To grow plants year-round, and because light exposure within a greenhouse can be significantly lower than outdoors, supplemental lighting is common. Traditionally, conventional light sources have been used for horticultural lighting, and they can be effective. The advent of light-­emitting diode (LED) lighting, however, has resulted in an extraordinary opportunity to reduce energy costs while increasing crop production and quality.

Plants need light but are only photosynthetically responsive to certain wavelengths in the visible light spectrum, specifically in the 400- to 700-­nanometer (nm) range in red and blue wavelengths. This is called photosynthetically active radiation (PAR).

The daily sum of light a plant receives is called the daily light integral (DLI), which expresses the number of photons of PAR falling on a 1 square meter area per day. This is a function of both the intensity of light and duration of light exposure. On an average day, an outdoor plant may receive anywhere from 5–60 moles of light per square foot, depending on location and the time of year. While some crops, such as African violets, grow well with a lower DLI, many plants don’t receive enough light during the winter months.

For this reason, electric lighting is often used to increase DLI by boosting the quantity of useful PAR falling on the plant as well as the duration of exposure. Plants require varying degrees of optimal DLI; proper dosage can have a big impact on root/shoot growth of seedling plugs, root development among cuttings, and final plant qualities, such as branching, number of flowers and stem thickness. Conversely, extending the photoperiod of some plants—such as chrysanthemums, which bloom when nights are longer—can be used to suppress flowering, which can be useful. While lamps such as high-pressure sodium, metal halide and linear fluorescent can be effective, they produce a significant amount of radiation, including heat, outside the PAR range, resulting in waste.

LEDs promise higher efficiency, longer life and no integral mercury or radiant heat output. While labor is the highest operating cost involved in greenhouse production, energy is second. The fact that LEDs produce less radiant heat than conventional lamps means they can be placed closer to the plants, which increases overall efficiency. Perhaps the most interesting feature of the LED source is the ability to establish precise spectral output, which can be adjusted through digital control for color tuning.

Plants respond to different wavelengths of light (associated with their human color perception) differently than people do. With plants, the delivery mechanism of light is the photon, which embodies varying degrees of energy based on wavelength. For example, red light sources are more efficient at delivering photons than blue sources. However, different varieties of plants may have varying levels of sensitivity to some light wavelengths, including blue. Because LED sources can be manufactured with precise spectral output, various packages of red and blue wavelengths can be optimized for specific plant photoreceptors. And, because LEDs are highly controllable and easily integrated with digital control, the source can be programmed to deliver custom intensities and spectral emission, including sunrise to sunset simulations.

The primary promise of the LED is, therefore, to facilitate production of healthy crops. Several academic studies support this claim.

Purdue University researchers studied the growth of flower varieties under an 85-to-15 red-blue LED mix and high-pressure sodium lamps. Results showed lower height, thicker stems and a higher quality index rating under the LED mix.

Another study at McGill University tested hydroponically grown tomato plants under various mixes of red and blue LEDs, all red LEDs, all high-pressure sodium and a 50-50 mix of LED and high-pressure sodium. A 5-to-1 red-blue LED mix performed well in every measured category, though the 50-50 mix of LED and high-pressure sodium produced the highest marketable fruit production.

While best practices are still being developed, solutions are available from a number of manufacturers, such as Cree, GE, Osram Sylvania and Philips. Osram’s Oslon 730-nm (“far red”) LED, expanding the product family of 450-nm (“deep blue”) and 660-nm (“hyper red”) LEDs, won the Technical Innovation Award at the 2015 Lightfair Innovation Awards.

Because of the large number of plants and LED options, it pays to partner with a manufacturer or consultant that can provide appropriate expertise. It is also generally advisable to conduct a trial installation of several luminaires and plants before commitment.