Cut gerbera production lit with HPS lights
Photo: Christopher J. Currey

While many scientists and inventors were experimenting with electrical lighting throughout the 1800s, no one developed a commercially viable lighting source until the latter half of the century when Thomas Edison brought the first incandescent light bulb to market. The horticulture industry’s interest in using electrical lighting to grow plants intensified soon after.

It’s been nearly 125 years since renowned botanist Liberty Hyde Bailey developed his initial study on the growth of plants under electrical light in detail, the first of its kind, and the start of a new era in the field of horticulture. Bailey, a journalist turned botanist, had visited Cambridge in the late 1880s and observed that plants growing near gas lamps grew differently than their counterparts further away; this was the impetus for his later investigation into the subject.

In 1901, Bailey published a paper about electrical lighting in protected environments entitled “Some Preliminary Studies of the Influence of the Electric Arc Lamp upon Greenhouse Plants,” according to a biography about Bailey by Harlan P. Banks.

“The horticulture and greenhouse industries have been early adopters of lighting technologies since the invention of the incandescent lamp,” says Dr. Gary Stutte, horticulturist and principal investigator at NASA’s Kennedy Space Center. “Electrical lighting has been around for a very long time, and it was primarily used to extend the growing season in the northern climates [where] it gets dark [early].” In the 1940s and 1950s, high pressure sodium (HPS) and metal halide lights came into wider use in the greenhouse.

“In the 1960s came some understanding of light control of plant responses,” says Stutte. That included the use of shade cloths and incandescent lights to control flowering in crops like poinsettias, and elicit different plant responses, he says. But overall, it was still being used to extend the growing season. In the 1970s, the industry saw more widespread use of electrical lighting, Stutte says.

In the late 1980s, NASA began looking into other lighting options besides fluorescent or incandescent lights to support plant growth in space. “NASA’s goal was to find a light source that was long-lasting, so it didn’t need to have replacements,” says Stutte. They were seeking a safe, reliable lamp that didn’t contain mercury or have special storage and disposal needs; this option was light-emitting diodes (LED). As LEDs become more commonplace in industrial and residential lighting and costs decrease, they could also eventually be a more feasible option for more greenhouse producers. “I think that people are rightly concerned about the adaptation of new technology, the initial cost of implementing [it],” Stutte says. But it could be worth it.

While it’s unlikely that commonly used light sources like HPS and fluorescent lamps will disappear from greenhouses any time soon, much of the current research is focusing on the opportunities that LEDs bring to the table. “LEDs are becoming a more significant component of sole-source and supplemental lighting in greenhouses and controlled environments,” Stutte says. “That’s driven by improved energy costs, disposal, safety, long life and the long-term economics of production.”

Today, we see growers using electrical lighting both for supplemental and sole-source lighting. To better understand how growers are using lighting to improve their operations, we bring you the inaugural State of Lighting Report. Click here to see the results of an extensive study that we conducted with Greenhouse Management and Produce Grower readers to find out how, why, where and when they’re utilizing electrical lighting. Next, you can read about two researchers who have been trialing different lighting sources in their growing operations. Rounding out the report, Ricardo Hernández, assistant professor in Controlled Environment and Sustainable Energy at North Carolina State University, gives his perspective on current lighting research and future possibilities, as seen at the 8th International Symposium on Light in Horticulture.

Useful terms and information relating to greenhouse lighting

  • Daily Light Integral (DLI): the cumulative amount of Photosynthetically Active Radiation (PAR) light received each day
  • Day-extension lighting: the practice of delivering light to extend the length of the natural day
  • Haitz’s Law: states that the amount of light coming out of a light-emitting diode (LED) will increase 20 times in the next 10 years (not to be confused with efficiency), and that the cost per diode will decrease 10 times in the next 10 years (Haitz and Tsao, 2011).
  • Horticultural Electrical Efficiency (HAR): the ratio of electrical power that is converted to photons of light (micromoles per second: µmol·m–2·s–1) in the PAR spectrum. In other words, it is measured by how many µmol a particular fixture emits every second (s) per every Watt (W) of electrical power provided. This number is either provided or can be directly calculated from the manufacturer fixture specifications.
  • Intensity: the rate at which light energy is delivered to a unit of surface, or energy per unit time per unit area
  • Light: radiation within the electromagnetic spectrum that has both wave and particle properties
  • Light spectrum: the frequency or wavelength of light. Different spectra or colors play different roles in plant development; red and blue play an important role in photosynthesis, and red and far-red in flowering and stem elongation.
  • Night-interruption lighting: the practice of providing light to plants during the middle of the night so that plants will perceive a short night/long day instead
  • Photons: individual particles of light
  • Photoperiod: the period of time in 24 hours that plants are exposed to light
  • Photoperiodic lighting: the use of electrical lighting to alter the daylength in order to affect plant growth and development; may be provided with day-extension or night-interruption lighting
  • Photoperiodism: a biological response to a change in the proportions of light and dark in a 24-hour daily cycle. Plant responses such as flowering can be classified as: short day (requires daylengths equal than or shorter than a certain critical daylength to flower); long day (requires daylengths equal than or longer than a certain critical daylength to flower); or day neutral (flowering is not affected by daylength).
  • Photosynthetically Active Radiation (PAR) light: wavelengths that are used for photosynthesis (400-700 nanometers)
  • Sole-source lighting: the use of electrical lighting as a complete replacement of ambient light
  • Supplemental lighting: the use of electrical lighting to add to available ambient light for quicker or better plant growth

Learn more by reading our 2016 State of Lighting Report Research.  

Sources: Christopher J. Currey; Philips Lighting; “Supplemental Light” by J. Raymond Kessler, Jr., accessed at www.ag.auburn.edu/landscape/supplementallight.html; “Luminous possibilities” by Ricardo Hernandez, April 2016 issue of Greenhouse Management; “Lighting Systems” accessed at https://courses.cit.cornell.edu/hort494/greenhouse/lighting/lightlft.html; “What are short day and long day plants?” accessed at http://extension.oregonstate.edu/gardening/what-are-short-day-and-long-day-plants; “Managing Photoperiod in the Greenhouse,” by Christopher J. Currey, Roberto G. Lopez, Erik S. Runkle, accessed at https://www.extension.purdue.edu/extmedia/HO/HO-253-W.pdf