Tomatoes growing under LED lights in a greenhouse
Photo: Erik Runkle

There are two types of supplemental lighting: day length lighting, also known as photoperiodic lighting, which regulates flowering; and high-intensity lighting, which supplements sunlight and increases growth, says Dr. Erik Runkle, professor and floriculture extension specialist at Michigan State University. When growers decide what they hope to accomplish with supplemental lighting, they come closer to selecting a company and product that works for them.

LEDs and high-intensity discharge [HID] lights are the most common supplemental lighting types, Runkle says. “HIDs include metal halide and high-pressure sodium [lights] — high-pressure sodium is much more common. But then there are LEDs that emit a sufficiently high intensity that can also be used,” he says. “Whereas with photoperiodic lighting, growers used to use the incandescent bulb, then some switched to compact fluorescent. Now there are LEDs that are meant specifically to regulate flowering.”

The question of when a grower should consider using one type of lighting over another is very situational, Runkle says. For a grower to determine if they should invest in supplemental lighting or upgrade their existing supplemental lighting, they should consider electricity costs, how many hours they light their crops per year, fixture costs and fixture efficiency.

While high-pressure sodium (HPS) lights have a low cost and high output, LEDs and their targeted light spectra are gaining favor with many greenhouse growers of ornamental and produce crops, he says. “I don’t think it’s a question of if LEDs will be in greenhouses — it’s a question of when, because the technology continues to advance,” he says. “The prices are coming down, so in the next decade or two, I think we’ll pretty much only see LEDs.”

When growers think about upgrading their supplemental lighting, Runkle says, they should consider several factors regarding the company and its processes, such as its reliability, product support, services and the durability of its products, Runkle says. “I am sure there are some LEDs on the market that are not developed for horticulture, and if you were to take those and put them in a greenhouse situation with high humidity, they may fail relatively quickly,” he says. “Make sure that they can tolerate the extremes of greenhouses.”

Crop type will help growers determine the best use of lighting to achieve a desirable outcome — such as if they should use LEDs to speed up or delay flowering, and when, Runkle says. Chrysanthemum and poinsettias require short days, so growers can provide light to them during the night to delay flowering, he says. Petunia and snapdragon require long days, so in the winter and early spring, growers can provide low-intensity lighting to accelerate flowering.

Working with a partner in the industry will help growers decide on a lighting solution, Runkle says. “It’s highly situational, so [sit] down and [speak] with someone, whether it’s the company folks, an academic or an extension educator, or someone who can help you come up with a decision that’s based on your specific growing application, your specific cost for electricity and how much light you [should] try to deliver,” he says. “It doesn’t take that much time to then come up with the information needed to be the basis of a decision.”