Grow light catalogs confront you with five different types of lights: fluorescent, compact fluorescent, metal halide, high-pressure sodium, and LEDs. Unfortunately, it can be hard for consumers to directly compare their performance. For the complete story, read The light plants need.
Incandescent bulbs, the type that Thomas Edison invented and that appear over cartoon characters’ heads when they have a brilliant idea, aren’t generally used for growing plants because they give out so much heat for the amount of photons they produce. I excluded incandescent bulbs from the list.
Fluorescent bulbs are those tubes that you remember hissing and flickering from school or the library. They’re glass tubes filled with mercury glass and coated inside with material that fluoresces—that is, lights up when it’s excited by ultraviolet light. When you turn a fluorescent light on, the electrical current makes the mercury atoms emit ultraviolet light, which makes the coating fluoresce. (Canon has a good animation of how this process works on their Science Lab pages.) Fluorescent bulbs can have different colors depending on what substance exactly you use for that coating. Older fluorescent fixtures flicker because they’re not putting enough electrical current through the bulb to get the mercury atoms going.
The reason people use fluorescent bulbs for lighting is that they convert a larger proportion of their electricity to light than incandescent bulbs, so they don’t require as much power to illuminate a spot. Since they’re using less power, they also generate less heat than incandescents. Lighting companies equivocate about exactly how hot their bulbs get, saying that it all depends on the environment around them, the fixture they’re in, etc. I took a laser thermometer to fluorescent bulbs in my house, measuring their temperature after running for two hours at 65ºF (we’re cheap with heating oil around here). The surface temperature of a T5 grow-light bulb was around 100ºF; the ancient T12 bulb in the laundry room was around 140ºF. Neither of them will burn the house down, but your tender seedlings wouldn’t appreciate it having the T12 bulb too close; water at 140ºF is hot enough to cause a third-degree burn in five seconds, according to the Burn Foundation.
Fluorescent tubes also last longer than incandescent bulbs, running for up to 20,000 hours, about ten times as long as incandescents. You’ll see references to T12, T8, and T5 fluorescent bulbs; the T just tells you that you’re measuring the diameter of the bulb in eighths of an inch. T12 bulbs are 1.5 inches in diameter, T8 bulbs are 1 inch, and T5 bulbs are ⅝ of an inch and are built a little differently. You can generally plug T8 and T12 tubes into the same fixtures, but not T5 tubes. The smaller-diameter fluorescent tubes put out more light per watt than the larger tubes.
Some T5 bulbs are T5HO, where HO stands for “high output.” The T5HO tubes do put out more light than the T5 bulbs, but they also use more energy to do so. You’ll get less light per watt from these bulbs, according to this chart put together by the Rensselaer Polytechnic Institute Lighting Research Center. Even more confusing, fluorescent bulbs’ light output varies with the room temperature. T8s give out more light at 77ºF, T5s at 95ºF. T5 performance falls off rapidly between 86-68º F (30-20ºC) from 95 percent to 75 percent of their maximum light output. At 59ºF (15ºC), you’ll only get 60 percent of the light out of a T5 as you would at 95ºF, while T8s are still putting out 90 percent of their maximum light.
The moral: Keep your seedlings warm. Fluorescents do eventually wear out. As the glass on the tube gradually absorbs mercury, there’s less mercury vapor in the tube to create ultraviolet light. Both T8 and T5 bulbs continue to put out about 90 percent of their maximum lumens over the course of their 20,000-hour lives, while T12 output falls to 75 percent by 5,000 hours. For that reason, and the sheer bulk of T12 lights, I excluded those bulbs from my sample.
Compact fluorescent bulbs are also available as grow lights, but they use more energy to create light than the tubes (more watts per lumen). They last about 10,000 hours, or half as long as the fluorescent tubes. The only real advantages to CFLs are that they can be plugged into conventional light sockets and that they contain less mercury than the tubes—but they do contain mercury and need to be disposed of carefully. Because of their inefficiency and relatively short useful life, I excluded them from the grow light sample.
Metal halide (MH) and high pressure sodium (HPS) lamps are both types of high intensity discharge (HID) lamps. HID lamps contain tungsten electrodes inside a tube filled with gas and metal salts. When electricity flows through the lamp, electricity arcs between the electrodes through the gas, melting and heating the metal salts until they form a glowing plasma. They put out a lot of light, but they use of energy (250 to 1000 watts per bulb) and consequently they give off a lot of heat. You can’t put a 600ºC HID bulb on top of your seedlings, or your laptop, or pretty much anything except another HID bulb. You need plenty of space and air circulation.
Metal halide lamps are filled with vaporized mercury and metal halides (compounds of bromine and/or iodide). HPS lights contain—surprise!—sodium instead. MH lights put out somewhat more light in the blue part of the spectrum than the HPS lights, which produce more yellow-to-red light. But, as Giacomelli said, most commercial applications use HPS because plants can do reasonably well without the additional blue light.
If you want to see what an HPS light looks like, go to a parking lot at night and look up. HPS lights are frequently used for outdoor lighting, giving everything that special orange glow. Many people find this particular hue aesthetically displeasing. If you’re going to be looking at your plants under lights, consider what color you’d like to see. That said, HPS bulbs are much more popular for horticulture, in part because the MH bulbs last just 10,000 hours on average, compared to about 18,000 hours for the HPS, and their light output decreases fairly rapidly with use. According to this chart, MH bulbs’ light output will deteriorate by about 25-51 percent before they’re halfway through their expected use. HPS lights will still put out 70 percent of their initial lumens at the end of their useful life. I eliminated MH bulbs from the sample.
LEDs shoot out photons at single wavelengths, not across a spectrum.
LEDs, or light-emitting diodes, are (relatively speaking) the hot new light. Well, really, they’re the cool new light, because they give off very little heat. As How Stuff Works writes, “[LEDs] are illuminated solely by the movement of electrons in a semiconductor material, and they last just as long as a standard transistor.”
That means that LEDs have very long useful lives: about 50,000 to 100,000 hours, in theory. As Bugbee said, “LEDs are too new a technology to be sure they’ll last 20 years.”
Unfortunately, there are two important things to know about LED lighting:
1) LEDs shoot out photons at single wavelengths, not across a spectrum.
2) LEDs can’t deliver as many photons per watt as other lights … yet.
The first point is important because plants need some light in both the “blue” spectrum (400-500 nm) and the “red” spectrum (600-700 nm) to survive and thrive (see The light plants need). But that may not be all they need. There’s research suggesting that plants growing under LEDs do better when they get some far-red light (700-740 nm) every day; lettuces grown with a half-hour of this far-red light had 28 percent more mass at the end than controls. Most LEDs don’t provide far-red light. Greenhouse growers like this, according to Harwood. “It helps with integrated pest management [IPM]” Harwood said. “LEDs with no infrared cut insect visibility.”
Bugs can’t see the leaves as well if you leave out near-red and green spectrum lights, Harwood said, and some diseases and funguses are inhibited if you leave out certain spectra as well. “White” LEDs are actually groups of three or more single-wavelength LEDs put together to make light that looks white to humans—and to make your plants look like they belong on Earth instead of Pandora.
The second point—that LEDs can’t deliver as many photons per watt for a given area—is partly due to the state of technology.
Bugs can’t see the leaves as well if you leave out near-red and green spectrum lights…
For incandescent, fluorescent, HPS, and MH lights, if you make a fixture that can withstand more current, you get a brighter light when you put more energy into the system. In LEDs, though, there seems to be a limit to how much light you can get out of a system. After a certain point, the excited electrons start running into each other and producing more heat instead of light, as Popular Science explains.
There’s plenty of research into making better LEDs (especially as the cannabis industry takes root), but for now, the options for the consumer aren’t very powerful. Instead of making LEDs higher watt, you have to add more of them to get more light to your plants, which costs more and takes up more space. Giacomelli tries to cluster LEDs together to look like an HPS light.
LED manufacturers also talk about the beam angle, or how far the light spreads from the center of the LED. The larger the beam angle, the more ground the light covers, but the less light you’re getting per unit area. For the purposes of this review, I’m assuming that you’ll be hanging your lights within a foot of your plants, so you’ll need a wide beam angle (90º) to get light to the edges of your seedling tray. If you’re hanging your light higher, you’ll want to choose a narrower beam angle to cover the same surface area.