With so many indoor lighting options available for your plants, it's easy to get overwhelmed. That's why we put together this overview of the four most common styles (fluorescent, high pressure sodium, metal halide and light emitting diode), detailing how they function and where they are best used.
Throughout this article, we discuss measures of light intensity and color temperature. For more details on the science of plant-available light, including reference charts, visit our page on measuring light intensity.
Let's get started!
How They Work
Fluorescent bulbs create light by passing an electrical current through an inert gas kept at low pressure, typically argon, that also includes trace amounts of mercury. Electrons in the mercury become excited by this current and begin to produce photons of ultraviolet light. As those photons come in contact with a phosphor coating inside the bulbs, the coating begins to glow. This is the light that we can see with our eyes.
Unlike an incandescent bulb that you may have in your ceiling or lamp at home, a fluorescent bulb requires a ballast to regulate the electrical current passing from your outlet to the bulb itself.
Fluorescent lights come in a range of sizes including T5, T8 and T12. The "T" in T5 indicates the bulb is tubular shaped, and the "5" represents that it has a 5/8 inch diameter. A T8 lamp has an 8/8 inch diameter (or 1 inch) and T12 has a 12/8 inch diameter (or an inch and a half). The more narrow the bulb, the more efficient and brighter it is, which is why T5s are the most popular fluorescent bulbs for growing plants.
Similar to fluorescents are Compact Fluorescent Lamps (or CFLs), but rather than a long, straight tube, CFL tubes are curved or folded to fit into the space of an incandescent bulb. Additionally, rather than requiring an external ballast to regulate electricity, they have a compact electronic ballast in the base of the lamp.
Fluorescent bulbs come in a large range of color temperatures, typically from 2700 Kelvin to 6500 Kelvin. This is achieved by adding different rare-earth elements to the phosphor coating within the bulb. Different levels of these elements provide either more greens and blues or more reds and oranges.
The most common bulbs are those with a color temperature of 6500 Kelvin. Because this temperature of light is most similar to what they would receive if they were growing outside during spring and early summer daylight hours, it's ideal for plants in their vegetative, seedling or clone stages of growth.
Another common color temperature in T5 bulbs is the 2700 Kelvin range, which includes more red and orange hues to the light. These warmer colors more closely mirror the light received from the setting sun or early fall when plants would typically be in a flowering stage.
A typical T5 bulb can produce anywhere from 70 to 100 lumens per watt. That gives you a reference point for its electrical efficiency, or ability to convert electricity into light.
As an example, a 2 foot bulb drawing 24 watts, with an output of 2,000 lumens would provide an efficiency of 83 lumens per watt.
Fluorescents can be a great light for plants through all stages of growth. They are fairly cost effective and readily available. If you're using this type of lighting, we'd recommend T5-sized bulbs in the 6500 Kelvin range. If you have a flowering or fruiting plant, we recommend switching to a 2700 Kelvin bulb when it's time for the flowering stage to begin.
High Pressure Sodium (HPS)
How They Work
High pressure sodium (HPS) lamps are members of the high intensity discharge (HID) lamp family. These bulbs contain a narrow arc tube made of aluminum oxide ceramic, supported by a frame. The arc tube is filled with a mixture of highly pressurize xenon, sodium and mercury.
A ballast is placed between your electrical outlet and the bulb in order to properly regulate current flow.
The most common way to start these lamps is with a pulse start. There is an ignition built into the ballast which sends a pulse of high voltage energy through the arc tube. As power is applied, temperature and pressure build gradually, causing heated vapors to enter into the arc and release light energy. This is why it can take several seconds for your HPS lights to turn on and about five minutes to reach full brightness.
The xenon gas ignites first, followed by the mercury, providing a blue light spectrum. Once the gas pressure and operating voltage have increased sufficiently, the sodium vapor ignites, producing light in the yellow spectrum.
If power is interrupted, even briefly, the lamp’s arc will extinguish. It is very important to know that the lamp must cool down for about 10-15 minutes before being turned back on.
HPS lamps come in one of two types, single-ended or double-ended, and are typically rated based on their wattage. In both cases, the base of the bulb that connects to the light fixture is different from a standard light socket in your house.
Single-ended HPS bulbs look similar to a normal lightbulb but are slightly larger and have a base called a Mogul. The empty space inside the bulb is maintained as a vacuum with no air present.
Double-ended HPS bulbs look more like fluorescent lights because they have pins on either side of the bulb that connect them to their fixtures and pass an electrical current from one end to the other. Unlike single-ended bulbs, the space within double ended bulbs is not a vacuum, but rather filled with nitrogen gas. This allows the bulb to operate at a higher-temperature, increasing its efficiency.
Most HPS bulbs come in a color temperature from 2000 Kelvin to 2700 Kelvin. This works well for plants in their fruiting and flowering stages because the light is more heavily on the yellow end of spectrum. These warmer colors more closely mirror the light received from the setting sun or early fall when plants would typically be flowering.
If your plants are in a vegetative state of growth, it's ideal to use a different style of bulb like Metal Halide, to get the blue end of the light spectrum.
HPS lamps are quite efficient and operate at about 100 lumens per watt. As the bulb's wattage increases, you can sometimes achieve 150 lumens per watt.
Due to their low color temperature, HPS bulbs are best used for plants in their fruiting and flowering stages. They also produce a significant amount of heat, so you want to be sure to have proper ventilation and cooling in your grow area. If you want a higher level of efficiency with a more even distribution of light, we'd recommend double-ended HPS bulbs. The one drawback is that they do run hotter and because of their nitrogen makeup, cannot be cooled by direct airflow on the bulbs without compromising their light output.
How They Work
Metal Halide lamps work when an electrical arc passes through a mix of gases, usually mercury, xenon and argon, and a variety of metal halides. Metal halides are produced by combining halogen with a metal (normally mixed with iodine or bromine). The type of metal halides used helps determine the color temperature of the light emitted.
The structure of a Metal Halide lamp has two main sections - the outer bulb and the inner quartz arc tube. There are two electrodes at either end of the arc tube. A current of electricity starts at one electrode and passes to the next. This begins to heat up the mercury, causing it to vaporize. As it continues to heat-up, the metal halides turn into gas causing the atoms to move away from the arc. This creates white light.
A ballast is required between your electrical outlet and the bulb in order to properly regulate current flow.
You can find MH lamps available in both single-ended (SE) and double-ended (DE) versions. Single-ended lamps have a screw at one end and are designed to be mounted in a mogul socket which is slightly larger then your standard light socket. Double-ended lamps, on the other hand, are designed to be mounted in a pair of sockets, one at each end.
For horticultural purposes, you will often find these lamps available in 150, 250, 400, and 1000-watt versions.
A variation to typical metal halides is the ceramic metal halide, or CMH grow light. This provides a blend of the spectrum you would typically receive in a metal halide with some of that from a high pressure sodium, or HPS, bulb. CMH uses a ceramic arc tube similar to those used in HPS grow lights instead of the quartz used in traditional metal halide grow lights. Because these ceramic arc tubes operate at higher pressure than quartz glass tubes, this allows for more precise variations of spectrum than with previous technologies.
Ceramic arc tubes are more resistant to breakdown, so they last a lot longer than metal halide and HPS bulbs. In fact, CMH grow lights tend to last over 24,000 hours while keeping at least 80% of their original intensity at the 20,000-hour mark. This overshadows the common 10,000-hour replacement rule for HPS and MH bulbs.
Metal Halide grow lights are best used for vegetative growth because of their high color temperature, typically 5500 Kelvin, 6500 Kelvin and 7200 Kelvin. This white to blue spectrum means they’re great for making your plant produce dense, lush canopies with very little internodal spacing. Consequently they are not ideal for plants in a flowering or fruiting stages where more red and orange light is preferable.
Metal halide lamps have high luminous efficiency of around 75 to 100 lumens per watt.
Due to their high color temperature, traditional metal halide bulbs are best used for plants in their vegetative stage of growth, but can be swapped out for high pressure sodium bulbs to provide a lower color temperature for flowering.
If you're looking for a solution that doesn't require swapping bulbs between phases of growth, ceramic metal halides are a good option. While they offer a longer lifespan and more versatility, they also come with a higher price tag.
Metal halide bulbs produce a significant amount of heat, so you want to be sure to have proper ventilation and cooling in your grow area.
Light Emitting Diode (LED)
How They Work
Light emitting diodes, or LEDs, are small electronic components called semiconductors which are covered in microscopic holes. These holes are formed by introducing very specific atoms to the semiconductors in a process called "doping." The atoms used in doping determine the size of the holes and ultimately the color of light produced by the LED. There are two sides of each LED, one that maintains a negative charge with more electrons and one with a positive charge that has more holes. When an electrical current starts to flow through the LED, the electrons combine with the holes, and energy is released in the form of light.
When LEDs were first introduced to the consumer market, they had a relatively low light output and limited selections of colors. As time progressed, new doping methods and phosphor coatings were created to allow just about any color combination imaginable, including infrared and ultraviolet spectrums. They also provide extreme efficiency and high intensity.
LEDs come in many shapes and sizes. Manufacturers can be very precise with the diodes they choose to essentially create any desired spectrum of light. Heat is the only real enemy of LEDs and residual heat emitted by diodes or current regulators must be allowed to escape, otherwise the output of the LEDs will slowly degrade. Depending on the design, LED fixtures will either include a motorized fan or have surface area sufficient to allow heat to dissipate into the surrounding environment.
Color variations of LED grow lights are directly connected to their intended purpose. A spectrum that contains all colors is considered to be full spectrum and mostly resembles sunlight. Adding other colors to the spectrum, such as green, far-red and deep-blue aids in giving plants more of what they desire in their respective stages of life.
The best way to determine an LED's output is by assessing the amount of photosynthetically active radiation (PAR) contacting the surface of your plants every second. This is called Photosynthetic Photon Flux Density (ppfd) and is measured in micromoles per meter squared per second (μMol/m2/s). PPFD values should be above 500μMol/m2/s to be effective for plant photosynthesis.
LED grow lights can use up to 50% less energy then HPS systems, and can last for approximately 50,000 hours of use. This is a big difference, considering you would want to replace an HPS bulb at least once a year.
We definitely think energy-efficient LED grow lights are the future of lighting for indoor cultivation. They're energy efficient, have a longer life span, produce less heat and can be made in basically any color spectrum desired, giving your plants some of the best light possible.