Plants need sunlight to grow.
(Groundbreaking stuff, I know.)
But what if you want to grow where there isn’t much (or any) sunlight?
- On your back porch
- In a dimly lit room
- In your basement
- In a grow tent
Then you need to invest in some grow lights.
But choosing the right grow lights for your setup isn’t as simple as going to Amazon and buying the first grow light you see.
There’s some science behind choosing the right grow lights for your crops, and in this guide we’ll explore the five main types:
- Fluorescent (CFL, T5, T8, and T12)
- High-intensity discharge (HID including HPS, MH, CMH, and HPS+MH)
- Double-ended (DE)
- Light-emitting diode (LED)
- Light-emitting plasma (LEP)
But first, let’s talk about how plants use light.
(I promise, this is crucial stuff to know before you start shopping.)
Light spectrums, wavelengths, and growth phases
Light is on a spectrum from UV (short wavelength) to infrared (long wavelength), and plants require different wavelengths for various cellular functions.
It’s easy to get lost down a confusing (albeit fascinating) rabbit hole of plant light requirements at various stages, but there are two important takeaways you should know:
- Cool blue light (short wavelength) promotes vegetative growth
- Warm red light (longer wavelength) promotes floral growth
Cool blue light (shorter wavelength) promotes vegetative growth
The image above clearly shows that both Chlorophyll A and Chlorophyll B absorb the most light at the blue end of the color spectrum. This means shorter wavelength blue light results in the highest rate of glucose production, which leads to faster growth rates.
Warm red light (longer wavelength) promotes floral growth
Let’s step away from grow lights for a moment and think about natural sunlight.
On a bright day with the sun overhead, we’re bombarded with a lot of short wavelength blue light. When the sun is either rising or setting, there’s far more long wavelength red light. Anecdotally, you can probably confirm this; sunrise and sunset are warmer and more red/orange than the middle of the day.
As the seasons change, the length of a day changes, too. Summers in the northern hemisphere are marked by longer days, which means more short wavelength blue light, whereas spring and fall have relatively less blue light and more red light.
Plants have evolved to sense the changing ratio of blue light to red light and now use those ratios as signals to begin flowering. This is crucial for farmers because you can capitalize on the plant’s natural responses to induce optimal flowering and fruit production by giving plants more long wavelength red light
This article does a fantastic job explaining the physiology of how light affects a plant’s cellular processes.
What this means for grow light selection: An optimal grow light setup is calibrated to the types of plants you’ll be growing and their respective stages of development. Young plants of all types require a lot of bright blue light for quick vegetative growth. But at some point, mature flowering plants like tomatoes and cannabis will need plenty of warm red light to induce flowering and deliver strong yields.
With our quick plant science lecture complete, let’s look at the five types of grow lights you can choose from.
1. Fluorescent grow lights
Fluorescent grow lights are an inexpensive, entry-level lighting option that’s best-suited for low-cost setups, especially those that don’t have many flowering plants.
There are two main types of fluorescent grow lights:
- Compact fluorescent lights (CFL)
- Tube fixtures (like T5, T8, and T12 grow lights)
Note: With tube fixtures, the number next to the “T” is the diameter of the tube in eighths of an inch. A T5 grow light has a 5/8″ diameter. T5 grow lights have the best output and are vastly superior to both T8 and T12 grow lights, so from here one out we’ll refer to these types of fluorescent grow lights as T5 lights.
CFL grow lights are the typical fluorescent lights you see at hardware stores. They don’t cover much area and are best suited for small-scale, beginner operations.
T5 grow lights are cost-efficient and cover a large area, making them great beginner grow lights as well.
Look for high-output (HO) grow lights and plan to change them every 18-24 months.
Advantages of Fluorescent Grow Lights
- Low initial cost and very energy-efficient
- Low radiated heat is safe for seedlings and allows lights to be hung close to greenery.
- Can be hung both horizontally and vertically for all types of configurations.
Disadvantages of Fluorescent Grow Lights
- Low yield per watt compared to other types of grow lights.
- Not strong enough to penetrate thick foliage.
2. High-intensity discharge (HID) grow lights
HID lights work by arcing a current across two tungsten electrodes at opposite ends of the light, igniting gas in the bulb to emit intense light capable of penetrating foliage to all parts of the plant.
There are three types of HID lights:
- High pressure sodium (HPS)
- Metal halide (MH)
- Dual arc hybrid (HPS+MH)
High pressure sodium (HPS) grow lights
HPS lights emit more light at the red/orange end of the spectrum and are better for the flowering phase.
Metal halide (MH) grow lights
MH lights emit more light at the blue end of the spectrum and are better for the vegetative phase.
Ceramic metal halide (CMH) lights replace the typical quartz arc tube with a ceramic arc tube that lasts longer and boasts 10-20% better efficiency with better color retention. You may also hear CMH lights referred to as light emitting ceramic (LEC) lights.
Dual arc hybrid (HPS+MH) grow lights
HPS+MH grow lights offer the benefits of both HPS and MH grow lights allowing you to use them for all plant growth phases from seedling through flowering.
Typically, they’ll include both a 600W MH tube and a 400W HPS tube, which are intended to replace a single 1000W light of either type. Obviously, the convenience of a single 1000W HPS+MH light is an advantage, but that convenience comes at the cost of overall growth and yield when compared to a setup with separate HPS and MH lights.
For hobby farmers, the convenience may be worth it. For more production-focused farmers, especially those growing cannabis, marginal gains in yield can be important, giving dedicated HPS and MH lights the advantage.
Advantages of HID Grow Lights
- Combining HPS and MH lights (or using dual arc lights) covers the plant’s needs for its entire life cycle.
- For cannabis production, MH lights produce significant UV wavelength light for better terpene, resin, and THC production.
- HID lights have been in use for decades and are well-researched.
Disadvantages of HID Grow Lights
- Need to be changed every year when used on a standard 12/12 light cycle.
- Generate a lot of heat, especially HPS lights, and require a lot of space.
- HPS lights generate more yellow and green light than the plants need, which is wasted energy.
- HID lights contain mercury.
3. Double-ended (DE) grow lights
DE lights are a type of HID light that connects to ports on both ends of the ballast as opposed to a single connecting port seen on standard single-ended (SE) HID grow lights.
They’re a fairly recent innovation that commercial growers have been using for 10-12 years and hobbyists for about five years.
DE lights have a shorter arc tube with thinner glass, which results in more light density for the same wattage from a more intense focal point.
However, DE lights are much pricier than their SE counterparts. Not only are they priced upwards of 50% higher for the same wattage, but you need an upgraded high-voltage ballast and a DE-specific reflector.
But while initial costs are higher, overall efficiency, production, and maintenance costs are lower.
Advantages of DE Grow Lights
- Produce more usable light for the same wattage.
- Need to be replaced every 18 months, which is less often than SE lights.
- Produce more UV light for better vegetative growth and, for cannabis farmers, better terpene, resin, and THC production.
- Tubes are made with higher-quality quartz vs standard borosilicate glass in SE lights.
- Tubes are filled with nitrogen instead of xenon for better thermal conductivity.
Disadvantages of DE Grow Lights
- Higher upfront costs with more expensive bulbs and specialized equipment required.
- Produce more heat than SE lights.
- Can’t handle direct air blowing on them, which affects the nitrogen inside the tubes and reduces bulb’s overall efficiency.
4. Light-emitting diode (LED) grow lights
LED lights last forever (relatively speaking), don’t use much energy, and don’t get hot, and emit all types of wavelengths across the light spectrum, making them great for grow light applications.
But there’s a catch: Watch out for the blue/purple LED grow lights as shown in the image above. In order to be full spectrum, you need white LED lights because white light is the blended result of every wavelength being present.
Advantages of LED Grow Lights
- Long lifetime with high efficiency and low energy use.
- Produce the same amount of light as other types of grow lights with lower wattage.
- Wide array of colors and wavelengths available for all plants in all growth phases.
- Produce very little heat.
- Installation is as simple as hanging them up and plugging them in.
- Most LED lights are both water-proof and dust-proof.
Disadvantages of LED Grow Lights
- High initial cost.
- Be wary of blue/purple LED lights that aren’t truly full spectrum.
- Must be sure you’re buying strong enough LEDs for the flowering phase with at least 2 micromoles per watt of energy.
- Extremely heavy due to built-in heat sink.
- May need to buy reflectors to concentrate light where you want it.
5. Light-emitting plasma (LEP) grow lights
Plasma grow lights are a relatively new, high-tech innovation with high upfront costs. They use a light-emitting plasma bulb (LEP) that contains a gas, which, when heated via electromagnetic induction, produces an energy-efficient, low-heat light that’s very similar to the sun.
They emit light at all parts of the spectrum with no electrodes and no moving parts. It’s kind of sciency, but this site has a good explanation.
Advantages of Plasma Grow Lights
- Produce a full light spectrum with both UVA and UVB lights that mimics the sun.
- Radiates almost zero heat.
- Extremely long lifespan though to be 3-4 times that of standard bulbs.
Disadvantages of Plasma Grow Lights
- A new type of lighting technology which still needs more study and comparison to truly know its benefits.
- Extremely high initial cost.
We’d love to hear about your experiences using these various types of grow lights. How have you used each and which are your favorites?