The Ultimate Grow Light FAQ
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The main differences between grow lights is the makeup of the light spectrum they emit, the intensity of the light that’s emitted, the coverage area, and the cost.
Most low-cost LED grow lights are “sufficient,” or would rank between 4-6 on a scale of 1-10. It’s just enough to grow you a normal cannabis plant. Most higher cost lights, would rank 7-10 on a scale of 1 to 10. They’d be enough to grow you a higher yielding cannabis plant.
As far as the different brands go, there’s not much to know aside from what they offer and how their customer service is. The brands are all competing for a share of the indoor growing market.
In general grow lights are safe. Many are UL or ETL certified, meaning they’re put through rigorous safety tests. If your grow lights generate heat, make sure they’re properly cooled.
During veg you can keep your grow light 18 inches above the canopy, or adjust higher if you notice heat stressed leaves (wavy curling upward)
A study found that cannabis sativa plants can handle up to 1500 uMols PPFD at 30 degrees celsius (86 fahrenheit). Yields decreased above those levels.
If you’re using a 20 hour light cycle in the vegetative phase, your plants will need a minimum of 278 uMols PPFD, which equals 20 moles of light per day.
If you’re using an 18 hour light cycle in the vegetative phase, your plants will need a minimum 308 uMols PPFD, which also equals 20 moles of light per day.
If you’re using a 12 hour light cycle in the flowering phase, your plants will need a minimum of 463 uMols PPFD, which equals 20 moles of light per day.
Cannabis plants need between 20 and 40 micromoles of light per day.
During flowering you can keep your grow light 18 inches above the canopy, or adjust higher if you notice heat stressed leaves (wavy curling upward)
It depends on the intensity of your grow light. Most lights would be good at 22-24 inches, however if you’re using a powerful grow light, you may want to raise it even higher. In terms of light intensity, ~300 micromoles per second PPFD is good for seedlings.
During veg you should expose your cannabis plants to blue light only if possible. If your grow light doesn’t have a separate color mode for veg, it’s okay.
During cannabis flowering phase you should use both red and blue (purple), or full white light spectrum.
Not all grow lights contain ultraviolet light wavelength.
UV-A (320-400 nm) and UV-B (290nm – 320nm) wavelengths have shown to be beneficial to plants¹. There is a link between exposure to these wavelengths and increased THC production in cannabis plants.
Not all grow lights contain UV wavelengths so make sure to check before you purchase.
Not all grow lights include infrared light.
Infrared light is important for helping your plant switch from vegetative to flowering, as well as the proper development of stems, node spacing. Infrared exposure can also increase plant yields.
Several studies show white light spectrum is beneficial to growing plants. (Read our article here)
Quantum board LED grow lights by HLG contain white spectrum.
Yes, grow lights can damage your eyes. You should wear safety glasses before approaching an operating grow light.
Grow lights are safe for pets as long as your pet isn’t staring into the grow light. Keep your pet in a separate area if it likes to go near your grow lights.
Grow lights are good for succulents because succulents are plants that benefit from sunlight.
Only if your grow light is providing the minimum photosynthetic photon flux density (PPFD) that your cannabis plants need to grow. Read our article on PAR/PPFD for more info.
Daily Light Integral (DLI) is the total amount of light your plant receives in one day. Usually measured in Moles.
One million micromoles (umol) = one Mole (mol). Cannabis plants need 20-40 moles of light per day. That equates to 460-900 umol intensity from your grow light. PPFD is the intensity of your grow light’s rays once they hit the surface of your plant. PPFD is measured in micromoles.
PAR is the name for photosynthesis-inducing wavelengths between 400 and 700nm.
Grow lights work by producing light with Photosynthetically Active Radiation that your plants use for photosynthesis and growth. This light mimics natural sunlight and ranges between 400-700nm.
It depends on the intensity of light being emitted, and what phase of plant growth your plant is in.
Most grow lights should hang at 18 inches. If your grow light is powerful (over 900 uMol), you can increase the hanging height above 18 inches. Coverage area will increase, but light intensity (PPFD) will weaken.
If your grow light is weak, you can consider decreasing the hanging height for more intense PPFD, but coverage area will decrease.
Many people typically adjust hanging height between different growth phases. For example, during the seedling and vegetative phases, your plant does not need such high intensity light. So if you have a strong PPFD light (>700 uMol), you can hang it higher during seedling & vegetative. If you have a weaker light, you don’t need to hang it higher due to the intensity already being on the less intense range.
COB LED grow lights contain densely placed LEDs on a single light. They’re more efficient and intense than non-COB LED grow lights.
Quantum boards are a specific design of LED grow light that utilize white spectrum and a bare-bones design. They don’t require cooling fans or heat sinks. They’re efficient and give COB LED grow lights a run for their money. These lights utilize drivers, and they are often dimmable.
The quantum board design was started by Horticulture Lighting Group and has since been replicated by other brands.
It depends on the size of your grow space and the power of the grow light. Read our article about understanding PPFD, and you’ll be able to determine how many grow lights you need for any size grow space.
Most are not waterproof. If you need a waterproof grow light, make sure it’s explicitly mentioned in the product description.
Most LED grow lights do not require ballasts.
Grow lights that use light bulbs, such as high pressure sodium and metal halide grow lights.
An HLG 550 V2 Quantum board LED grow light would take care of it.
If that’s not for you, a COB LED or another high power grow light that’ll provide sufficient PPFD readings all the way to the edge of your tent. We wrote a whole article about it. At the end of that article you’ll find a helpful chart featuring various grow light models, and how many of each you’d need to sufficiently cover a 4×4 grow tent.
You can build your own grow light but it’s more complicated than simply buying one. You can find DIY kits for sale here
It depends on the heat output. Some low power grow lights as well as quantum board grow lights don’t generate much heat, so they don’t require fans.
Blue wavelengths 400-500nm are best for plant’s vegetation phase.
A plant’s flowering phase requires the whole PAR spectrum of light which is 400-700nm.
A recent study found that HPS lights resulted in 200% higher yield than MH grow lights.²
Strong and balanced blue and red wavelengths are ideal. I’d avoid a light that only has blue or only has red, unless the red only light is a small “booster” grow light meant to boost PPFD during flower phase.
You can, but it’s not recommended. Most people do 18 on, 6 off, or 12 on, 12 off.
I imagine the only point of keeping your grow light on 24 hours is to increase your total Mole/day of light absorption. In that case you probably need another grow light – not 24 hours of light.
People generally aim to grow cannabis plants with shorter stems and more abundant flowers. A weak grow light will cause your plant to stretch by growing longer stems so the flowers can ultimately be closer to your light and absorb a higher amount. A high power grow light will prevent this.
Most LEDs within grow lights are rated to last 50,000 hours. Some LEDs are rated for 100,000 hours.
1.5 micromoles per joule of energy is considered efficient. High end grow lights will produce 2.3 micromoles per joule, or better.
- Lydon, J. et al (1987). UV‐B Radiation Effects on Photosynthesis, Growth, and Cannabinoid Production of Two Cannabis sativa Chemotypes. Photochemistry and Photobiology, 46(2), 201–206.
- Backer, R., Schwinghamer, T., Rosenbaum, P., McCarty, V., Eichhorn Bilodeau, S., Lyu, D., Ahmed, M., Robinson, G., Lefsrud, M., Wilkins, O. and Smith, D. (2019). Closing the Yield Gap for Cannabis: A Meta-Analysis of Factors Determining Cannabis Yield. Frontiers in Plant Science, 10.