Watch the episode · 6:23This article contains paid links to GrowAce, a US shop. We may earn a commission at no cost to you.
For decades, indoor growers flowered their plants under the orange glow of a high-pressure sodium lamp: HPS for short. Now most new grow lights are LEDs. But plenty of growers still swear the old lamp grows bigger buds. So what did side-by-side trials actually find?
The short answer ▶ 0:29
Per square metre, the sodium lamp actually grew a little more flower. But per unit of electricity, the LED won easily. And there are two situations where the old lamp still makes sense.
| HPS | LED | |
|---|---|---|
| Flower per m² | A LITTLE MORE | A LITTLE LESS |
| FLOWER PER kWh | LESS | FAR MORE |
The clearest test comes from a grow room at Utah State University: same plants, same amount of light, with white LEDs on one side and a sodium lamp on the other.
Source · Westmoreland, Kusuma & Bugbee, PLOS ONE (2021)
What the plants did ▶ 0:53
In that 2021 study, the team grew hemp in five 1-metre chambers, three times over. Every chamber got exactly the same light level, and only the type of lamp changed.
Source · Westmoreland, Kusuma & Bugbee, PLOS ONE (2021)
The result: the sodium lamp grew 4.6% more dried flower per square metre than the most efficient LED.
Why? It comes down to blue. Sodium light is only about 4% blue; the LEDs ranged from 10% to 20%. And in all three runs, the more blue in the light, the less flower: about 12% less going from 4% to 20% blue.
Source · Westmoreland, Kusuma & Bugbee, PLOS ONE (2021), Fig. 3
Blue light drives photosynthesis a little less efficiently than red, and plants under lots of blue tend to grow smaller leaves. So there's no magic in the sodium lamp: it's mostly the lack of blue. And LEDs can copy that. Lights with white plus extra red already have less blue.
An earlier trial in Italy, with a high-THC variety, found the same pattern. Plants under the sodium lamp grew taller and gave more flower: in one run, about 27 grams a plant, against about 23 under the LEDs.
Source · Magagnini, Grassi & Kotiranta, Med. Cannabis Cannabinoids (2018)
But the THC went the other way. In the first run, flowers under the sodium lamp tested at 9.5% THC; under one of the LEDs, 15.4%. So the total cannabinoids per plant came out about the same.
Source · Magagnini, Grassi & Kotiranta, Med. Cannabis Cannabinoids (2018)
In the Utah study, though, the light made no difference to THC or CBD at all. So do LEDs make stronger buds? One trial says yes, one says no.
The electricity bill ▶ 2:26
As we worked out in what size LED your tent needs, a good LED makes about 2.5 micromoles of light from every joule of electricity. A double-ended sodium lamp makes about 1.7.
The most efficient LED grew 27% more flower for each unit of electricity than the sodium lamp.
Source · Westmoreland, Kusuma & Bugbee, PLOS ONE (2021)
On the study's chart, every step up in efficiency gave more flower for the same power. The authors' conclusion: a light's efficiency mattered more than its colour.
Source · Westmoreland, Kusuma & Bugbee, PLOS ONE (2021), Fig. 7
| LED 2.5 | HPS 1.7 | |
|---|---|---|
| 1 M² at 800 | 320 W | 470 W |
| 8-week flower | 215 kWh | 315 kWh |
| Extra per grow | +100 kWh |
Take 1 square metre at a light level of 800. Over an 8-week flower, on 12 hours a day, a good LED uses about 215 kWh. The sodium lamp uses about 315 kWh. That's 100 extra kilowatt hours every grow, and all of it ends up as heat in your room. Multiply by your own price per kilowatt hour.
It wasn't always like this. In 2014, the same Utah lab tested 22 fixtures. The best LED and the best sodium lamp were dead level, at 1.7, and the LEDs cost 5 to 10 times more to buy. Back then, sodium was the smart choice.
Source · Nelson & Bugbee, PLOS ONE (2014)
Heat, colour and lamp life ▶ 3:35
Every watt a lamp uses ends up as heat in your room. But where it goes is different. A sodium lamp sends much of its heat straight down onto the plants, as infrared. An LED's heat goes into its metal fins and rises off the back, away from the plants.
Source · Nelson & Bugbee, PLOS ONE (2015)
Through a thermal camera, the Utah canopy under a bare sodium lamp ran about 1 °C warmer than under the LEDs.
Source · Westmoreland, Kusuma & Bugbee, PLOS ONE (2021), S3 Fig.
Under stress, the gap grows. A Utah model found that with thirsty plants and still air, leaves could run 12 °C above the air under a sodium lamp, against 8 °C under LEDs. With watered plants and a fan, the difference is small.
Source · Nelson & Bugbee, PLOS ONE (2015)
That's why growers who switch to LED often report running the room a little warmer. And because a good LED draws less power, there's less heat to get rid of in the first place.
One more difference you can see. Under sodium light everything looks orange: its colour accuracy score (CRI) is 43 out of 100. White LEDs usually score over 70. So pests and yellowing leaves are easier to spot.
Source · Westmoreland, Kusuma & Bugbee, PLOS ONE (2021)
Then there's lifespan. A double-ended sodium lamp is rated for about 10,000 hours, so on 12 hours a day you'd replace the lamp every two years or so. LEDs are typically rated for about 50,000 hours before they fade to 70% of their light.
Source · Nelson & Bugbee, PLOS ONE (2014)
When HPS still makes sense ▶ 4:56
In a cold room. If your grow space is a chilly garage or loft in winter, a double-ended sodium lamp's heat isn't waste. It warms the canopy, and the Utah team called that useful in the heating season. With an LED in the same room, you may need to add heat.
Source · Nelson & Bugbee, PLOS ONE (2014)
And when you already own one. A working sodium lamp you've paid for still grows good flower. It just costs about a quarter more electricity for every gram.
Source · Westmoreland, Kusuma & Bugbee, PLOS ONE (2021)
What about ceramic metal halide (CMH), often sold as the middle ground? In that 2014 test, the best one made 1.46 micromoles per joule: less light per watt than the sodium lamp.
Source · Nelson & Bugbee, PLOS ONE (2014)
The sodium lamp grew slightly more per square metre. The LED grew far more per unit of electricity, put less heat on the plants and shows you their true colours. If you're choosing a light, compare light per watt, not the watts on the box.
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Sources
- Westmoreland, F.M., Kusuma, P. & Bugbee, B. (2021). "Cannabis lighting: Decreasing blue photon fraction increases yield but efficacy is more important for cost effective production of cannabinoids." PLOS ONE 16(3): e0248988. doi:10.1371/journal.pone.0248988 (CC BY 4.0)
- Magagnini, G., Grassi, G. & Kotiranta, S. (2018). "The Effect of Light Spectrum on the Morphology and Cannabinoid Content of Cannabis sativa L." Medical Cannabis and Cannabinoids 1(1): 19-27. doi:10.1159/000489030
- Nelson, J.A. & Bugbee, B. (2014). "Economic Analysis of Greenhouse Lighting: Light Emitting Diodes vs. High Intensity Discharge Fixtures." PLOS ONE 9(6): e99010. doi:10.1371/journal.pone.0099010
- Nelson, J.A. & Bugbee, B. (2015). "Analysis of Environmental Effects on Leaf Temperature under Sunlight, High Pressure Sodium and Light Emitting Diodes." PLOS ONE 10(10): e0138930. doi:10.1371/journal.pone.0138930
- Kusuma, P., Pattison, P.M. & Bugbee, B. (2020). "From physics to fixtures to food: current and potential LED efficacy." Horticulture Research 7: 56. doi:10.1038/s41438-020-0283-7
- Seed Science EP43 (What Size LED Do You Actually Need for Your Tent?).
Every claim in this article comes from the episode script, which is checked against these papers. Prices on Seedscanner are dated observations; this page is not growing or legal advice.
