LED or HPS: which is better for cannabis?

Q: LED or HPS: which is better for cannabis?

LED versus HPS for cannabis quality and yield per photon is an open question at GenetixBay: perhaps. At equal PPFD, research shows similar yields, while LED wins clearly on running cost, about 2.5–3 µmol/J against roughly 1.7 µmol/J for HPS, and on leaf heat load. One small trial found more flower mass under HPS but higher cannabinoid concentration under LED. A Bay Trial will test it.

Short answer

On running cost, LED wins: about 2.5–3 µmol/J against roughly 1.7 µmol/J for HPS ^[2]. On yield and quality per photon, the GenetixBay answer is "perhaps": at equal PPFD the research shows broadly similar results, the trials are few, and the question is down for a controlled Bay Trial. Contested (house position: open question).

Why

Photon quantity drives cannabis yield far more than colour. Across three studies, a lower blue fraction (HPS is about 4 % blue) slightly raised yield, but the authors concluded fixture efficacy mattered more for cost ^[1].

In a 2018 trial at about 450 µmol/m²/s, plants under HPS were taller and made more flower dry weight than under two LED spectra, while the LED plants had higher THC and CBD concentrations; total cannabinoid yield didn't differ ^[3]. It was small and funded by an LED maker, so treat it as one data point.

HPS puts more radiant heat on the leaf; LED leaves usually run cooler than the air ^[4]. That changes the climate you need, not just the bill.

The numbers

	White LED	HPS
Efficacy	2.5–3+ µmol/J ^[2]	~1.7 µmol/J ^[2]
Electricity for the same PPFD	~35–45 % less	baseline
Leaf temperature vs air	usually 1–3 °C (2–5 °F) cooler	near air or warmer ^[4]
Blue fraction	~5–15 % typical	~4 % ^[1]
Yield at equal PPFD	similar in most work	similar; more mass in one small trial ^[3]
Cannabinoid concentration	higher in one trial ^[3]	lower in that trial
Lamp replacement	none for years	bulbs lose output

Verdict: pick LED for cost, heat and control; whether HPS buds are better per photon is unproven either way, so GenetixBay is testing it in a Bay Trial.

What to do

Compare fixtures by µmol/J and PPFD map, not lamp type alone.
Switching to LED? Raise air temperature 1–3 °C and re-check leaf VPD.
Running HPS? Keep it; just measure leaf temperature and cool accordingly.
Want to know for your cultivar? Run a side-by-side at the same PPFD and log grams and lab results.

Common mistakes

Comparing an LED at 600 µmol/m²/s with HPS at 1,000.
Keeping HPS room settings after switching to LED.
Treating one manufacturer-funded trial as settled.
Adding Cal-Mag "because it's LED" (Myth).

Full article: Light spectrum
Are purple (blurple) LEDs as good as white LEDs?
Should I run my room warmer with LEDs?
PPFD and DLI by stage
Terms: LED vs HPS · Bay Trials

An LED (light-emitting diode) grow light uses solid-state diodes to produce light; modern fixtures are the most efficient grow lights, at roughly 2.5–3 µmol/J.Photon efficacy is how many micromoles of PAR photons a fixture produces per joule of electricity (µmol/J); higher means more light for the same power bill and heat.HPS (high-pressure sodium) is a gas-discharge lamp with orange-yellow light peaking near 589 nm; it was the standard flowering light before LEDs.Photosynthetic photon flux density: the number of PAR photons landing on one square metre each second, in µmol/m²/s; the standard measure of light intensity at the canopy.Bay Trials are GenetixBay's member-run controlled grow experiments that test growing claims side by side and publish the results openly.Blue light is the 400–500 nm band; it keeps plants compact with shorter internodes and thicker leaves, but a high blue fraction slightly lowers cannabis yield.The weight of cannabis after drying (and usually trimming), the basis growers use for yield; it is typically about 20-25% of trimmed wet weight.THC (Δ9-tetrahydrocannabinol) is the neutral, decarboxylated cannabinoid mainly responsible for cannabis's reported intoxicating effects; the living plant makes it almost entirely as THCA.CBD (cannabidiol) is a non-intoxicating neutral cannabinoid formed when CBDA decarboxylates; hemp-type and CBD-dominant cannabis make it mainly as CBDA in their trichomes.A cannabinoid is any compound that binds the body's cannabinoid receptors or shares the typical cannabis terpenophenolic structure; the plant-made ones are phytocannabinoids such as THCA and CBDA.Leaf temperature is the actual surface temperature of a leaf, which often differs from air temperature by 1–3 °C depending on light source and transpiration.Vapour pressure deficit: how much more water vapour the air could hold at its temperature, in kPa; it is the main environmental driver of transpiration.Cal-Mag is a supplement supplying calcium and magnesium, usually as nitrates; need depends on cultivar, water source (RO vs tap) and medium (coco), not on LED lighting.

Follow-up questions

Does HPS give better buds than LED?

Whether HPS gives better cannabis buds than LED is an open question; the GenetixBay answer is 'perhaps', and it is slated for a controlled Bay Trial. One 2018 trial at about 450 µmol/m²/s found more flower dry weight under HPS but higher THC and CBD concentration under the LEDs, with equal total cannabinoid yield. Other work finds similar yield at equal PPFD. Evidence is thin.

Is LED cheaper to run than HPS?

Yes, LED is cheaper to run than HPS for the same light on the canopy. Good current LED fixtures exceed 2.5–3 µmol of photosynthetic photons per joule, against about 1.7 µmol/J for HPS, so an LED needs roughly 35–45 % less electricity for the same PPFD. Less wasted heat also means smaller cooling bills. This part of the comparison is established.

Do I need to change my grow room settings when switching from HPS to LED?

Yes, switching from HPS to LED changes leaf temperature. HPS radiates heat that warms leaves to around air temperature or above, while LED leaves usually run 1–3 °C cooler than the air. Many growers raise air temperature by 1–3 °C under LED to keep leaf temperature and VPD where they were. Measure leaf temperature with an IR thermometer rather than guessing.

Sources

Westmoreland FM, 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. pmc.ncbi.nlm.nih.gov/articles/PMC7987162/

Kusuma P, Pattison PM, Bugbee B, 2020. From physics to fixtures to food: current and potential LED efficacy. Horticulture Research 7:56. doi.org/10.1038/s41438-020-0283-7

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.org/10.1159/000489030

Nelson JA, Bugbee B, 2015. Analysis of environmental effects on leaf temperature under sunlight, HPS and LEDs. PLoS ONE 10(10): e0138930. pmc.ncbi.nlm.nih.gov/articles/PMC4598144/