Should I run my room warmer with LEDs than with HPS?

Q: Should I run my room warmer with LEDs than with HPS?

Yes, run an LED room about 1–3 °C (2–5 °F) warmer than an HPS room. Under LEDs, well-watered leaves usually sit 1–3 °C below air temperature; under HPS they sit near or above it, because HPS radiates more heat onto the leaf. For a target leaf temperature of about 24–27 °C (75–81 °F), that means roughly 26–28 °C (79–82 °F) air under LED. Measure leaves with an IR thermometer.

Short answer

Yes. Run an LED room about 1–3 °C (2–5 °F) warmer than you would with HPS. Under LED, healthy leaves usually sit 1–3 °C below air; under HPS they sit near or above it ^[1]. For a leaf at about 24–27 °C (75–81 °F), that means roughly 26–28 °C (79–82 °F) air under LED. Supported.

Why

HPS lamps emit a lot of infrared and radiant heat that lands directly on the leaf. LEDs put far less radiant heat on the canopy for the same PPFD, so a transpiring leaf cools below the air around it ^[1].

The plant responds to leaf temperature, not air temperature. Cannabis photosynthesis peaks around 25–30 °C leaf temperature under strong light ^[2]. A leaf that's 3 °C colder than you think is working below its best.

Leaf temperature also sets leaf VPD ^[3]. Cool leaves under LED mean a lower real VPD than an air-based reading suggests, so LED rooms run at HPS settings often end up effectively too humid. That, not the light, is the real story behind the Cal-Mag-under-LED Myth.

The numbers

	LED	HPS
Leaf vs air (well-watered, moving air)	−1 to −3 °C (−2 to −5 °F)	0 to +2 °C (0 to +4 °F)
Veg air for a good leaf temperature	26–28 °C (79–82 °F)	24–27 °C (75–81 °F)
Mid-flower air	24–27 °C (75–81 °F)	22–26 °C (72–79 °F)
Late-flower air	21–25 °C (70–77 °F)	20–24 °C (68–75 °F)
Upper air limit without CO₂	~30 °C (86 °F)	~30 °C (86 °F)

What to do

Measure leaf temperature with an IR thermometer (emissivity ~0.95) on lit top leaves.
Compare with air at canopy height.
Raise air temperature until leaves read about 24–27 °C in veg and early flower.
Recalculate leaf VPD and adjust RH to hit the stage band.
Re-check after any change in light intensity or fan setup.

Common mistakes

Keeping HPS setpoints after switching to LED.
Using air VPD from an app with LEDs.
Adding Cal-Mag instead of fixing leaf temperature and VPD.
Measuring leaf temperature in shade or from too far away.

Full article: Temperature by stage
LED or HPS: which is better?
VPD explained
How do I lower the temperature in my grow tent?
Terms: leaf temperature · IR thermometer

Follow-up questions

How do I measure leaf temperature?

Measure cannabis leaf temperature with an infrared thermometer set to emissivity about 0.95, aimed at a fully lit top leaf from 10–20 cm away so the spot covers only leaf. Take several readings across the canopy and compare with air temperature at the same height. Under LED a healthy, transpiring leaf usually reads 1–3 °C below air; above air suggests water stress or still air.

Does leaf temperature change VPD under LED?

Yes, cooler leaves under LED lower the real leaf VPD. At 26 °C air and 60 % RH, an app using air temperature shows about 1.34 kPa, but a leaf at 24 °C is only at about 0.97 kPa. Growers who ignore this run LED rooms too humid. Raising air temperature 1–3 °C, or lowering RH, brings leaf VPD back into the 1.0–1.5 kPa flower band.

Is 28 °C too hot under LED?

28 °C (82 °F) air is fine for cannabis under LED in veg and early flower, because leaves usually sit 1–3 °C cooler, near the 25–30 °C leaf optimum. Without added CO₂, keep air below about 30 °C (86 °F). In late flower, 20–24 °C (68–75 °F) air is the usual target. Check leaf temperature and leaf VPD rather than air alone.

Sources

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/

Chandra S, Lata H, Khan IA, ElSohly MA, 2008. Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions. Physiology and Molecular Biology of Plants 14(4):299–306. doi.org/10.1007/s12298-008-0027-x

Allen RG, Pereira LS, Raes D, Smith M, 1998. Crop evapotranspiration: guidelines for computing crop water requirements (FAO Irrigation and Drainage Paper 56), Chapter 3. FAO. fao.org/4/x0490e/x0490e07.htm