Temperature, humidity and CO₂ set the ceiling on what your lights and nutrients can do. The cannabis-specific evidence we’ve found so far comes mostly from leaf-level measurements rather than whole-plant harvests.
University of Mississippi researchers measured photosynthesis in high-THC cannabis leaves across light levels, temperatures and CO₂ concentrations. Photosynthesis was strongest at moderate temperatures and fell off above about 30 °C (86 °F). In a follow-up study, elevated CO₂ improved water-use efficiency by reducing water lost through the leaves.
Leaf measurements are useful but limited: a separate lighting study showed that leaf photosynthesis can max out long before whole-plant yield does. Treat these numbers as a guide to the environment, not a promise of bigger harvests.
Gaps we’re still filling: controlled whole-plant studies of VPD, humidity and day/night temperature for drug-type cannabis.
Key terms
VPD
Vapor pressure deficit: a measure combining temperature and humidity that describes how strongly the air pulls water from leaves.
Water-use efficiency
How much a plant photosynthesizes for each unit of water it loses through its leaves.
PPFD
Photosynthetic photon flux density: how much usable light reaches a surface each second, measured in µmol·m⁻²·s⁻¹. Usually measured at the top of the canopy.
Studies
2 studies
Photosynthetic response of Cannabis sativa L., an important medicinal plant, to elevated levels of CO2
Physiology and Molecular Biology of Plants · 2011
Drug-type cannabis
Lab study
Open access
Elevated CO₂ improved water-use efficiency by reducing water loss through the leaves.
Authors: Suman Chandra, Hemant Lata, Ikhlas A. Khan, Mahmoud A. ElSohly
17(3):291–295
Study plant: Leaves of four high-THC varieties, short-term gas-exchange measurements
Leaves of four high-THC varieties were measured at roughly ambient (390 ppm) and elevated (545 and 700 ppm) CO₂. Under elevated CO₂ the leaves partly closed their pores and lost less water, which raised their water-use efficiency.
Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions
Physiology and Molecular Biology of Plants · 2008
Drug-type cannabis
Lab study
Open access
Leaf photosynthesis was strongest at moderate temperatures and fell off above about 30 °C (86 °F).
Authors: Suman Chandra, Hemant Lata, Ikhlas A. Khan, Mahmoud A. ElSohly
14(4):299–306
Study plant: Leaves of a high-THC Mexican variety, measured in a leaf chamber
Using a leaf chamber, researchers measured photosynthesis and water-use efficiency in a high-THC variety across light levels up to 2,000 µmol·m⁻²·s⁻¹, temperatures from 20 °C to 40 °C, and CO₂ from 250 to 750 ppm. Photosynthesis and water-use efficiency rose with light at the cooler temperatures (20–25 °C), and temperatures above about 30 °C reduced photosynthesis. It remains a widely cited baseline for cannabis environment targets.