Leaf temperature is the temperature of the leaf tissue itself. It is set by a balance of energy gained (absorbed light and radiant heat) and lost (transpiration cooling, convection to air, radiation to cooler surfaces). It is the temperature that governs photosynthesis rate and leaf VPD, so it matters more than the room thermometer.
In practice
Under LED, which emits little radiant heat, a well-watered, transpiring leaf often sits 1–3 °C (2–5 °F) below air temperature. Under HPS, the extra infrared often pushes leaves to or above air temperature. Nelson and Bugbee (2015) quantified this. It is the main reason a grower who switches from HPS to LED should raise room temperature: the goal is a similar leaf temperature, typically 24–28 °C (75–82 °F) in flower.
Leaf temperature rising towards or above air temperature under LED can mean stomata are closing, a sign of root problems, drought or VPD set too high.
Don't confuse with
- Air temperature: what the room thermometer reads.
- Canopy-top temperature near the fixture: can be hotter than leaves lower down.
- Low-stress training (LST): an unrelated training method sharing the abbreviation; prefer "leaf temp" to avoid confusion.
The evaporation of water from a plant, mainly through open stomata, which pulls water and dissolved nutrients up from the roots.The process in chloroplasts that uses light energy to turn carbon dioxide and water into sugars and oxygen; cannabis uses the C3 pathway.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.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.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.Microscopic pores, mostly on the underside of leaves, each opened and closed by two guard cells to control gas exchange and water loss.LST (low-stress training) bends and ties stems so the plant grows wide and flat, exposing more bud sites to light without cutting or breaking tissue.Leaf VPD is the vapour pressure deficit between the inside of a leaf, at leaf temperature, and the surrounding air; it is what actually drives transpiration.An IR (infrared) thermometer reads surface temperature from emitted infrared radiation; growers use it to measure leaf temperature for leaf VPD.In growing, temperature usually means grow-room air temperature; cannabis grows best around 24–28 °C (75–82 °F) by day and a few degrees cooler at night.LED vs HPS is the debate over whether LED or high-pressure sodium lights grow better cannabis; LEDs are clearly more efficient, while final-quality differences remain an open question.Heat stress is damage or slowed growth when leaf or canopy temperature rises above what the plant can handle, typically above about 30–32 °C (86–90 °F) in cannabis.
