What is VPD and why does your plant care?
VPD stands for Vapor Pressure Deficit. It's the difference between how much water vapor the air could hold at a given temperature and how much it actually holds. In plain terms: it measures how "thirsty" the air in your grow space is.
When VPD is in range, your plant's stomata open up, the plant transpires actively, and it pulls nutrients up from the roots through mass-flow. Out of range, calcium and magnesium uptake gets restricted, stomata clamp shut, or free moisture sits on your buds — setting up ideal conditions for mold or mites.
How do you calculate VPD?
The calculator uses the Magnus equation (Alduchov & Eskridge 1996), valid between 0 and 60 °C:
- SVP (kPa) = 0.61078 × exp(17.2694 × T / (T + 237.29))
- AVP = SVP × (RH / 100)
- Air VPD = SVP − AVP
- Leaf VPD = SVP(T_leaf) − AVP — where T_leaf = T_air − 2 °C under LED (standard offset)
Leaf VPD is what actually matters for management decisions; an air-sensor reading can overestimate the real value by up to 28% under LEDs, because those fixtures don't heat the leaf surface the way an HPS does (Nelson & Bugbee, Utah State University, 2015).
What are the optimal VPD ranges by stage?
The recommended leaf VPD ranges (Corredor-Perilla et al. 2025, Frontiers in Plant Science, DOI: 10.3389/fpls.2025.1678142) are as follows:
- Seedling / Clone: 0.4–0.8 kPa — temp 22–26 °C, relative humidity 65–75%
- Veg: 0.7–1.2 kPa — temp 22–28 °C, relative humidity 55–70%
- Early bloom: 1.0–1.3 kPa — temp 22–26 °C, relative humidity 50–60%
- Late bloom: 1.2–1.5 kPa — temp 18–25 °C, relative humidity 40–50%
What happens when VPD is too low?
A VPD that's too low (RH 78–98%, VPD ≈ 0.05–0.25 kPa) is one of the most costly mistakes in indoor growing. The Corredor-Perilla et al. 2025 study measured a 71% reduction in floral biomass and a three-week delay in flowering onset under controlled conditions where relative humidity went unmanaged.
On the pathogen side, VPD below 0.4 kPa leaves free moisture on leaves and buds — ideal conditions for Botrytis cinerea. Active air circulation around the canopy reduces incidence by 66% to 92% (Punja, 2023, DOI: 10.1139/cjb-2022-0139).
What happens when VPD is too high?
At VPD above 1.6 kPa, stomata close to conserve water. The plant stops transpiring, mass-flow nutrient delivery shuts down, and you start seeing heat/drought stress: leaf tip burn, brown tips, upward leaf curl.
Above 2.0 kPa, the hot dry environment accelerates the reproductive cycle of Tetranychus (spider mites): at temperatures above 27 °C and RH below 40%, they can go from egg to adult in 5–7 days.
How do you adjust VPD in practice?
The general rule: move humidity first, temperature second. It's faster to humidify or dehumidify than to shift the temperature of the whole space, and swinging temps affects other parameters (CO₂, transpiration, photosynthesis).
- VPD too low: lower relative humidity or nudge temperature up slightly within the healthy range for your stage.
- VPD too high: raise humidity or lower temperature; in late bloom, prioritize dropping RH only down to the stage floor (40–50%) to stay out of mite territory.
- At lights-off, temps drop and humidity climbs: VPD can crash below 0.4 kPa. Keep the dehumidifier running through the dark period too.
- Under LED, always calculate leaf VPD (T_leaf = T_air − 2 °C) so you don't overestimate the real value.
What are the most common VPD mistakes?
- Tracking temperature and humidity separately instead of VPD: 25 °C and 60% RH "sounds fine" but VPD is the number that integrates both variables into what the plant actually experiences.
- Using the same VPD target all cycle long: a clone needs 0.4–0.8 kPa; that same range in late flower is an open invitation for mold.
- Ignoring nighttime VPD: lights go off, VPD crashes, nobody checks.
- Not accounting for the leaf temperature offset under LED: real VPD can be up to 28% lower than your air sensor reads (Nelson & Bugbee, Utah State University, 2015).
