Cannabis Humidity Chart: RH and VPD by Growth Stage, Day and Night
Humidity affects cannabis at every stage, but it matters most during flower, when dense bud structures create microclimates that pathogens exploit and when the plant's own transpiration can raise tent humidity well above ambient levels. Vapour pressure deficit — a combined measure of temperature and humidity — gives a far more useful picture than relative humidity alone. This guide covers what the research confirms, what experienced growers target in practice, and the important distinction between the two.
Cannabis humidity chart by stage, lights on and lights off
The chart puts the ranges from this guide in one place. Lights-off humidity is listed separately because that is where most tents drift out of range: the air cools, the same amount of water vapour now makes up a bigger share of what the air can hold, and RH climbs even though nothing new was added. Readings should be taken at canopy height. The figures are commonly cited grower ranges, not results of a cannabis-specific trial, and the night column is a ceiling to stay under rather than a target to aim for.
| Stage | RH, lights on | RH, lights off (stay under) | VPD, lights on |
|---|---|---|---|
| Seedling / clone | 65 – 80% | 80% | 0.4 – 0.8 kPa |
| Vegetative | 55 – 70% | 70% | 0.8 – 1.2 kPa |
| Early – mid flower | 45 – 60% | 60% | 1.0 – 1.5 kPa |
| Late flower (last 2–3 weeks) | 40 – 50% | 55% | 1.2 – 1.6 kPa |
VPD — why it matters more than relative humidity alone
Relative humidity changes with every temperature swing, so a reading of sixty percent at twenty degrees Celsius means something very different than sixty percent at twenty-eight degrees. Vapour pressure deficit, expressed in kilopascals, combines temperature and humidity into a single number that reflects the actual drying force on the leaf surface. A high VPD pulls moisture aggressively from the leaves and drives transpiration; a low VPD means the surrounding air is nearly saturated and the plant struggles to release water vapour, which slows the internal nutrient transport system. For cannabis, which moves a large volume of water through its tissues during rapid growth, maintaining VPD in the right window keeps both photosynthesis and nutrient delivery running at full speed.
Stage-by-stage VPD ranges — grower consensus, not a research table
There is no peer-reviewed scientific table establishing precise VPD targets for cannabis by growth stage. The ranges below are drawn from the accumulated experience of commercial and home growers and are commonly referenced across the cultivation community — they should be treated as practical guidelines, not as research-validated thresholds. For seedlings and clones, whose tiny root systems cannot keep up with rapid water loss, a low VPD of roughly 0.4 to 0.8 kPa (corresponding to high humidity around seventy to eighty percent) keeps transpiration slow while roots establish. During vegetative growth, a VPD of approximately 0.8 to 1.2 kPa supports steady transpiration and nutrient flow as the root system expands. In flower, many growers push VPD toward 1.0 to 1.6 kPa by lowering humidity to the fifty to sixty percent range, reducing the still-air pockets inside buds where fungal pathogens thrive.
What research does confirm about cannabis and humidity
A peer-reviewed study on Cannabis sativa found that elevated relative humidity significantly decreases cannabinoid concentrations while also delaying flowering development. This confirms that humidity is not just a comfort variable — it directly impacts the chemical quality of the harvest. The study did not produce a VPD chart by stage, but it provides the strongest published evidence that letting humidity drift too high during flower has measurable consequences on potency.
Managing humidity swings across the light cycle
Cannabis plants transpire heavily while the lights are on and slow down sharply in the dark, which means humidity can spike at lights-off as the air cools and its capacity to hold moisture drops. In a sealed tent during late flower, this spike can push humidity above seventy percent within an hour of lights-out, exactly when the buds are most susceptible to botrytis. The most common mitigations are a dehumidifier set to the target range for the current stage, an exhaust fan that ramps up when humidity crosses a threshold, and defoliation of the lower canopy to improve airflow through the plant. None of these replaces monitoring — a sensor logging VPD across the full 24-hour cycle reveals spikes that a single daytime reading misses entirely.
Why humidity spikes in the first two hours after lights-off
The jump is mostly physics. Air at 26°C and 55% RH holds about 1.85 kPa of water vapour, and its VPD is around 1.5 kPa. Cool that same air to 20°C at lights-off without removing any water and RH rises to roughly 79%, while VPD falls to about 0.5 kPa. The dew point of that air is near 16°C, and bud and leaf surfaces in a dark tent can run a degree or two below air temperature, so the gap to condensation inside dense colas gets small. On top of that, the plants are still releasing water they took up during the day, and in many tents the exhaust drops to a night setting at the same moment. The result is a peak in the first one to two hours of the dark period that a daytime reading never shows. Botrytis needs free water or near-saturated air on the flower surface for several hours to infect, which is why this window matters most in late flower, when buds are at their densest.
How to flatten the lights-off spike
Start the dehumidifier and keep the exhaust at its day speed for the first hour or two of the dark period instead of letting a timer drop both at lights-off; that is the stretch where they do the most good. Make the temperature drop gentle: in late flower a night that is only 3–5°C cooler than the day produces a much smaller RH jump than a 10°C fall. Keep the circulation fans running all night so there is no still air inside the canopy. Water early in the lights-on period rather than just before lights-off, so the wet medium is not evaporating into cooling air. Lower-canopy defoliation and enough space between plants help air reach the inside of the colas. Check the result with a logging hygrometer at canopy height: if the night maximum still crosses the ceiling in the chart, the fix is not finished.
How long plants take to recover after you fix humidity or VPD
No published study gives a recovery timeline for cannabis after a humidity or VPD correction, so what follows is grower experience and general plant physiology. Stomata and transpiration respond to a new VPD within minutes to hours, and drooping caused by a too-high VPD usually eases within a day once the air is corrected and the root zone has water. Leaves already damaged by long periods of wrong humidity — crisp edges from very dry air, or spots from condensation — do not repair themselves; judge the fix by the new growth over the next five to ten days. Mould is different: a bud that has started to rot will not recover when humidity comes down, and the affected part should be removed carefully so spores do not spread to the rest of the plant.
How GrowScope helps
GrowScope's climate log files each temperature and humidity reading you enter under lights-on or lights-off, computes leaf VPD for it, and warns about high night humidity in flower, so growers can see whether their flower room spends the dark period in the danger zone for pathogens. Over a cycle, the log lets them correlate humidity patterns with growth rate and plant health.
Key takeaway
Use the chart as a guide — roughly 65-80% RH for seedlings, 55-70% in veg, 45-60% in flower and 40-50% in late flower — and watch the first two hours after lights-off, when cooling air pushes RH up fastest and the bud rot risk peaks.