VPD and Climate: Which Setpoints Actually Help Your Club

Every grow room has a thermo-hygrometer these days, and most have a VPD chart pinned to the wall somewhere. But when a member asks tomorrow why you run 50 % humidity in bloom, what do you say? “It says so on the internet” is not a good answer. Let’s look at what the research actually gives us.

VPD, vapour pressure deficit, has become the grow scene’s favourite term. The value bundles temperature and relative humidity into a single number and describes how strongly the air pulls water “out of the leaves”. That is handy, because it lets you steer the climate without juggling temperature and humidity separately in your head. The problem: the neatly gridded VPD charts that circulate everywhere hardly come from cannabis studies. For a club that wants to grow reproducibly and traceably, an honest look at what is proven, and what only sounds good, is worth it.

What VPD Is Actually About

The plant opens its stomata to take in CO₂ for photosynthesis. In doing so it loses water through the same openings. How much depends on the vapour pressure deficit: if the air is dry and warm (high VPD), a lot evaporates; if it is humid and cool (low VPD), little does. Both extremes cost yield. At too high a VPD the plant closes its stomata to avoid drying out, and thereby also shuts down CO₂ uptake, which slows photosynthesis. At too low a VPD, moisture builds up, transpiration as the “engine” for nutrient transport from the roots weakens, and the fungal risk rises.

You can calculate VPD yourself, the formula is simple and built into every climate controller: from the leaf temperature (roughly air temperature) you get the saturation vapour pressure, from which you subtract the actual vapour pressure that follows from relative humidity. The result in kilopascals (kPa) is the VPD. Important in practice: because temperature is part of it, “60 % humidity” at 22 °C is a completely different VPD than at 28 °C. That is exactly why switching to a VPD value instead of pure humidity targets pays off.

What Is Actually Proven in Cannabis, and What Is Transferred

Here comes the uncomfortable part. If you go through the cannabis literature systematically (we did this in our own setpoint review), you find surprisingly little direct VPD research. The only clean cannabis VPD study comes from Sheldon et al. (2021): they tested 13 hemp varieties in a walk-in climate chamber against four VPD levels and found that the threshold at which the stomata close is variety-dependent. In roughly 5 of 13 varieties they close much earlier. Two catches for us: these are fiber hemp trials, not THC-rich genetics, and the study sits behind a paywall. Fiber hemp and medical cannabis differ enough in leaf anatomy and stomata architecture that you should not take absolute threshold values one to one.

The second solid cannabis pillar is older: Chandra et al. (2008) measured photosynthesis response curves against temperature, CO₂ and light. That is the source of the often-cited finding that photosynthesis reaches its optimum at around 25 to 30 °C and saturates at roughly 750 ppm CO₂. It is not a VPD study, but it marks out the temperature corridor you should even be moving in.

The rest of the VPD “wisdom” is transferred from better-researched C3 plants: tomato, lettuce, cucumber. Like these, cannabis is a C3 plant, so the mechanisms transfer, but the absolute numbers do not. The most important pieces carried over from C3 model plants:

• Setpoint systematics: Shamshiri et al. (2018) provide evidence-based T, RH and VPD ranges per growth stage for tomato, neatly graded into “optimal / borderline / critical”. Exactly this kind of synthesis is completely missing for cannabis. The framework carries over, but you have to adapt the actual values yourself.
• The stomata mechanism: Amitrano et al. (2021) showed in lettuce that low VPD leads to more and smaller stomata and raises net photosynthesis by around 18 %. The mechanism holds across C3 plants.
• The upper limit: Zhong et al. (2023) quantified a VPD threshold at ecosystem level (around 3.5 to 4.0 hPa) above which photosynthesis begins to tip. Together with Sheldon’s hemp thresholds, that gives a plausible basis for arguing an upper VPD limit. The absolute number from an ecosystem study is not, however, directly transferable to the grow room.
• The most exciting open field: Amitrano et al. (2021, second paper) found in lettuce that under different VPD (0.69 vs. 1.76 kPa) the profile of bioactive compounds shifts. The obvious hypothesis, that VPD could also influence cannabinoid and terpene profiles, is simply not yet tested for cannabis.

Practical Setpoints, With an Honest Evidence Label

The following tiering sums up what comes together from cannabis studies, C3 transfer and industry consensus. Read the last column too: it tells you how firm the ground is under each value. “Proven” means direct cannabis evidence, “transferred” means derived from C3 plants, “consensus” means practical experience without a clean study.

Stage	Temperature (day)	rel. humidity	VPD (day)	Confidence
Clones / rooting	22 to 25 °C	70 to 80 %	0.4 to 0.7 kPa	transferred
Vegetative	24 to 28 °C	60 to 70 %	0.8 to 1.2 kPa	partly proven (T), rest transferred
Early bloom	24 to 26 °C	55 to 65 %	1.0 to 1.3 kPa	consensus
Late bloom	22 to 24 °C	45 to 55 %	1.2 to 1.6 kPa	humidity from fungal risk, VPD transferred

A few things that really stand out from the studies and that you can pass on to a member:

The temperature corridor is the best proven. Chandra et al. (2008) back 25 to 30 °C for maximum photosynthesis in the growth phase. That many growers drop to 22 to 24 °C in late bloom is, by contrast, mostly industry knowledge. The idea that cooler nights trigger colour and trichomes is plausible, but not cleanly proven by a cannabis study.

The humidity targets in bloom do not come from plant physiology, but from fungal risk. That is the most important point for a club. The often-cited value “below 60 % relative humidity in bloom” derives almost entirely from Botrytis management, not from a yield optimum. More on that in a moment.

Variety dependence is real. If Sheldon et al. (2021) show for hemp that varieties close their stomata at different points, then for you that means: a single VPD setpoint for all varieties in the room is a compromise. Anyone running mixed genetics will never hit the optimum for all of them at once.

The Coupling That Matters Most in the Club: Climate and Fungal Pressure

For a cultivation association, the most interesting climate question is rarely “how do I squeeze out the last percent of yield”, but “how do I avoid the flower rotting on me before handout”. And that is exactly where climate control becomes the central lever. Botrytis cinerea, the agent of bud rot, destroys flowers according to Mahmoud et al. (2023) especially fast at relative humidity above 70 % and moderate temperatures of 17 to 24 °C. That is precisely the range a dense, mature flower likes to land in during late bloom when dehumidification cannot keep up.

Buirs & Punja (2024) list climate modulation, that is targeted control of humidity, VPD and air movement, as one of the central measures in integrated pathogen management. The honest addition: an exact, empirically confirmed VPD or humidity threshold for cannabis, above which the Botrytis risk tips, does not exist. The value “below 60 %, better 45 to 55 % in late bloom” is qualitatively well founded, but the exact threshold is a matter of experience, not study data.

What does that mean concretely for the club? Your humidity targets in bloom are, first and foremost, an insurance policy against crop loss and contamination, and thus directly a quality and handout-safety issue. Anyone who reliably keeps relative humidity in the dense late bloom below about 55 % and provides air movement in the canopy noticeably lowers the bud rot risk. That is not fine-tuning for the last gram, it is basic protection.

The Catch Hardly Anyone Talks About: Energy

Low VPD in bloom means: actively dehumidify. And dehumidification costs electricity, depending on the setup quite a lot. Mehboob et al. (2020) modelled the energy consumption of an indoor cannabis facility; HVAC and dehumidification are large items in it, alongside lighting. The awkward conflict: the humidity range that minimises your fungal risk is exactly the one that makes your dehumidifiers work hardest. For a club on a limited budget that is a real trade-off, not only an agronomic one but a business one. So far no cannabis study has cleanly worked through this conflict between climate optimum and energy cost; anyone planning here works from experience and rules of thumb.

What We Do Not Yet Know

So that nobody passes this text on as “settled truth”, here are the gaps in black and white:

• There is no systematic VPD setpoint review for cannabis with solid evidence per growth stage. Everything that looks like one is transfer or consensus.
• The only direct VPD study concerns fiber hemp, not THC-rich medical genetics.
• Whether VPD shifts the cannabinoid and terpene profile is an open, plausible hypothesis, shown in lettuce, untested in cannabis.
• Quantitative humidity/VPD thresholds for Botrytis in cannabis are missing; the values given are qualitatively founded.
• All empirical studies run in small chambers. A walk-in test chamber does not scale linearly to a real grow room with its microclimate corners and uneven air movement.

Takeaway for the Club

Start with what is proven, and treat the rest as well-founded starting values, not as a law of nature. Concretely:

1. Switch to VPD instead of pure humidity. Because temperature is part of it, a VPD target is more consistent across different room conditions than a fixed humidity percentage.
2. Hold the temperature corridor roughly at 25 to 28 °C in the growth phase and 22 to 24 °C in late bloom, the best-proven part.
3. Treat bloom humidity as fungal insurance. Below about 55 % relative humidity in the dense late bloom plus good air movement beats any yield fine-tuning if it gets the harvest through the handout audit safely.
4. Factor in the dehumidification cost before you commit to a very low humidity setpoint, especially on a smaller club budget.
5. Keep records per variety. If you run mixed genetics, note which variety reacts how at which climate. The variety dependence the research points to is something you can only map out yourself, for your own genetics.

[Image: VPD chart with colour-marked stage target ranges, placeholder]

Sources

1. Own setpoint review (T, RH, VPD, CO₂ per stage), 2026-05-04
2. Chandra et al. (2008), Physiol. Mol. Biol. Plants. doi:10.1007/s12298-008-0027-x
3. Sheldon et al. (2021), J. Crop Improv. doi:10.1080/15427528.2021.1883175
4. Shamshiri et al. (2018), Int. Agrophysics. doi:10.1515/intag-2017-0005
5. Amitrano et al. (2021), Agronomy 11, 1396. doi:10.3390/agronomy11071396
6. Amitrano et al. (2021), Horticulturae 7, 32. doi:10.3390/horticulturae7020032
7. Zhong et al. (2023), Science Advances. doi:10.1126/sciadv.adf3166
8. Mahmoud et al. (2023), Botany. doi:10.1139/cjb-2022-0139
9. Buirs & Punja (2024), Plants 13, 786. doi:10.3390/plants13060786
10. Mehboob et al. (2020), IEEE OAJPE. doi:10.1109/oajpe.2020.3003540