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 for cannabis has become the grow scene’s favourite topic. VPD, vapour pressure deficit, 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. But here is the catch: VPD for cannabis is far less well researched than the neatly gridded charts on grow room walls suggest. Exactly one direct cannabis study sits behind the topic, everything else is transfer from better-studied plants or industry consensus. 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 In short: VPD describes how much water the air can still pull out of the plant. It is given in kilopascals (kPa) and rises as the air gets warmer or drier. 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. VPD for Cannabis: What Is Proven 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 VPD for cannabis looks like once you put cannabis studies, C3 transfer and industry consensus side by side. 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
