Cultivation

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

In the 1970s, botanists could not even agree on whether cannabis was one species or several. Today there are plants with three times as many chromosome sets as the original, and, in a first study, up to 65 percent more total cannabinoids. How we got here is a short history of plant breeding in fast-forward. The Start: A Plant Nobody Really Knew In 1976, two botanists drew a line under a 250-year-old debate. Ernest Small and Arthur Cronquist published a paper in the journal Taxon with the plain title “A practical and natural taxonomy for Cannabis” (Small & Cronquist, 1976). Their conclusion: there is only one biological species, Cannabis sativa L., with two subspecies and a wild and a cultivated variety each. The labels “Sativa” and “Indica” marketed today are, taxonomically, more of a marketing construct than a botanical finding, a view that contemporary reviews confirm (McPartland, 2018). What existed as breeding at that time ran largely underground. In California and the Netherlands, hobby breeders crossed landraces (regional, naturally adapted original strains) from Afghanistan, Mexico and Thailand and named the results Skunk #1, Northern Lights or Haze. Scientific genetics happened in parallel elsewhere: on fiber hemp in Wageningen, on industrial hemp in Bologna. The Mendel Turn: One Locus Governs the THC-CBD Ratio In 2003, Etienne de Meijer and his Italian team delivered one of the most important findings in cannabis genetics. In the journal Genetics, they showed that the ratio of THC to CBD in a plant is controlled by a single locus with two co-dominant alleles (de Meijer et al., 2003). That sounds technical, but in plain terms it means: cross a THC-dominant with a CBD-dominant plant and you can predict the cannabinoid profile of the offspring like Mendel’s peas. That opened the door to targeted variety development. Not by chance, a first wave of CBD-dominant hemp varieties appeared on the market in the years that followed, and the idea of cleanly defining “chemical varieties” or chemotypes became the standard (Russo, 2019). 2011: Cannabis Gets a Genome The next big leap came out of Toronto. Harm van Bakel and colleagues sequenced the genome of the drug strain Purple Kush in 2011 and compared it with the fiber hemp Finola (van Bakel et al., 2011). The result: around 534 megabases, 30,000 genes and, at the molecular level, clear differences in the cannabinoid biosynthesis pathways. Four years later, a team in Canada backed this up with marker data: hemp and marijuana are genetically clearly separate populations, but variety names often correlate surprisingly poorly with the underlying genetics (Sawler et al., 2015). Then, in 2018, the keystone: a high-resolution map showed that the genes for THC and CBD synthase sit in extremely rearranged genome regions, packed with jumping elements (Laverty et al., 2019). That explains why cannabinoid profiles can vary so strongly between varieties, and it provides the basis for modern marker-assisted selection. The fiber hemp field kept pace in parallel with its own reviews and Wageningen programs (Salentijn et al., 2014). Domestication: Older and More East Asian Than Thought For a long time, Central Asia counted as the cradle of cannabis. In 2021, Guangpeng Ren and colleagues presented a resequencing of 110 accessions worldwide in Science Advances and shifted the story eastward (Ren et al., 2021). Cannabis was, accordingly, domesticated around 12,000 years ago in East Asia; the sativa– and indica-type lines known today are the result of later selection. The original wild forms are largely extinct, which makes highland landraces from Afghanistan and the Hindu Kush a genetic treasure (McPartland & Small, 2020). If you want the full picture of genome research, you will find the roadmap in a review article: available assemblies, GWAS studies, sex determination, cannabinoid clusters (Hurgobin et al., 2020). The Cannabis sativa of 2026 is genetically better understood than many classic crops. Polyploids: The Latest Wave That leaves the current trend, and it is old and new at once. Polyploidy means a plant has three (triploid), four (tetraploid) or even six (hexaploid) chromosome sets instead of the usual two. This has worked in many crops for decades; in cannabis there were attempts early on, but it stayed a niche (Parsons et al., 2019). Triploids have a practical charm: they are largely sterile and barely form seeds. For flower production that is a real advantage: no unwanted pollination and more even flower mass. A controlled study from Connecticut confirmed in 2024 that triploid cannabis plants keep pace with diploids in growth and flower yield, but their seed formation is dramatically reduced (Kurtz et al., 2024). And then comes the latest paper, fresh from Korea: Tae Hyun Ha and his team presented the first stable hexaploid cannabis plants in early 2026 (Ha et al., 2026). With an optimized colchicine treatment and multi-generation selfing, they arrived at plants whose hexaploid status is cleanly confirmed by flow cytometry and chromosome counting. The study reports roughly 65 percent higher total cannabinoid content, about 60 percent more plant height and 2.7 times the dry flower mass compared with diploids. Whether this holds up in practice is for follow-up research to show, but the trend is clear. What Has Changed in 50 Years The plant is the same, what we know about it is not. In 1976 there was an argument over whether cannabis is one species. In 2026, cannabinoid profiles can be dialed in by marker selection, the domestication history reconstructed and varieties bred with triple the chromosome count. What remains open: most variety names on the market still correlate poorly with the actual genetics, and the diversity of the original landraces is under threat (Andre et al., 2016). So keep an eye on the next wave of cannabis research. It will probably not come from Wageningen or Toronto, but from where polyploids, domestication genetics and variety identity meet. Take-Aways One species, many chemotypes: cannabis is botanically a single species; the THC-CBD ratio depends on a single locus. Genome since 2011, map since 2018: modern cannabis breeding is marker-assisted and no