A 2025 controlled study grew one CBD-dominant cultivar in growth chambers at 78–98% relative humidity and at 37–58% relative humidity, with mean air temperature nearly the same in both. Under high humidity, total CBD fell 78.2%, CBDA fell 79.5% and flower biomass fell 71.0%, and visible flowering started three weeks later. The paper reports no disease. Instead it describes physiological symptoms: guttation, tip burn, leaf rot and early leaf senescence. Common grower advice holds that high humidity matters mainly because it invites botrytis. This trial suggests humidity may also cost yield and cannabinoids before any fungus arrives. It was run at 20–22 °C, not in monsoon conditions, so for Thai growers it is a result worth testing, not a settled one.
1. The numbers
Corredor-Perilla, Kwon & Park, Frontiers in Plant Science 16 (2025), doi:10.3389/fpls.2025.1678142. [verified — Frontiers | Elevated relative humidity significantly decreases cannabinoid concentrations while delaying flowering development in Cannabis sativa L.]
One Type III (CBD-dominant) cultivar, Cherry Berry, with 10 clones per treatment:
| Low RH | High RH | |
|---|---|---|
| Canopy relative humidity | 37–58% | 78–98% |
| Air VPD, flowering stage (calculated) | 0.92 kPa | 0.25 kPa |
| Air temperature (week-14 mean) | 20.3 ± 3.6 °C | 21.6 ± 1.4 °C |
| CBDA | 27.80 mg/g | 5.70 mg/g (−79.5%) |
| Total CBD | 2.80% | 0.61% (−78.2%) |
| Total CBC | 0.42% | 0.04% (about −90%) |
| Flower biomass | baseline | −71.0% |
| Total biomass | baseline | −75.3% |
| Visible flowering onset | week 10 | week 13 |
| Harvest | week 14 | week 15 |
How to read these figures:
- VPD. The paper calculated VPD from air temperature and RH. At 20.3 °C, 37–58% RH works out to roughly 1.0–1.5 kPa, while 0.92 kPa corresponds to about 61% RH. The paper does not say over what period its stage values were averaged, so treat 0.92 kPa as approximate.
- CBC and biomass. The CBC percentage is my calculation from the reported values. The abstract’s “1.5-fold” CBC figure does not match those values. The biomass figures come from the abstract, and the results text gives a smaller total-biomass gap (2.71-fold, about −63%).
- Baseline potency. The paper describes this cultivar as typically 14–18% CBD, yet the low-RH baseline here was only 2.8% total CBD, a hemp-like level. How high-potency Type I or Type III cultivars would respond is unknown.
The temperature line is the one to read twice. Mean air temperature was 20.3 °C in one chamber and 21.6 °C in the other. This is not a heat study with humidity as a passenger. The paper attributes the losses to humidity and the low VPD it produces.
2. Why this reframes the usual advice
Existing topics on Asiannabis about VPD and botrytis (see topic 1046, 1245) treat high humidity mainly as a disease risk. That framing is correct, and the mould risk is real.
The usual framing also suggests that a crop that got through without mould got away with it. This study suggests otherwise. Plants with no reported disease still lost most of their biomass and cannabinoid content. They did show warning signs, though: guttation, tip burn and leaf rot.
Mechanistically this is not surprising. At 0.25 kPa, transpiration slows sharply. Transpiration moves water, calcium and other mass-flow nutrients from root to leaf, and it is how the leaf sheds heat. Tip burn fits that picture. [inferred — the study reports the outcome; this mechanism is standard plant physiology, not something the paper establishes]
3. What the study does not license anyone to claim
One cultivar, low baseline. [verified as a limitation] This was a single Type III variety at 2.8% total CBD, so the result may not transfer to THC-dominant or high-CBD cultivars.
Low temperature. Both chambers ran at about 20–22 °C. That is not a Thai room. I could not find any test of 85% RH combined with 31 °C, which is the actual Thai wet-season condition. The combination could be worse than either factor alone, or different in kind. [unverified]
“Three weeks later” means flowering onset, not harvest. Visible flowering started at week 10 in the low-RH chamber and at week 13 under high RH, with weeks counted from the start of the trial. The photoperiod was stepped down in stages (15 h, then 13.5 h, then 12.5 h), so this is not a clean count of days from the flip. Harvest came only one week later (week 15 against week 14), and the authors chose that date. The high-RH plants therefore had less time in visible flower before harvest, and part of the gap may reflect immaturity rather than humidity alone. [inferred from the reported timeline]
Concentration and biomass fell together. The cannabinoid yield per plant therefore fell more than either figure alone. The paper reports concentration and biomass separately, and they are not multiplied into a single headline number here.
4. Fixing it in Thailand
Reaching the low-RH regime in a Thai flowering room means continuous refrigerant dehumidification. When intake air is around 30 °C and 80–85% RH, air exchange has little capacity left to carry moisture out. [inferred from the physics] Refrigerant dehumidifiers also return heat to the room. One grower on a Rollitup thread about Thai flowering rooms reports about 2 °C of room warming. [anecdotal — grower reports, Dehumidifier Planning in Humid Thailand (Flowering Room) | Rollitup] Humidity and temperature therefore have to be planned as one system.
5. Where the money goes first, if it is limited
In my view (a judgement from the evidence, not a finding):
Flowering takes priority, especially the second half. [inferred] The losses accumulated through flowering. If you can condition only one space, make it the flowering room.
Drying needs its own space. [inferred] Do not plan to dry in the flowering room. UVM Extension estimates about 1,390 lb of water removed per US ton of harvested crop, roughly 0.7 litres of water per kg of wet material. That is a general figure from a hops example, assuming about 80% moisture at harvest dried to 10%. [verified source; metric conversion mine — Drying and Curing Herbs and Flowers (Hops, Hemp, and Everything Else!) – UVM Extension Ag Engineering] That load arrives all at once.
Below-canopy airflow is the cheapest partial measure, not a substitute. Air moving across the leaves raises the local vapour gradient even when room RH is unchanged. It costs a fan, but it will not get you to the low-RH regime. [inferred]
Clearing the lower canopy earns its place twice. It reduces the humid, still air where botrytis starts. It also removes shaded leaf area that probably contributes little while the plant is transpiration-limited. [inferred]
Genetics: unknown. I could not find published work on cultivar variation in humidity tolerance. Variation seems likely, but that is an expectation, not evidence. [inferred]
6. What to record so this becomes knowledge instead of anecdote
- VPD, not RH alone. RH without temperature cannot be interpreted.
- Daily VPD minimum through flowering. A sustained low may matter more than the average.
- Total cannabinoid yield per plant, not concentration alone.
- Days from flip to visible flowering, and to harvest. The flowering delay needs no lab to observe.
- Whether botrytis appeared, and whether guttation or tip burn appeared. A clean crop with poor numbers is the case this study describes, and common grower advice does not address it.
Has anyone here brought a wet-season crop through clean, with no botrytis and no powdery mildew, and still had it test well below the same genetics in the dry season? This study suggests that may happen, and it is worth testing under Thai conditions. If it happens, the daily VPD minimum and the flip-to-flower and flip-to-harvest day counts are the numbers that would show it.
This article is technical horticultural information for readers operating legally in their jurisdiction. Cannabis cultivation is regulated differently in every country. In Thailand, cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis) B.E. 2568, and cultivation and sale require licensing. Nothing here is legal advice.