If your air is cooler and you have a humidifier you can achieve some decent temp and rh control by turning on the humidifier when RH is low and venting when your temp is high or RH is high.
As soon as my lights go out not only the air temp drops. The leaf temp drops even further… and this is used to calculate the VPD, right? Therefore it’s almost impossible for me too keep up the VPD like on daytime…
One way you might be able to counteract this is by ramping set points. You can prime your plants for the night period by bringing the light, and perhaps also temp and humidity down, if your controller allows for it. By ramping the lights down, there will be less leaf heating from the lights, and my guess is that the mechanisms they use to shed water in order to keep themselves cool will ramp down as well.
Also, as the plants get further into the night period, they transpire much less and you’ll see less leaf cooling so the effect you notice is transient, but I’m not sure by how much (maybe an hour?)
Another idea is to warm the room for the first hour or so of the night period, assuming your dehus can’t keep up with the spike of demand at the transition. Peak dehumidifier load is the first hour or so of lights out before the plants “shut down” and the AC is no longer participating in dehumidification. Dehus are significantly more efficient at warmer temperatures, so it will remove more pints, but also the warmer temperature will result in a higher VPD even if the humidity doesn’t change.
But I don’t think night vpd matters very much. I get good results by lowering humidity to 50 at night and this seems to give good results regardless of the night temperature. The studies referenced in the Ultimate VPD guide mainly warned against major and frequent fluctuations in vpd and if I remember correctly none of them were specifically cannabis studies. A single day/night fluctuation from 1.0 to 1.5 sounds like a lot, but is pretty minor compared to most of the studies. Gas exchange also isn’t everything; I use the night as an opportunity to get a deep dryback so I like a higher vpd. I want O2 to get into the roots, which is more important to me than getting O2 into the leaves.
If you look at CO2 rise during the night in a sealed room, you can see that after about 4-6 hours the rate goes way down. I don’t think cannabis has enough reserves to take advantage of high gas exchange all night anyway.
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Hi there im sitting at 30C and just got a dehumidifier i know its not ideal and im not expecting much but according to the charts they should be good sitting around that temp with humidity at around 60%. has anyone had good crops at that high temp?
30C is totally OK for veg and the first maybe 2 weeks of flower, but is at the upper end for what is acceptable for most strains in mid flower; I recommend keeping plants under 28C for weeks 3 through the end of 6. For the end of flower (weeks 7-9), you will not see proper ripening or resin formation or color unless you can drop the temps; cannabis is an annual and expects the end of season signals to complete its life cycle. My recommendation for most strains in weeks 7-9 is to target ~23C during the day and 20 at night, or if you cannot swing then target the average of those two (~21-22).
Secondary reasoning for my advice is that buds do not cool themselves the way that leaves do, and typically near the end of flower the plants are very close to the lights. Both of these factors lead to the plant feeling much warmer at the same temperature than when they were in veg. So you may see perfectly amazing growth at 30C for all of veg, and see signs of stress (leathery leaves, spotted leaves, purple stems) during flower at those same temperatures (since the plant might be seeing 35C at the top near the light).
If your plant is healthy, not crowded/good airflow, and not near any other plants with active diseases (like powdery mildew) or bugs, then you can safely keep vpd low into late flower just like it were early flower. I often end flower around a vpd of 1.0 and still get good results. I grow small “nug” style strains (like runtz, lemon cherry gelato, SFV) not the big bud baseball bat style plants so take my advice with that in mind.
Lastly, if your AC can’t keep up, consider lowering your light intensity. If your PPFD is above 800, you won’t notice a huge difference between your lights being at 80% vs 100%, especially if it means keeping your plants healthy. Heat stressed buds are typically small, no matter how much light and nutrients they get.
PS: if you must keep your room warm (like if you’re using fresh air and growing in the summer), then just keep your lights distant from your plants to avoid that effect I mentioned earlier. If you put your hand at the level of your upper canopy and you can feel your hand roasting because of the light, then you probably need to back off or dim. You might also want to use other “end of season” signals in this case, like lowering fertilizer, deeper drybacks, growing with organic nutrients (that naturally get depleted) or cutting out nitrogen entirely in late flower.
Don’t forget PH! Loving this post btw
Hello @peet, thanks for a great article! Super clear and helpful.
I am calculating VPD for a research project, and I am wondering if you have a specific reference for your VPD equation? I followed the references at the end of your article, but the coefficients seemed to be different among the articles, and I am wondering how you got yours?
Thanks!
Hi Peet,
I’m Sunny, an RDWC specialist working with high-performance cannabis systems.
In my own runs I consistently operate at VPD levels above commonly published recommendations. Due to RDWC’s significantly higher transpiration capacity, this has produced very stable results from a plant-response perspective.
Visually, the plants remain in a constant “pray-mode”: leaf angles are typically lifted around 45–60°, indicating high stomatal activity and strong photosynthetic drive.
I can also clearly identify the upper limit — when VPD is pushed too far, leaf angles increase dramatically (≈85–88°), which coincides with early stress signaling rather than productive transpiration.
Based on these observations, it appears that RDWC systems tolerate and even benefit from higher VPD targets, provided root oxygenation and nutrient flow are not limiting factors.
I’d be very interested in how this aligns with the data you’re seeing at Pulse, particularly regarding stomatal regulation and long-term plant balance.
Best regards
Sunny
RDWC is definitely a bit on the fringe. I don’t know about your observations but in my experience you can’t even feed the plants the same as in substrate. Nutrient brands often say to feed with 3.0 EC, which has never given me good results. I usually feed with 1.8-2.2 and that seems to be a sweet spot for coco. But with DWC, I couldn’t go above 1.8 without signs of over feeding; I think I usually stuck with 1.6.
I can’t say I ever used high vpd though since every system I’ve ever run couldn’t get above 1.3 past mid flower.
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Yes, that’s correct. We maintain our EC levels between 1.4-1.6mS depending on sativa- or indica-dominant characteristics.
I’ve seen and tried all kinds of different hydro/substrate setups an according to my observations, I could clearly correlate the increased growth and yield from each system performance, directly to the transpirations rates. Pulse pro was a key element in this study for me. Things dramatically changed once I had insights about the transpiration and respiration rates during light/dark cycles, of course for the good.
My studies reflect thereby following results in increased growth/transpiration rates according to each system:
1x Soil
1.25x Coco/Soil, Coco
1.5x Rockwool
1.5-1.75x Aeroponics / NFT / DWC
2-2.25x RDWC
I believe that these are critical factors which any pro should be aware of, especially when identifying and dialing in the optimal VPD sweet spot, for each cultivation system.
Visual inspections and plant response always confirmed my observations and theory.
Therefore, I am actively seeking experts in this field who would consider system-specific transpiration dynamics when calculating and defining VPD targets for different cultivation system types, rather than applying generalized VPD models.
Do you know someone in this field whom I could contact or be put in touch with?
That would be highly appreciated.