mystic_roots Posted March 27 Share Posted March 27 I can't believe that I'm only just learning about VPD, but every grow year is a learning one. I suppose my question is how much of a difference VPD has on your plants if it's not within a certain range and does it make any difference if they're photoperiod or autoflower? I know for veg you're looking around 1.0 kPa, late veg/early flower around 1.2kPa and then late flower around 1.55kPa, but what methods are best for controlling this, because in my current grow my ACI controller is giving me 1.55kPa and that's with temps around 27/28 and RH at 65%. Link to comment Share on other sites More sharing options...
sweettooth Posted March 27 Share Posted March 27 Tough question to answer. for years I never bother monitoring my vpd and always had good crops amd good yeilds. I think if you keep your humidity within a good range, the plants will do just as good. Personally I think true vpd is rather anal and only really applicable if your pushing the envelope and need everything to be spot on, environment, water, nutes, co2, EVERYTHING. One knows what humidity levels are too dry or too humid. I think it can cause too many issues as you become fixated on getting the vpd right when actually the range is quite large, would you see that much difference by keeping it in the sweet spot on home grows??? 2 Link to comment Share on other sites More sharing options...
smokinggolden Posted March 27 Share Posted March 27 (edited) I think it's good to bear in mind. If you are keeping humidity and temperature optimal for the different growth stages and grow good healthy plants. Then you probably already must have good VPD! Or else your plants would not transpire correctly and have problems. eta: tip: I like to use an IR thermometer pointed at the walls to get ambient temp, then point at the top leaves to make sure they read a couple of degrees C lower. The flowers do not transpire and so should read slightly higher than the leaves. The flowers can also 'cook' whilst the leaves around them survive the temperature by transpiring super fast thereby cooling themselves with the evaporative effect (assuming they do not run out of water or the salt level rises too much in the pot). Edited March 27 by smokinggolden Link to comment Share on other sites More sharing options...
GSZZ Posted March 27 Share Posted March 27 1 hour ago, mystic_roots said: I suppose my question is how much of a difference VPD has on your plants The most common misunderstanding growers have with VPD is this right here - that VPD is the driving force of transpiration, when infact the reality is VPD is the result of transpiration. Light is solely responsible for your plant transpiring. VPD is another tool that growers can use to balance all of their environmental inputs with their plants. 1 Link to comment Share on other sites More sharing options...
inceywinceyspider Posted March 27 Share Posted March 27 I think its useful to understand what the range is and where your environment is on that as if you're not always able to have optimum environmental conditions its going to be best to adjust the light intensity accordingly. Link to comment Share on other sites More sharing options...
MrSlatersParrot Posted March 28 Share Posted March 28 VPD is another way to measure humidity. You can't measure it directly, it has to be calculated. It's a better way of setting the humidity levels if you're comparing grows at different temps. For plant growth the leaf temp is often taken into account because it's cooler than the air if the plant is transpiring effectively, because of evaporation. An IR thermometer is a useful thing to have. If you know that your plants want eg 25°C and 60% RH then give them that, you don't really need to worry about VPD. There are online calculators that let you play with this if you want. Transpiration is affected by different humidities, too dry and the plant will try to close it's stomata to limit water loss, which also limits gas exchange and therefore CO2 intake, so photosynthesis is reduced, too humid and the water doesn't evaporate off the leaves fast enough and transpiration is reduced and so is growth. Plants do transpire in the dark, I believe about 10-15% of lights on rate. Link to comment Share on other sites More sharing options...
KC Posted March 28 Share Posted March 28 I had to setup a temporary veg tent the other week, being the lazy sod that I am I thought fuck the extractor i'll leave it out and just stick a little led in there instead of the 315cmh. As you can imagine it got more humid than usual with no extractor. I did notice my root growth has absolutely exploded even though the temps were quite low, I hadn't even put a heat mat in at the time either. Is that because my vpd was on point? I'll be honest it's aterm I've only heard of recently Link to comment Share on other sites More sharing options...
MrSlatersParrot Posted March 28 Share Posted March 28 (edited) Yeah plants put out loads of water vapour! You can soon tell if your extractor's not working, with biggish plants the place will be dripping. Don't know about your plants' roots though! Not sure about copying stuff from other websites, is it allowed? Well anyway, here's a sample VPD chart, unfortunately in Fahrenheit. There's more info and a calculator on their website. If the VPD is the same, the plant is getting the same conditions for water vapour exchange. So on that basis, 84°F and 82% RH is equivalent to 59°F and 58% RH because the VPD is the same. Edited March 28 by MrSlatersParrot Add reply toKC 1 Link to comment Share on other sites More sharing options...
MrSlatersParrot Posted March 28 Share Posted March 28 OOps I've taken the pic down, I think I just did something bad! Hang on a min Link to comment Share on other sites More sharing options...
MrSlatersParrot Posted March 28 Share Posted March 28 Er right, I think this is OK, I've taken off all references to their website. But you should visit the blog on the pulse website for more info, because I've nicked their chart. Link to comment Share on other sites More sharing options...
NezA Posted March 28 Share Posted March 28 That chart needs a LST offset in order to be accurate it isn't accurate in and of itself. I can't see the LST value in that picture but I believe their sample chart was set to -2 just be aware that you will need to generate your own chart with your own LST offset value because every grow room is different. 2 Link to comment Share on other sites More sharing options...
MrSlatersParrot Posted March 28 Share Posted March 28 Yes quite right I posted it just to illustrate what I was talking about. The bit I've chopped off says 'Leaf temperature adjustment 0°F' Link to comment Share on other sites More sharing options...
sweettooth Posted March 28 Share Posted March 28 3 hours ago, KC said: I had to setup a temporary veg tent the other week, being the lazy sod that I am I thought fuck the extractor i'll leave it out and just stick a little led in there instead of the 315cmh. As you can imagine it got more humid than usual with no extractor. I did notice my root growth has absolutely exploded even though the temps were quite low, I hadn't even put a heat mat in at the time either. Is that because my vpd was on point? I'll be honest it's aterm I've only heard of recently I do this to root out pots after potting up. works great, it's due to the roots seeking out the moisture in the air, as its full 90% or higher, the roots love it 1 Link to comment Share on other sites More sharing options...
The Villan Posted April 4 Share Posted April 4 (edited) Sorry, I found the answer just after posting, sorry to have tagged. Edited April 4 by The Villan Link to comment Share on other sites More sharing options...
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