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Old 03-29-2010, 12:21 PM
Billy Liar
Posts: n/a
Default Humidity and Vapour Pressure Deficit (VPD)

Hi peeps,
here is an article I've been aware of for a couple of years, it makes for some good reading.
I was wondering if shanti may share his feelings on the relativity of this info to Indoors?? and also if he uses this method in his gardens??

Humidity and Vapour Pressure Deficit (VPD)
For years Relative Humidity (RH) has been used as a measure of how much water vapour is present in the air and is probably still the preferred method used by experienced growers. In a greenhouse, the amount of water vapour present has a direct effect on a plants ability to transpire and hence grow.

Another measure called vapour pressure deficit (VPD) is also used to indicate humidity and is felt to be more directly related to a plants wellbeing. VPD combines the effects of both humidity AND temperature into one value and so gives a good indication of plant wellbeing without the need for the grower to do any mental arithmetic. VPD values run in the opposite way to RH values so when RH is high VPD is low.

If humidity is too low (i.e. high VPD), the stomata on the leaves tend to close in order to limit transpiration and prevent wilting. This closing of the stomata will also limit the rate of CO2 uptake and hence limit photosynthesis and consequently plant growth. Low humidity also reduces turgidity (water pressure within the plant cells) and this in turn also restricts growth. Blossom end rot in tomatoes and capsicum can also be attributed to low humidity (high VPD).

Conversely, if humidity is too high (i.e. low VPD) the stomata will fully open but even so the plants will be unable to evaporate enough water to carry minerals into the plant and so again, growth will be impeded and mineral deficiencies (particularly calcium) may occur. In addition, the plants may exhibit soft growth, fungal problems and mineral deficiency symptoms.

It is frequently stated that VPD more closely matches what the plant "feels" in relation to temperature and humidity and therefore forms a better basis for environment control. Unfortunately, VPD is extremely difficult to determine accurately as it is necessary to know the leaf tissue temperature. Attempts to measure leaf temperature reliably on an ongoing basis have often ended in disaster. One of the problems is that the plants leaves are in differing amounts of sun with some leaves in full sun, some in partial sun and others in full shade. This makes the concept of "leaf tissue temperature" particularly complex.

By measuring the temperature and relative humidity within the crop canopy the calculated VPD is still a useful measure as it combines both temperature and humidity into a single measure in a way that approximates the well-being of the crop. As an example, for many crops it is suggested that RH should be kept between the following limits at the stated temperatures:-

You can see from the table that the higher the temperature is the more humidity is required by the plants. The above makes it difficult to specify control parameters as different RH settings are required at different temperatures.

Now look how much simpler this is made by using VPD as the whole of the above table is contained in just three VPD values as follows

AutoVent 2 and 3 environment controllers estimate the VPD based on the air temperature and humidity in the crop canopy. It will only be close to the true figure for a healthy transpiring crop. The VPD calculator below allows the VPD to be estimated based on both air and leaf temperatures. This clearly shows the possible error in VPD due to just a 1 deg C difference between air and leaf temperature.

As a general rule, most plants grow well at VPDs of between 0.8 to 0.95 KPa

Fogging or other humdification is usually applied at VPDs above 1.25KPa and heating and dehumidification at VPDs below 0.45KPa

Use the on-line calculator below calculate the VPD from Air Temperature and Relative Humidity (with and without leaf temnperature). You need to be connected to the internet to use this link.


I would also like to thank LeMarcel for the info he gave me about C3, C4, and CAM type photosynthesis, which make humidity make more sense for a stress free plant.
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Old 03-29-2010, 01:11 PM
L33t's Avatar
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Join Date: Apr 2007
Posts: 3,541

Very interesting article BillyLiar

Its nice to see a tool that calculates the ideal conditions.

Some comments on the calculator, Here is something that I see with the VPD calculator and the general rule that most plants "grow well at VPDs of between 0.8 and 0.95 KPa (ideal KPa 0.85)" in regards to indoor grown and specifically flowering cannabis.

When I use the calculator (without leaf temperature) and enter the ambient canopy temperature that I usually use for growing sativas indoors (28C) the calculator says I must have 75-79%RH.

I find this is too humid for indoor plants that flower as there is high risk of mold.

The pictures above show that fogging should be applied if VPD is over 1.25 but that would be apply fogging if RH is 67% or less..that too humid.

Outdoors plants can grow with higher RH levels and not mold as there is always a lot of fresh air. So the ideal VPD RH recomended values in an indoor room for cannabis with 28C ambient temp would be closer to 1.5-1.9 (50-60%RH 28C) when it comes to flowering. For veg I agree with the values proposed although still indoors I think its always safer to be on the drier side of things.

I have noticed that drier conditions may cause extra frost on the plants in flowering and it also all depends on strain , some can tolerate and thrive in humid/wet conditions while others are more adapted to dry climates , cannabis is one of the plants that has spread and survived almost everywhere from equatorial areas to really north ones.

Thanks for sharing
cheers , Haz3
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Old 03-29-2010, 05:18 PM
Billy Liar
Posts: n/a

Hi l33t, thanks for taking the time to have a look and make a comment.
heres some info I came across a few weeks ago, after LeMarcel told me about C3, C4, And CAM type plants. This is from memory so bare with me here..
LeM told me that cannabis is a C4 plant, but C4 plants have the ability to work in C3 when grown in water (hydroponic (the way I grow)), but work in C4 when grown in organic soil.
From what I can work out, When working in C4 the plant will close its stomata in low humidity, but will still be able to photosynthesise effectively and efficiently. When working in C3 the plant will not be able to photosynthesise effectively in a lower humidity. Growth would then be affected.
I researched alot of what Krusty said about plant stress, and he says high humidity is a must (to create a zero stress environment). a couple other growers agree, most STRONGLY DISAGREE...
this brings me to the issue of mold or bud rot. Bud rot is the development of an airborne mould spore??? Which will thrive in a humid enviroment. and even more so in the dense heavy bud of an Indica Plant. but no so much in the light "airy" bud of a Sativa Plant.. Again this brings me back to what Krusty said, about regimental cleaning and disinfecting of the Grow space. Even using Sulphur burners early in flowering. to kill the mould spores whilst young flowers are still developing..
So if photosynthesis and transpiration mainly take place in the leaf then surely a staiva and an indica plant should react in a similar fashion with the correct VPD?????
I think that the lower humidity belief, is due to bud rot issues. I also think when people measure RH in their room its prob lower than it is among the canopy (which IMO affects the underside of the leaf) , so they think they are doing fine with very low humidity, when it could be alot higher than they believe. (just My Opinion). Sativa plants are prob an exception to this.

So here is the info I have read on "What's the difference between C3, C4, and CAM plants":

The difference occurs in the second part of photosynthesis, the Calvin-Benson cycle, which "fixes" CO2 into carbohydrates.

The Calvin-Benson cycle (in "normal", C3 plants) consists of three processes:
1. The fixation of CO2 onto a 5-carbon "receptor" (ribulose 1,5-bisphosphate, better known as RuBP), which results in two 3-carbon molecules ( a sugar-phospate called 3-phosphoglycerate, or 3PG), a reaction catalyzed by the protein rubisco.
2. The reduction of 3PG to form a carbohydrate, glyceraldehyde 3-phosphate (G3P).
3. Regeneration of the original receptor, RuBP.

Every "turn" of this cycle, one CO2 is fixed.

The problem comes in the first part of the cycle, where rubisco is used. Rubisco can either grab onto CO2..._or_ O2. If it latches onto CO2 as it should, then the first part of the cycle produces 2x 3PG, as it should. If it latches onto O2 instead, then the first part of the cycle produces one 3PG, and one glycolate. Now, C3 plants have evolved ways to reclaim at least some of the carbons channeled away as glycolate, by feeding glycolate through a peroxisome and a mitochondrion, where it undergoes several transformations and some of it is released back out as CO2 (this is the pathway called photorespiration). However, it reduces the net carbon fixation by about 25%.

Rubisco has about 10x more affinity for CO2 than it does for O2, so under normal circumstances this is not a problem. However, on very hot, dry days plants close the stomata in their leaves in order to minimize the loss of water -- and this interferes with gas exchange as well. As CO2 is used up by the normal Calvin-Benson cycle, the balance of CO2:O2 inside the leaf alters in favor of O2, and rubsico starts to grab it instead. This both slows down photosynthesis and reduces its carbon fixation overall.

The C4 plants have introduced an extra bit into the Calvin-Benson cycle, an extra early reaction that fixes CO2 into not *3*-carbon sugars, but *4*-carbon sugars called oxaloacetate (hence the names, by the way, C3 for 3-carbon and C4 for 4-carbon sugars) -- by plunking CO2 onto a different receptor molecule (phosphoenolpyruvate, or PEP) by way of the enzyme PEP carboxylase.

PEP carboxylase has two advantages over rubisco: it has no affinity for O2 at all, and it finds and fixes CO2 even at very low CO2 levels. And oxaloacetate has an advantage over 3PG, in low-CO2 circumstances -- some of it degrades to form CO2 again in the mesophyll, the cells which carry CO2 to rubisco.

As a result, the C4 plants can close their stomata to retain moisture under hot, dry conditions, but still keep photosynthesis ticking over at good efficiency.

CAM plants (from "crassulacean acid metabolism", because this mechanism was first described in members of plant family Crassulaceae) are a different kind of C4 plant. In the C4 plants described above, the fixation of CO2 into 4-carbon sugars and the further fixation of CO2 into 3-carbon sugars happens in different cells, separated in space but at the same time. In CAM plants, the two different kinds of CO2-fixation happen in the same cells, but separated in time. In CAM plants the fixation of CO2 into oxaloacetate happens at night, when it is cooler and the stomata can open to ensure a plentiful supply of CO2, and then the oxaloacetate is stored as malic acid. Then, during the day, the stomata close to minimize moisture loss, and the stored malic acid is reclaimed and turned back into CO2 to power the normal Calvin-Benson cycle.

I hope that answers your question.

Just for added info, C3 plants include roses, wheat, rice, barley, oats, rye, and Kentucky bluegrass. C4 plants include corn (maize), sugarcane, and crabgrass (which is why crabgrass thrives in the hot days of August, when Kentucky bluegrass withers). CAM plants include many kinds of cacti, and pineapples.
I'm not trying to disagree with anyone or prove that I'm right. just trying to get a discussion going and find out what the truth is.
Hope we can all pick holes in these theories, and maybe dispel any myths, I hope shanti can chime in at some point as he is one of the most qualified people on the subject of greenhouse growing and indoor growing of cannabis, from a biological point of view..
either way its a good way to get our feelings on the subject of humidity across..
Peace and Love
Billy Liar
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