Growers Network was created as a resource for adults in the cannabis industry.

Please verify your age to enter.

Growers Spotlight - VPD FOR CANNABIS CULTIVATION

#1

Check out our interview with Chris Vaughn on VPD:
http://growersnetwork.org/cultivation/vpd-for-cannabis-cultivation/

7 Likes
#2

I have a couple of questions.

Very high humidity levels, how do you control Botritis? Or is this not an issue in Cannabis production?

When I hear humidity levels of 70% or greater, this is above the grmination level for Botritis. Have you gone back to old practices like volatilizing sulfur? Like the old carnations growers did up until the 1950’s? Or do some of the newer fungicide not affect the plants at the budding stage?

One last thought mite control at these high humidity levels must also be a challenge.

I understand you try to move large volumes of air through the grow area, but does this not spreed mite? I think I can find a paper on the fluid dynamics in mite dissemination. It was written in the 1960’s or early 1970’s when high speed fans started showing up in glasshouses.

I don’t see any free lunch. We want the crop to ripen as quickly as possible with the fewest input.

What do the cost/benefit analysis look like? I apologize if the questions sounds flippant.

Ethan

6 Likes
#3

Hi @ethan! I’ll forward your questions to Chris.

3 Likes
#4

Hey @ethan! I got a response from Chris:

[quote=Chris Vaughn]Botrytis is seldom a problem with most cultivars todays. If you’re concerned try building up the phyllosphere with beneficial bacteria foliar sprays early in the bloom cycle (I recommend OG Biowar Foliar). Never vollatize sulfur in bloom cycle. Also the user is misunderstanding VPD use. The key is to start with humidity levels high and slowly reduce them through the flowering phase while adjusting temperature accordingly. Also, moving your humidity range out of optimal VPD by 5% or so isn’t “harmful” to the plants. If you’re concerned about molds bring the humidity slightly out of range. The point of VPD is to teach growers that humidity matters, contrary to popular “wisdom” that lower is better. Yes, pathogens like PM can be an issue but only if you’re IPM is lacking, or technique is poor. Remember this is an advanced technique.

Mites: if you have mites, especially easy ones like spider mites, you’re already doing something wrong. Certain mites like higher humidity but so do the parasitic fungi used to fight them (hint hint).

Benefits of VPD: over all health and vigor = more trichome production, bigger yields, higher quality[/quote]

3 Likes
#5

Thank you for your response. I am not a current cannabis grower, but a very old Horticulturist. I was invited to the group to help with technical prospective on horticulture and information technology.

The VPD was a cutting edge concept when I was in graduate school at Mizzu. The plant physiologist where creating vapor pressure monitoring.

I am a retired old fart.

Please read my biography.

5 Likes
#6

Found greenhouse controllers for this Water Pro VDP controllers

4 Likes
#7

What about at night ? Even though they are not accumulating radiation do you think VPD plays as big of a role as it does during the day?

4 Likes
#8

I’m trying to get a handle on why VDP is considered when RH illustrates the end result of VDP. In my opinion, there is a great misunderstanding of RH even in engineering text books. First, air does NOT hold water. RH is the ratio of the rate that water vapor comes from the air and sticks to a surface divided by the rate at which water molecules leave a surface to exist as vapor in the air. This is defined in text books as the ratio of the vapor pressures. When RH equals 100% nothing will dry because the arrival rate equals the condensation rate so there is no net movement of water to or from a surface. So RH tells us how long it takes to dry something in air.

Plants differ from engineering surfaces (like the walls of your house) in that the surfaces are a source of water. It is further complicated by the fact that the inner surfaces of mature bud cannot lose water at a rate commensurate with RH or VDP because it is a tortuous path between the air and the inner surfaces of the bud. Therefore, without air movement, the RH inside a really large dense bud (the kind we want) is close to 100%. The whole purpose of my reply is to make the point the water movement between the air and the plant is very complicated. I don’t think that VDP gives us anything new that we didn’t already know from RH.

4 Likes
#9

From what I’ve learned, VPD is important for water/nutrient uptake regardless of light, and it’s important to prevent your grow from reaching the dewpoint, when you could start experiencing mold/mildew problems.

@kirkcollier said:

I’m trying to get a handle on why VDP is considered when RH illustrates the end result of VDP. In my opinion, there is a great misunderstanding of RH even in engineering text books. First, air does NOT hold water. RH is the ratio of the rate that water vapor comes from the air and sticks to a surface divided by the rate at which water molecules leave a surface to exist as vapor in the air. This is defined in text books as the ratio of the vapor pressures. When RH equals 100% nothing will dry because the arrival rate equals the condensation rate so there is no net movement of water to or from a surface. So RH tells us how long it takes to dry something in air.

The reason I think VPD might be useful is because RH is a relative measurement and will rarely tell you the range within which the plant can be safely held. RH is great for “at-a-glance” adjustments, but it doesn’t tell you much about the “optimal” window.

3 Likes
#10

Could you please explain why RH is relative and VPD is optimal? I’m not seeing the physics.

3 Likes
#11

What I meant was that RH simply tells you what is. It doesn’t necessarily tell you an appropriate range of environmental conditions you can change to based on your current environment.

In other words, RH is useful for the here and now, but not as useful when it comes to planning, because it is reliant on a lot of different factors. Because VPD is an absolute measurement, it’s more useful for planning purposes.

3 Likes
#12

Hello all, the VPD is a critical measurement that helps the cultivator achieve stomata control with higher brix levels. Your plants will transpire at lights out if the VPD is not correct.Transpiration will cause a host of negative issues. Water can form on the leaves and that will cause yellowing of leaves and a host of other issues such as bud rot ( Botrytis Cenera) I am enclosing some links for your learning pleasure.

Karma Up

https://garden.org/onlinecourse/PartI47.htm

5 Likes
#13

From wikipedia
"Unlike relative humidity, vapour-pressure deficit has a simple nearly straight-line relationship to the rate of evapotranspiration and other measures of evaporation."

Evaporation rates are affected by temp in a nonlinear way . This table shows VPD values for a grid of RH and temp.
image

The chart shows how humidity and temperature can be manipulated to drive transpiration in the best way (green area). Higher temps won’t dry out your plant if the RH is high enough. But this isn’t linear and the tolerance for error shrinks at high temps.

So VPD is a change of reference frame to slightly simplify the nonlinear relationship between temp and humidity with respect to transpiration.

4 Likes
#14

Thanks Terry, I can understand that VPD would be applicable to gases like oxygen, etc. However, water transport goes both ways. That is, water is being condensed on the leaf surface at the same time that it is evaporating from the leaf surface. Like a chemical reaction where the arrows go both ways. If the relative humidity is 100% or the vapor pressure difference is zero, then both processes happen at the same rate and no net water is transferred. ie nothing dries out. When the relative humidity is zero, only evaporation takes place and VPD would be appropriate.

If you really want to know the net rate at which water leaves a leaf, then you must know the rate that water is condensing back onto the leaf. This all happens because water is a polar molecule and not all of the water in the air is the same temperature (kinetic theory of gases. This is how I was taught in engineering

2 Likes
#15

Agreed, and the net (total) rate of transpiration averages evaporation/condensation to summarize this. Transpiration depends on temperature and humidity in a nonlinear fashion, but VPD simplifies our measurement of transpiration with only single number (higher is more, lower is less)

For another physics analogy Max Yield
“VPD can also be described as the potential of the surrounding air to pull water vapor from the foliage (i.e., transpiration)”

Agreed, and for a given temperature, by reducing the RH we increase the VPD.
All else being equal, plants open stoma to increase the flux of water transportation and diffuse the pressure. Higher temps won’t dry out your plant if the RH is high enough… but plants will get “stressed out as it can’t evaporate enough water from its foliage to cool its tissue and it will overheat.”

Bottom line: If you can control temperature and humidity, it could make sense to consider VPD to contextualize these environmental stats in terms of the phenomena driving plant transpiration/respiration/cooling/nutrient uptake.
VPD does this with one simple stat related linearly (simply) with the thing you are most interested in.

2 Likes
#16

The reason why VPD is much more informative then RH is due the fact that VPD is quantitative and describes the direct physical force that pulls water from the plants. At 50% RH and 40c VPD is: 3.692 kPa while at 10c it’s: 0.614 kPa. The plants experience very different forces, and you couldn’t know that if you have only RH % and that force could also be used later on to calculate evapotranspiration, which you can’t do with RH.

2 Likes
#17

Thanks you guys for the great responses. I think where we disagree is that VPD assumes that water only leaves a surface due to the partial pressure difference between the water on the plant and the water vapor in the air. If water is also being adsorbed on the leaf surface, VPD will overpredict the net rate that water leaves the surface.

One of my pet peeves with definitions of RH is the concept that air “holds” water. It does not. Water acts under its own physics regardless of air molecules. In actuality, 100% RH is when the rate of water release from a surface is equal the the rate of water condensation on that surface when they are both at the same temperature. 0% RH is when there is no water condensation on the surface. All transport is evaporation and VPD is very accurate in determining the rate of water removal.

3 Likes
#18

Indeed a great discussion :).
I have to disagree with you Kirk though, first of all, VPD is the water vapor pressure at saturation for a given temperature minus the actual water vapor pressure content. There is no assumption in VPD, it’s a physical definition, the assumption comes in when trying to predict transpiration from plants, then you assume that INSIDE the plant(and not ON), in the substomatal cavity, the air is saturated, and so the force driving water out of the plants is VPD.

3 Likes
#19

Thanks Tai for the great response. I totally agree with your definition of VPD. If water transport were truly one way then VDP would describe the transport of water from the leaf to the air. However, my point is that water will also travel from the air to the leaf even though the vapor pressure is higher on the leaf than the air. The reason is the forces involved with adsorption, that is a very thin layer of water. That energy is higher than the VPD predicted by the equations defining it. That is saturation pressure. If there is a large amount of water on the leaf such that the water layer is thick enough for adsorption energy to not be a consideration, then VPD would be accurate. I don’t think that the water film thickness on a leaf is enough to negate adsorption energy. I think that VPD treats leaves like a bucket of water which is not correct IMO.

2 Likes
#20

I agree with you that there’s an inaccurate assumption, another one is that the leaf and air are at the same temperature which we know isn’t true, but I’m still confused about what you said.
I see your point about adsorption to the leaf surface, but water isn’t lost through the leaf’s surface, the cuticle, rather through the air under the stomatas, so how does that effect that?

2 Likes