Vapour-pressure deficit

[David Pilling]

I got a new Bluetooth temperature/humidity sensor - Govee - whose phone app. wants to tell me the VPD - Vapour-pressure deficit.

"...the difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated."

Did anyone ever bother about this in their greenhouse - seemingly it can have an effect on plant growth - see:


https://en.wikipedia.org/wiki/Vapour-pressure_deficit

[petershaw]

VPD is not really new but its becoming a much more acceptable way to monitor plant growth and health. I think the weed growers were instrumental in the becoming more wide spread.

Another definition of VPD is vapor pressure difference which really looks at the difference between what's happening in the leaf compared to the air surrounding the leaf, as compared to the difference determined by the air relative humidity and temperature. Leaf temperate rather than air temperature.

Seems like a small difference but its not to the plant. Leaves receiving sunlight will be significantly warmer than the surrounding air temperature resulting in a higher energy level of the water in the leaf than in the air, allowing water vapor to move to an area of lower energy, cooling the leaf and allowing transpiration even in a humid environment.

The difficulty of course is how to measure leaf temperature accurately and have the environmental computer do something about it. Thus normal greenhouses use the air temperature to trigger fogging or other techniques to cool the air. Though the Dutch greenhouses use way more sensors than most greenhouses in the US.

If you are curious, as I was, the book "Plant Empowerment" was transformation for me in understanding how plants really interact with their environment. The people that developed this wrote the book below and how have a consulting company.

https://www.letsgrow.com/plant-empowerment/

[Steve Marak]

As far as I can tell, if you have the basic information - temperature, air pressure, and partial pressure of water vapor - then RH, dewpoint, and VPD are different ways of looking at the same thing. There are formulas and tables that let you get to any of them or convert between them. I grew up with RH. The weather people now, at least in the US, like dewpoint, for technical reasons, and the crop science people like VPD, also for technical reasons.

I also use Bluetooth enabled sensors (SensorPush), and it lets me display any or all of those three values. I tend to look at RH first since I'm most familiar with it, but I've got the VPD display turned on and am trying to get used to looking at it for the greenhouse sensor.

I agree strongly with Peter that leaf temperature is in many, maybe even most, cases, more important in knowing whether the plant is too hot than is air temperature, especially with C3 plants. High levels of photorespiration can become detrimental very quickly. (What I can practically do about that in my growing situation, other than more shade, is unfortunately a whole other question.)

The only real use I make of any of those numbers is deciding whether to spray water on the GH floor to increase RH/reduce VPD for the cuttings and seedlings, since they're so sensitive to water stress.

Thanks for the link to "Plant Empowerment", Peter. Looks like some good information there.

[David Pilling]

The Govee app says

VPD Instructions

The Vapor Pressure Deficit (VPD) is the difference between the saturated water vapor pressure and the actual water vapor pressure in the air at a certain temperature. It indicates the extent to which the actual air is far from the water vapor saturation state i.e. the degree of dryness of the air.

VPD has an impact on the closure of stomata, thus affecting the physiological processes such as transpiration and photosynthesis. It has an important impact on the evaportranspiration process and water use efficiency in forest ecosystems.

I found a quote:

"The optimal vapor pressure deficit range for most plants is considered to be between 0.4 kPa and 1.6 kPa. This, of course, differs among plants. Each plant has its own sweet spot for different stages of development. As a rule of thumb, VPD should be on the higher side of the optimal range during later stages."

No surprise that in my greenhouse max 2.82 kPa min 0.22 kPa

[petershaw]

I am not sure how important it is for a hobby greenhouse and there are few ways to control it without installing some sort of fogging system (not mist).

Where it is critical is in production food greenhouses. If you are going something like cucumbers or tomatoes in a highwire system the crops need to be keep in balance producing both new leaves and flowers. They usually refer to this as generative or vegetative growth. (Any high value crop can be negatively influenced as well. Cannabis is one where I see growers taking note of what's happening, though in a closed warehouse they are not interested in a fogging system due to potential powdery mildew.)

While teaching hydroponic and substrate production my student growers would often try to produce more fruit without monitoring the vegetative portion of the plants. Tomato crops started late summer would be producing fruit going into shorter days with decreasing light. By January the plants would get out of balance without reducing the fruit load and if I was not paying attention (I was also the department chair) the plants would stop flowering. This only happened one time but we had to remove a ton of fruit, change the watering schedule, the level of fertility, and wait at least 4 weeks for the plants to start flowering again, then 8 weeks for fruit. Great teaching opportunity..I installed a high pressure fogging system in the new greenhouse that really helped in the summer when the VPD could climb way out of range.

[David Pilling]

Peter - really interesting to hear about professional greenhouse users.