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humidity and vpd in a greenhouse #819756

Asked February 04, 2023, 6:12 PM EST

I hope your able to help me with my question. I have a small 8*8 8mm twinwall poly carb greenhouse in my yard used for fruits, veggies and perennial flowers. I struggle with keeping my humidity up when the temp raises either in the summer or with my propane heater that I use. I the spring I will be installing a mister system. is there a good way to keep my humidity up another way? I already have a large bucket of water and I saw no difference. would misting in the morning or throughout the day help keep that humidity up? on the other hand I am learning more about vpd. it seems with seedling/ cutting you want it lower to prevent fungal infections? how do I control that as well. currently my greenhouse is unheated. my max temp on 1/4 was 82 degrees. with a low humidity point of 22% and a vpd of 2.8. it seems that the lower the humidity the higher the vpd. is that how it works? if i keep the humidity up when the temp goes up with i more effectively control the vpd? thanks

Kent County Michigan

Expert Response

Hi Casey,

My name is Jeremy and I'm a greenhouse agent with MSU Extension. Those are all great questions!

Is there a good way to keep my humidity up another way?  If we're looking to increase the relative humidity (RH) of a growing environment, we'll need to add water vapor to the system and then try to keep it from escaping. Most greenhouses aren't airtight, so growers need to consistently add water vapor over time. Commercial growers will often construct tents to help keep RH higher in specific areas -- usually when they're rooting cuttings. I have attached photos from three different greenhouses that illustrate this concept. Note that the materials vary in permeability. Sheet plastic isn't very permeable, so it will trap more moisture than woven fabric.

Would misting in the morning or throughout the day help keep that humidity up? You're on the right track with misters. Yes, using them throughout the day should increase RH (depending on the air temp, of course).

On the other hand I am learning more about vpd. It seems with seedling/ cutting you want it lower to prevent fungal infections? No and yes. New vegetative cuttings do not have a root system, but they're still alive and photosynthesizing, so growers try to keep RH pretty high to reduce transpiration rates. Seedlings in plug trays with root systems, on the other hand, don't need nearly has high of an RH. In fact, rooted seedlings are usually better off with a lower RH and higher VPD.

Yes, a high RH environment is usually more favorable for pathogen development. New cuttings need a high RH, though, so it becomes a bit of a balancing act. Commercial growers take cuttings off the mist as soon as they develop some roots. Part of the reason is to reduce the chance of disease.

As you are learning, RH is dramatically affected by air temperature, so VPD can be a useful metric because it removes temperature from the equation.

It seems that the lower the humidity the higher the vpd. is that how it works? Yep, pretty much!

I'm not sure if you've read these yet, but these two articles do a nice job of explaining VPD.

https://www.canr.msu.edu/resources/water-vpd

https://www.canr.msu.edu/resources/vpd_vs_relative_humidity

I hope that helps. Let me know if I can be of further assistance!

Warmly,

Jeremy

An Ask Extension Expert Replied February 09, 2023, 9:52 AM EST
Thank you so much for your information. It definitely helps. Couple more questions if you don't mind
1) in my situation I will mostly have seedlings. Either vegetables or perennial seeds. It sounds like I would want a middle to lower the level and .7-1.5 vpd correct? When growing said seeds I have a heat mat under my trays. I also keep domes on my trays. Should I continue to do this? 
2) in my green house I have a fan and a very that opens automatically. I find that it some times helps control the temp but not all the time, most of the high temps happen in the summer. 
3) is vpd and humidity always an inverse relationship? Say my temp is around 80 degrees, my humidity is going to drop and vpd rise. If I mist would that bring up the humidity and lower the vpd? Basically I have to experiment with how to better stabilize the vpd to be ~.5 or higher. 
4) I also have a couple potted immature citrus trees I will be keeping in my greenhouse till it's warm enough to go outside. If I'm trying to optimize conditions for my seedlings will this effecty citrus trees negatively? 

I really appreciate your help in this. I am actually going to be taking the msu master Gardener program this spring 

On Thu, Feb 9, 2023, 09:52 Ask Extension <<personal data hidden>> wrote:
The Question Asker Replied February 09, 2023, 12:31 PM EST

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