Greenhouser, what humidity levels are we talking about? And so we can calculate the VPD, what temps at the same time?

On cloudy days of which we get quite a few this time of year the humidity will be between 75 and 95%. On a sunny day like now, around 70%. The minimun temp at night is 59F and the warmest in the day can reach 85%, if the sun is shining. The auto-vents open/close at 70F. Heat comes on at 62F but some really cold nights it hit 59F.

Greenhouser,
This is really interesting thread. Thanks for posting this. It will be good for a newbie like me to learn this. Thanks Bunches

What flooring material are you using? is it different from the HFGH?
Perhaps the excess humidity is being drawn from the ground and just becomes trapped inside the GH.
I would imagine that it would be the same case for the HFGH if you have the same flooring material. Unless the HFGH is sitting on "dry ground" compared to the Rion.

80%+ humidity is a bit high, even for my GH which houses mostly orchids.
Perhaps you could set up an exhaust fan to periodically purge the excess moisture from the GH?

Outside temperatures may limit the times when you could do this, as running the exhaust fan will also draw in cold air from the outside and into the GH. So it becomes a balancing act between causing a cooldown in your GH, or keeping it damp with all the excess humidity.

Also, what are the humidity readings outside the GH? in relation to the inside humidity readings?

Is it possible that perhaps the Spider Mites did it?

1eyedJack and the Dawg

What flooring material are you using? is it different from the HFGH?
Perhaps the excess humidity is being drawn from the ground and just becomes trapped inside the GH.
I would imagine that it would be the same case for the HFGH if you have the same flooring material. Unless the HFGH is sitting on "dry ground" compared to the Rion.

Both have gravel floors and are only 4' apart.

80%+ humidity is a bit high, even for my GH which houses mostly orchids.
Perhaps you could set up an exhaust fan to periodically purge the excess moisture from the GH?

I can't see how. Sucking in bitter cold air is out of the question. It would also make the heating bill soar.

Outside temperatures may limit the times when you could do this, as running the exhaust fan will also draw in cold air from the outside and into the GH. So it becomes a balancing act between causing a cooldown in your GH, or keeping it damp with all the excess humidity.

That wont work. It gets too cold here to purge the stale heated air. It would also be cost prohibitive even if the plants could stand the constant temp drops.

Also, what are the humidity readings outside the GH? in relation to the inside humidity readings?

Right now the humidity outside is at 61% and it's 39F.

I assume you meant 85F.

I'm going to assume that the high temps are associated with the lower RH...this is usually the case. At 59F with 95% RH, your VPD is way too low at 0.09 kPa. At 85F and 70% RH your VPD is much more acceptable at 1.23 kPa (that may even be a little high). You want the VPD to be over 0.4 or 0.5 to prevent fungus survival, and at least over 0.2 kPa to prevent fungal growth.

So lets assume your temps are stable, we can't change that. At a low of 59F, you don't want the RH over about 76%, this is a VPD of about 0.4 kPa. The RH will drop during the day when it warms up. At 85F, you don't want the RH over 90%. But, as I said, when it warms up the RH will go down. Your problem mainly seems to be at night when it's cooler.

The main way to lower humidity is to ventilate in cooler air (which holds less moisture), then reheat the air.

Here is a link to a VPD calculator I published on zoho. If the link doesn't work, go to www.zoho.com, then Zoho Sheets, then click the "Public Sheets" link, then put "dewpoint" in the search box and it should pop right up.

I wrote a detailed post on GH humidity for the other forum, I could copy and paste if anyone is interested.

Here is a link that might be useful: VPD calculator link

BTW, you can't just go by RH. Plants don't sense RH, as we have discussed. They sense absolute humidity, which is usually measured as the VPD.

The link works :-)

Very interested please....... barb

I just rechecked at 6:09 CST. The humidity outside is 76% and in the GH it's 70%, probably because the temp outside dropped and all 3 heaters are running.

OK, but what is the inside temp and what is the outside temp? You can't go by RH alone, bec the outside air is colder and therefore holds far less moisture. You have to know the temps at the same time in order to know how much moisture you have in the air.

Stressbaby:

I assume you meant 85F.

Sorrry, yes.

I'm going to assume that the high temps are associated with the lower RH...this is usually the case. At 59F with 95% RH, your VPD is way too low at 0.09 kPa. At 85F and 70% RH your VPD is much more acceptable at 1.23 kPa (that may even be a little high). You want the VPD to be over 0.4 or 0.5 to prevent fungus survival, and at least over 0.2 kPa to prevent fungal growth.

I have no idea what VPD or kPa means. ????

So lets assume your temps are stable, we can't change that. At a low of 59F, you don't want the RH over about 76%, this is a VPD of about 0.4 kPa. The RH will drop during the day when it warms up. At 85F, you don't want the RH over 90%. But, as I said, when it warms up the RH will go down. Your problem mainly seems to be at night when it's cooler.

The RH went down tonight, probably due to the 3 heaters running.

The main way to lower humidity is to ventilate in cooler air (which holds less moisture), then reheat the air.

That wont work as it's 70% in the GH at this time and 76% outside. Also I can't have bitter cold air blowing over the plants dropping the temperature into the 20s or 30sF. I also couldn't possibly afford to keep sucking out the heat with the stale air. The heaters would never shut off. :(

Here is a link to a VPD calculator I published on zoho. If the link doesn't work, go to www.zoho.com, then Zoho Sheets, then click the "Public Sheets" link, then put "dewpoint" in the search box and it should pop right up.

I wrote a detailed post on GH humidity for the other forum, I could copy and paste if anyone is interested.

That sounds like a great idea.

OK, but what is the inside temp and what is the outside temp? You can't go by RH alone, bec the outside air is colder and therefore holds far less moisture. You have to know the temps at the same time in order to know how much moisture you have in the air.

At this time (6:36 PM CST) it's 61F in the GH with a humidity of 70%.
Outside it's 36F and and the humidity is 76%.

So there is more moisture in the outside air than in the GH.

That wont work as it's 70% in the GH at this time and 76% outside.

GHer, it will work and if you will tell me your simultaneous inside and outside temps that go with the RH you give above, I will do the math and show you.

BTW, we're not talking about replacing all the air at once. We're talking about replacing just a little bit at a time.

Here is the other post, hope it helps someone:

There have been many questions recently on humidity in the GH. I have written a couple of things on this subject and I synthesized them here in an effort to answer as many questions as I can. Sorry if long, overly technical or boring.

Proper greenhouse humidity is important both in preventing plant diseases and promoting healthy plant growth. High humidity can have several effects. It may promote the Botrytis and other fungal diseases. It restricts plant transpiration, which in turn limits evaporative leaf cooling and can lead to overheating of plant foliage. If high humidity persists for a long time, the restriction of transpiration can limit the "transpiration stream" of nutrients and can lead to nutrient deficiencies.

Low humidity has it's own problems. It may increase foliar transpiration to the extent that the root system cannot keep up, or to the point that the leaf stomata close in the plant. This stomata closure minimizes the water loss, but it also reduces CO2 going into the leaf, decreasing photosynthesis and bringing growth to a standstill.

Plants don't care about relative humidity. This is what most devices, including mine, measure. Plants care about absolute moisture content. Scientists measure this as the vapor pressure deficit, or VPD. This, in absolute numbers, is the difference between the amount of moisture the air can hold when saturated and the amount of moisture the air actually contains. The higher the VPD, the drier the air. The lower the VPD, the more humid the air. You can think of this as the "drying power" of the greenhouse air. To convert between RH and VPD you have to use a chart or complicated mathematical formulas. Here is a spreadsheet calculator that will do the conversion for you; go here: [HYPERLINK@sheet.zoho.com] and search for or click "Calc-dewpoint-VPD."

So, where do you want to keep the GH humidity? Fungi survive at a VPD of less than 0.4kPa and a VPD of less than 0.2kPa promotes fungal growth. High VPD (drier air) corresponds to a VPD of greater than 1.5 or 2.0. Ideal VPD for most plants varies but most sources list somewhere from 0.5kPa to 0.95kPa. Using the chart in the links I have below, you can see that a VPD of 0.2-1.0 kPa corresponds to a RH of about 40-87% at 15*C (59*F) but corresponds to a RH of 77-96% at 30*C (86*F).

Low humidity can be managed with misters and foggers. It is also useful to shade plants under conditions of low humidity to reduce the rate of transpiration.

Managing high humidity starts with ventilation...replacing warmer, humid greenhouse air with cooler, drier outside air. Ventilation also involves significant energy losses, and therefore ventilation often must be accompanied by heating. Therefore, lowering greenhouse humidity with a combination of ventilation and heating can cost a lot of energy.

My experience is that humidity is the hardest of the GH elements to control. Low humidity is not so much a problem as high humidity. Humidity can be raised with foggers, misters, and generally you'll do this in the summer. Dropping the humidity is more of a challenge and is generally done either by venting and heating. Most growers simply aim to avoid the extremes of humidity. Over most temperature ranges, a GH humidity of 50-85% is generally safe.

I hope that helps someone out. ;-)

http://www.hortnet.co.nz/publications/science/n/neder/humid01.htm#top

http://ohioline.osu.edu/aex-fact/0804.html

There is nothing on that page that says "public sheets."

When I put in temp and humidity on the thing there and hit enter, nothing happens. How does it work?

GH, you are incorrect. The inside air holds more moisture. Go the the calculator and put in the numbers.

Your mistake is in going by RH alone. Inside you have water vapor pressure of 12.8 hPa. This is a measure of the absolute amount of water in the air. Outside, the water vapor pressure is 5.4 hPa. This means that the air inside the greenhouse holds less than one half of the water held by the air inside. That is why ventilating works.

In case the other post was lost. This forum makes following threads difficult:

Outside the greenhouse: temp 35F - humidity 76%
INSIDE the GH: temp 61F - humidity 70%

I had to remove the begonias today as they were covered in black furry mold.

GHer, it's www.zoho.com -> Zoho sheets -> Public sheets -> "dewpoint" in the search box.

Nothing there says "Public Sheets."

How do I type in dewpoint if I can get the page that says "public sheets." What is the exact URL for "Public Sheets?"

OK, I found the sheet thing again and entered the temp and humidity, hit enter and nothing happened for the second time. What do I click on to make it work? Clicking on calc-dewpoint does nothing - does it work with Mozilla and pop-up blockers?

Thank you for this information and the links. I have my homework to do but at least I now have a glimmer of what VPD means and how important it is.

For those of you who can't figure out the calculator. First the public sheets is located on the menu at the bottom of the webpage.

Once you put in your temp and RH, after you click outside the column the number in the VPD line will change.

SB.. I don't seem to be able to wrap my head around this sentence.
(Fungi survive at a VPD of less than 0.4kPa and a VPD of less than 0.2kPa promotes fungal growth.)

Does this mean that any VPD above .4kPa (What is kPa?)would be "safe?" .2 grows fungi and -.4 to .2 it lives but doesn't thrive.

Not very scientific am I?

Thank you,
J

"Does this mean that any VPD above .4kPa (What is kPa?)would be "safe?" .2 grows fungi and -.4 to .2 it lives but doesn't thrive."

I think you've got it! Safest thing would be to stay over 0.4kPa.

Hi been reading these threads with interest. I built a greenhouse last year, 14'x9' about 8ft tall above ground and sunk 2' into the ground. I bought a 680m3/h exhaust fan and 200m3/h intake fan, controlled by a fan controller where I set the temperature. I had a fan heater which I set up on a thermostat to switch off at a certain temperature. Plan was to maintain around 70f for my tomatoes, chilli, avocado and citrus and grow year round. In May the problem was not enough moisture, the tomatoes were dry setting so I rectified that with auto-watering allowing the pots to overflow. That worked well for a while then I got grey mould so stopped the over watering but still I struggle. I got my electric bill for using the fan heater that amounted to 1200 so heating as well as ventilation is a no go. However a dehumidifyer uses a fraction of the cost .25 as opposed to 3kw. Does anyone know whether a dehumidifyer with the above fan set up would solve the issue without resorting to heaters? In winter as they give off heat and I understand the process produces heat how viable would it be to keep it running/use instead of heaters?

You would have to monitor it pretty closely to see if they give off enough heat to keep your plants from freezing. A couple of degrees above freezing is usually enough. But then with the cooler temps, you have the issue of not enough humidity in winter for the plants to thrive.

I'm not a scientific person and have been following this thread with some difficulty, but I went to the site recommended by stressbaby and will be doing some testing of my own this winter to see what works best. I have a heating and ventilation system and a swamp cooler for summer, but I am also installing a misting system to keep humidity levels higher in summer and winter.

I have consistently had very dry air, in spite of the swamp cooler, and an ongoing battle with mildew. I went to a local garden shop, Archies Gardenland, where they have a large greenhouse, and asked them what they use to prevent mold, mildew and pest infestations, and they showed me to a raft of chemicals especially for greenhouses. I confess I fell off the organic wagon to save my plumerias, palms and citrus trees.

Winter of 2009/2010 was my first winter with my greenhouse and I learned a lot by observing, taking notes on what worked and what didn't, and also consulting the company that built my greenhouse, Texas Greenhouse Company. They know a lot about how best to manage the greenhouse and have helped me with many of my issues.

Funny, before I had this greenhouse, I thought it would simplify my gardening. Not so, it is a fun, fascinating, interesting, challenging, frustrating FULL TIME JOB!!! Ya gotta love it. Cheryl