Making humidity

What can I do to make the humidity level higher in my greenhouse? It is not really dry in there but there is not a whole lot of humidity either. What do most greenhouses do to keep the humidity level higher? Do you buy a standard himidifier or is there something like that made especially for greenhouses. And also, what would be a good humidity level for my plants? I have tropicals, cacti, succulents and african violets?

Thanks,
Jesse

What kind of construction is your green house made of? You can get meters pretty inexpensively that will tell you exactly what they temp and humidity is. The more sealed your green house is, the warmer and more humid it will be. Warmth and humidity kinda go hand in hand I think... but not always. You generally don't want it too humid... depends on what you have growing. But if your green house is sealed more... water can't evaporate from it and thus will make it more humid.

Donna

I would respectfully disagree with Donna with regard to "warmth and humidity kinda go hand in hand..."

The water holding capacity of air rises exponentially with air temperature. This is why the relative humidity in the summer is 100% at 10*C but 75% at 15*C with the same amount of water in the air. So relative humidity varies inversely with temperature.

The short, less accurate answer to your question, Jesse, is 60-85% relative humidity.

The longer, more accurate answer, for most plants, is a vapor pressure deficit of 0.2-1.0 kPa.

I suppose an explanation is in order...

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. This is the deficit, 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.

VPD of less than 0.2 promotes fungal diease. Also, low VPD decrease transpirational water loss from the plant. This reduces "draw" from the roots and can lead to nutrient deficiencies. The failure of the plant to transpire results in less evaporative cooling and can lead to increase temperatures in the plant. Low VPD can also lead to increased internodal stretch.

High VPD (drier air), say at VPD of greater than 1.5 or 2.0, closes stomata in the plant. This reduces water loss, but the closure of stomata also reduces CO2 into the leaf, decreasing photosynthesis.

To convert between RH and VPD you have to use a chart or complicated mathematical formulas. 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). You get the idea.

There are GH controllers that measure and help control VPD...I saw one recently for $1999 (gulp).

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. Check out this link and the related links: http://www.hortnet.co.nz/publications/science/n/neder/humid03.htm#top

SB

Well... down here in the South... it's hot and humid!!!

I know, Donna, it's kind of counterintuitive...the weatherman says "hot and humid" and so we assume that heat and humidity go together. But a year of reading and hygrometer observations has finally convinced me otherwise.

What we (humans) really sense is the dew point...the temperature to which the air must be cooled for the water to condense. The closer the actual temp is to the dew point, the higher the relative humidity. Most people become very uncomfortable when the dew point is above 70*F.

So you folks in Florida don't necessarily have us beat in relative humidity, but you do have us beat in dew points! Check out the maps: http://docs.lib.noaa.gov/rescue/mwr/093/mwr-093-02-0113.pdf#search=%22average%20dew%20points%22

I have noticed that the humidity is higher in my sunroom in the winter than it is in the summer. I don't keep any plant in there in the summer either but still, I heard somewhere that cooler temps is more humid. Sorry stressbaby, that was a little over my head.:)

I realize that if you take an electric heater and place it anywhere, the humidity will drop. So, I was wrong about heat in general and humidity going together... but in a green house, at least mine, when it is hot out, it is extremely humid in there. Right now I have the door and vent open.

Donna

Um, sorry, pdoyle! :-)

You've both got the physics of it...there rest is just math and measurement. And as I said earlier, this is really hard to control. Usually we're more worried about temperature and light than about humidity. The humidity that plants like generally corresponds to a RH of 60-85% within the temperature ranges we generally have in the GH. Many of the things you do to cool the GH in the summer have the added benefit of helping bring up the humidity when you need it most (when it is 40% RH and 100*F at 2pm). With the possible exception of high humidity in a warm GH in the winter, I personally think that trying to separately manage the humidity is more trouble than it's worth for a hobbyist.

I guess I need to buy a humidity gauge of some kind so I can see how much I actually have. It feels nice in my greenhouse which makes me think there is no humidity at all. But since it does not actually have all to do with heat, I have no idea what it is. Does anyone know where I can get an accurate humidity gauge so that I can monitor the humidity in the gh?

Jesse

I have a thermometer with a humidity meter? on it (looks like the thermometer) Yesterday when I got home it read somewhere between 50 and 60.

I have seen one of those at HD. I will have to get one of those so I can see what my humidity is. What would you recommend the humidity be set at in my gh? I have tropicals, cacti, succulents, and african violets. I will keep the temp at 65. How high does my humidity need to be? And if too low, how can I make it highter?

Jesse

I can not answer that. I know someone will come along and help. All I can say is during the winter I store my ficus tree, asparagus ferns, boston ferns, spider plants, christmas cactus, rubber plants, tropical hibiscus, aloe, a brug, ponytail palms. I brought everything in on the dry side and watered them ONCE during the whole winter. I checked on them periodically (stuck my finger in the dirt) and they were all pretty moist. I would venture and say this was because the humidity was high.
Dawn

Coping with low humidity: http://www.hortnet.co.nz/publications/science/n/neder/humid07.htm#top

Now I know what happened to a plant that I just bought. It died when I repotted it and now I know why. I over watered because I did not know to give much less water when it is cold. I was just watering as normal but noticed that plant stayed wet and then started to die. I don't know what to do with it now since it is mostly dead. Maybe I will take it out of the pot and let it dry out a bit and see if I can save it.

Jesse

Yes, you got it...at cooler temps, the VPD will tend to be lower, the transpirational losses lower, rate of photosynthesis lower, and the water needs lower.

Are the roots brown and mushy? You may be able to salvage it. Pull the plant, trim any brown mushy roots, spray the roots with an anitfungal (I use Daconil) and repot in a well aerated growing media. Reduce the metabolic demands on the plant by keeping it in more shade than usual.

Always an adventure!

Thanks for the information. I think I will do that and see if I can save it. I'm trying to learn all these new terms like transpirational. Having a greenhouse is a bit more complicated than I thought. But I love to learn new things. I'm going to go take that plant out of the soil it is in and repot and not water so it can dry out some.

Jesse

Plantcrazii,

I have an Oregon Scientific weather station up. Hooks up to the computer and sends data across my network. I have freeware software that I use for it and you can even set up alarms for like if the temp drops to a certain level...etc. It really is nice. I don't have a unit for it in my green house YET... but I am going to get one that interfaces with it. You can also just buy a single unit to measure such things for as little as $15.

If worse comes to worse... and you need humidity... a humidifier may be in order.

Donna