Heating

I have a 10' x 10' greenhouse... it's a "portable" type... it has the clear thick plastic with the green lines going through it. Don't know what it's called. Any how... I'm growing seeds in it and am assuming that the temp inside of it should be at least above 50 f. I'm looking for a good economical way of keeping the heat up, at night especially. I have a monitor for the temp and humidity out there so I can keep tabs on it. It's a good ways from the house so I'm assuming I'll have to run some sort of extension cord out to it to power the heat source. So far, the temp during the day is fine... 10 degrees above outside... but it actually does drop lower (like by 10 degrees) than the outside temp at times during the night... the humidity is very high (95%) at night and I'm assuming that has to do with the temp. No complaints from any of the plants or seedlings that are in there yet.

Any suggestions?

Thanks,
Donna

You might try heat mats under your seedling trays instead of trying to heat the whole greenhouse--those portable greenhouses are not insulated very well, so to try and keep the temperature up in them with a heater may not work too well. They're better for keeping the frost off if you have a night that gets a little chilly, or for getting a headstart on things in the spring, not for holding a 50 degree temperature when it's 25 or 30 degrees out. But if you don't mind using more energy than you should and you have a fairly small greenhouse you might be able to keep the temp where you want it.

Hiya Donna =)

In order to get a really good idea of what you'll need to keep that greenhouse at 50 on those cold nights, you'll really need to know the type and thickness of the covering. After you find that info, grab your favorite search engine, and look for "greenhouse btuh calculator". There are a lot of javascript ones out there. They'll tell you how many BTU's per Hour that you're going to need to meet your goal. From that, you can find a heating system that will provide that warmth. The heat mats may be a good idea, too. It would depend on what you're planning on starting. I'm guessing you'll need to suppliment that, even in your location, though. At least a few times, during the winter.

Also, keep in mind that long extension cords are VERY energy-inefficient. They may be a quick, temporary fix, but for cost and safety's sake, shouldn't be used long-term.

HTH
Eggs

Thanks for the info guys... I figure I'll only need to use heating until the beginning of March. And not that much really... I fret over a few degrees under 50. I'm a perfectionist I guess. I have considered the mats. But I have many, many seeds to start. From daylilies to herbs. Plus, I am also overwintering my annuals (impatiens, coleus, begonias)... as well... I have some pepper plants that are still putting out peppers in there.... so as long as they'll put out...I want to keep them cozy.

So, it's not just a single thing I'm doing. I realize that most plants will be fine in there with a simple 100 light bulb going. That's usually enough to keep them from freezing. We don't get too many nights of 32 or below thank goodness. This is my first year doing this with the greenhouse... so, all is kinda new to me. The thing is, I always read, sow seeds when temp can't fall below 50 degrees. I mean, how close to that do I have to be?

I made a note about my humidity... it seems high to me. Could cause problems like fungus and such. Should I worry about that?

Thanks,
Donna

Now I'm seeing that the dewpoint is exceeding the the temp in the greenhouse with 97% humidity. I suppose it's really foggy in there!!! Thing is the humidity and dewpoint drop during the day, of course... but the humidity seems to steadily remain at least 80% or higher. I do currently have plastic down on the bottom... perhaps that should be changed to something that will allow watter to pass through. When I know that the temp over night is not going to be cold... such as in the days coming... I do open it up and allow it to air out for as many days as I can. It's not really all that cold here yet.

Thanks,
Donna

Donna, what kind of temperatures and dew points are we talking about?

"I made a note about my humidity... it seems high to me. Could cause problems like fungus and such. Should I worry about that?"
In a word, yes. Over the range of most greenhouse temperatures, you get best growth without fungal diseases at relative humidities of 60-85%. Depending on the temperature, a relative humidity of >85% could promote fungal growth.

Keep in mind that plants don't care about relative humidity, they only care about the absolute level of moisture in the air. In the trade this is quantified by the vapor pressure deficit or VPD. Fungal growth is particularly damaging at VPD below 0.2 kPa. How this translates into relative humidity depends on the temperature. So at 50*F that is a RH of 83% but at 86*F that is a RH of >95%.

BTW, this relates to your heating in the sense that one way to reduce GH humidity is to heat and ventilate.

Donna,

I can't say about other plants but for daylily seeds the recommended temperature is 78. Considered the best for germination though I'm sure many will sprout with less than that. I think 50 will be far too cold for them to sprout at all though.

MollyD

MollyD1953,

I'm germinating (growing roots) the daylily seeds inside. I will plant into containers and move into the greenhouse.

Thanks,
Donna

stressbaby,

We're talking of temp of like 63 f at night with a dew point of 65... humidity 97%. During the day, like now, the temp is 73, dew point 69 and humidity of 89%... I'm going out there and doing some work any how... will let it air out a bit and see.

Thanks,
Donna

This message was edited Nov 5, 2006 1:42 PM

And I really don't understand how it could be colder in the greenhouse by 5 - 10 degrees than outside.

Donna

I think one of your thermometers is probably off--only other thing I can think of is if the greenhouse is shaded then it might keep the inside of it cooler than the outside during the day, and if you looked at the inside vs outside temperatures fairly early in the evening it might not have had time to equalize yet. Thermometer placement (at least on the outside one) can be a little tricky too--at my old house the thermostat had a sensor that would tell you how hot/cold it was outside the house as well as inside, and the outside measurement was always way off--the sun beat down on it during the day so it would always say it was hotter than it actually was, and it was in a somewhat windy spot so at night it would always read colder than the real temperature

I also don't understand how the temperature can be below the dew point. When the temp equals the dew point the RH is 100%. When it goes below, the RH stays at 100% and the water vapor condenses out of the air. Donna, are you getting readings from two different devices?

ecrane3,

I am looking at 2 diff thermometers. One is mounted on the side of the house in the shade... that gives me the outdoor temp. My greenhouse isn't in the shade at all... and during the day, the temp is higher than the other thermometer reading. As expected. It's only at night. So, tonight I left the door up a bit and opened the vent on the back of my greenhouse just to see the diff (since it's not going to be cold tonight). At the present time, there is on 1 degree in difference, but the greenhouse is the one who is lower... but the humidity is higher. The wind is blowing... so lots of air getting circulated. A degree different I could understand and accept... but I had it sealed up tight last night and before... very different temps.

The only thing I can figure is that the build up of moisture and such is causing the temp to drop... also the themometer is sitting on a piece of concrete. Other than those 2 things... I dunno. I don't have a breathable bottom... it is plastic. That may make a difference. I dunno.

Donna

stressbaby,

The temp, humidity and dew point in the greenhouse come from one device. When I went out there today, there was much condensation on the walls and roof of the greenhouse. So, since the temp is not going to be cold tonight, I've left it open to get air... and there is a good constant breeze going.

Donna

Oh, and just to add ... the temp device in the greenhouse, I tested it inside my house for a few days. It was dead on the same as my indoor sensors. So, it's probably right. I don't know how exactly the dew point is calculated as such. But I do live 10 miles from the Atlantic. So, I'm maybe 10' above sea level. Now, my hubby, when he programmed the base unit put 0 for sea level. That may be messing with the dew point values. But I've never seen the outdoor dew point go above the temp. I have noticed that the closer the 2 get, the higher the humidity gets. It's been very humid around here the past few days, today it rained all day.

Donna