Can we talk about greenhouses for a minute?

MiaOKCOctober 29, 2012

Good morning, all! I inherited this little 6x8 hobby greenhouse with my new house and it has seen better days. However, isn't the old adage that any greenhouse is better than none? Ok, that's not an adage, I just made that up.

Anyhooo, I cleaned it out and reattached all the missing panels. The previous owner had used a foil tape to try and seal it up better, but the tape isn't used throughout. I don't have an auto vent opener (yet) but have it on the wish list. Right now I can open the door for ventilation and that's it. I put a min/max thermometer in there at the beginning of summer, and it has gotten HOOOOOT, even with four of the panels missing, so I had thought installing the missing panels would make it perfect to store my hibiscus and palm trees for the winter (and relieve my garage of this duty so I could park my car in there instead). Wrong!

The min/max showed me it got just as cold in the greenhouse as it did in the outdoor air temps over the weekend (29 was our lowest, I think). Of course, during the day, it's a tropical paradise/oven - it was 80 and 90 degrees in there over the weekend with the door shut and pretty cool daytime temps outside.

Is there a way to make it warm enough to overwinter my tender plants? Without a heater, anyway? Looking for a low-cost, low-fire-hazard solution here. I've considered using the foil tape all over and trying to make it airtight. Or replacing the flimsy polycarbonate plastic panels with something sturdier/thicker/hopefully more thermal-preserving, like a polygal panel (like corrugated plastic, sometimes you see political/real estate signs made out of this).


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Mia, my friend in Tulsa has a greenhouse attached to her house. It gets heated in the winter, and all openings open and all plants are out in the summer.

She has a gazillion potted plants she always brings in for the winter. You can hardly get around in her house.

Last year she got a little portable greenhouse, and set it up in her driveway. No, I couldn't really see a difference in the amount of plants she has in the house, LOL, but she was planning on putting it up again this year.

She heats both green houses. She also can not leave for a couple of days in the winter, cause the plants would fry in the daytime without some ventilation. Nothing is motorized there.

I'll attach a file, where I have pictures of her yard and greenhouse.


Chris' yard:!cpZZ1QQtppZZ28

her green house:

Here is a link that might be useful: potted plants in the house, and the greenhouse

    Bookmark   October 29, 2012 at 11:52AM
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Wow, Moni, how beautiful! This reminds me of my mom's half basement in winter - it's like being in a nursery! I will look at options for adding heat, but fear it will be too $$$.

    Bookmark   October 29, 2012 at 12:35PM
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Mia, Chris and her hubby have even gotten a WHOLE house generator now, mostly to keep their green houses warm in case of a power failure.

You don't even want to know how long it takes to water daily in drought conditions like we had last summer, and this summer, or how high the water bill is.


    Bookmark   October 29, 2012 at 1:50PM
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jbclem(z9b Topanga, Ca)

When I lived in Malibu, Ca, I had a large glass greenhouse. It doesn't get that cold in Malibu, near the beach, but one thing was obvious...on cold winter days, the minute the sun went down it was almost like being outdoors, the glass transmitted in the cold very easily. Since you have plastic, you won't have to worry about glass, but I wouldn't be tempted to switch to glass, even though it's much nicer looking. There are websites that sell plastic panels, multi-layer, for greenhouses.

One good idea...people using bubble wrap to line the inside of their glass greenhouses. Its inexpensive and effective. It would work with plastic also and cost much less than the special multi-layer plastic panels. And it can be removed for the summer. This bubble wrap is like the type used to cover swimming pools, heavier duty that the mailing/shipping bubble wrap.

For the summer I think you have to use shade cloth or have some way to cut the power of the sun. Shade cloth suspended six inches above the roof so some air can flow between the shade cloth and the plastic/glass panels.


    Bookmark   October 30, 2012 at 6:33AM
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Okiedawn OK Zone 7


You've already seen than your plastic panels offer virtually no heat retention and glass wouldn't give you any more than they do I don't think.

If you go to a greenhouse supply store (one place to start online would be to google and find the greenhouse megastore) you can find the kind of bubble-wrap insulation manufactured for use as insulation in greenhouses, or go to the Greenhouse forum here at GW and you'll find many discussions there. Many of the folks there use a specific brand of swimming pool cover like John described, but I can't remember the brand they buy.

So far this autumn, my greenhouse, which is only covered with 6 mm greenhouse plastic and Aluminet shadecloth, has stayed barely above freezing, but I consider that a victory because for the last 4 nights our overnight lows at our house have reached 28, 28, 26, and 36. Sadly, our forecast was for 33, 34, 36 and 44. lol With the forecasted temperatures, I didn't expect the greenhouse plants to freeze but with the actual temperatures I did.

Last year, my greenhouse's overnight temperatures generally went exactly as low as the outdoor temperatures, but I didn't have the Aluminet shadecloth on the greenhouse at that point either. This year the greenhouse's overnight low temperatures are just a tiny bit higher than the outside air temps so far, and I give partial credit for that to the 18 molasses cattle feed tubs in the greenhouse that are filled with water. I put them in there hoping they would serve as solar heat collectors during the day and then would give the greenhouse a little warmer at night. So far, it seems like that is working. However, I am not fooled into thinking they will keep it warm for long. Right now, the daytime temperatures are high enough that if I close the greenhouse vents and doors around 4 p.m. on a warm, sunny day, then the heat inside can built up to about 85-90 degrees before the sun begins to set. If I closed the doors and vents earlier, it likely would get hotter, but I can't let the lettuce get too hot or it will bolt.

Between the built-up heat, the warmish water in the molasses feed tubs, and the aluminet shade cloth, the greenhouse has stayed just warm enough to keep the plants in there from freezing at night. The plants mostly are tomato plants, peppers and herbs in containers and they are fine as long as the air inside the greenhouse stays above freezing. I don't have anything tropical in there that prefers warmer nighttime temperatures. I do have lettuce and cool-season greens that tolerate colder temperatures. If the tomatoes and peppers produce another week or two or three, I'll be happy. They would have frozen last week if they weren't in the greenhouse.

My tropicals are either on the patio during the day and the garage at night, or in the house. I don't think it would be cost-effective to heat a greenhouse the size of mine throughout the winter. We built it as a season extender, expecting it would help us keep some things going a little later in fall and get some things growing a little earlier in spring. If I had intended to overwinter plants in it all winter, I would have used twinwall 6-mm or 8-mm poly panels. You can find them at most greenhouse supply stores or at Farm-Tek, and I still would have known I'd need a heater to overwinter plants in there.

From the minute we finished building the greenhouse (in fact, before we even finished it), Tim and I both found ourselves describing things we'd do differently next time. One of the main purposes of this simple, hoophouse-style greenhouse was to learn from it so that when we build our "real" and larger greenhouse, we'll know more about what we want and how to achieve that objective. From that standpoint, I know that what I want in the next greenhouse is (a) good walls that hold heat in as well as possible, and (b) a heating system that is cost effective. Because we have a large woodland on our acreage, I think we'll likely go with a wood-burning greenhouse heater.


    Bookmark   October 30, 2012 at 10:05AM
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I agree with the consensus here that you can't keep a greenhouse that small above freezing overnight in Oklahoma winters without a heater. Our greenhouse is 16x24 with no insulation anywhere. The north wall and half of the east and west wall are tin as is the north roof. The south roof is single wall Polycarb and the walls are 4x6 recycled operable windows. There is a walkway of concrete pavers that covers a 4x8 foot area and there is a gravel path and several gallon jugs of water. Last winter with no heat at all the lowest it got inside was 24 F on a night that got 12 F outside. This was after a sunny afternoon. We covered our lettuce, spinach and other cold hardy plants with a double layer of sheets and none froze. On cloudy days followed by cold nights it stays only a couple degrees warmer inside. We built an 8x8 ft "warm room" inside it which we plan to heat with a small electric heater--maybe. Time will tell. I am beginning to think that it's not worth it to try to grow tomatoes in the winter, but if I can find a couple quava bushes, I would love to have them.

    Bookmark   October 30, 2012 at 11:33AM
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Thanks for all the info, everyone! I've fallen down the rabbit hole of the Greenhouse forum (which I didn't know about - thanks, Dawn!) and also ran across a really helpful link there that directed me to American Plant Products & Services, a wholesale supply place in OKC that someone got replacement polycarb panels, shade cloth, etc. Will definitely be giving them a call next. I also see several heaters that are portable and less than $100 (electric or electric/oil-filled radiator style), which would probably be a worthwhile investment to get my garage back... am thinking of scraping ice off the windshield every morning and comparing that to an extra few minutes of sleep in my warm bed - could be very worthwhile!... Mostly scared of burning something down - I've never left a space heater on without being in the room with it. Don't know if I could sleep knowing there was a heater chugging along in the greenhouse all night!

I also have to consider I'd been planning to pack in 6 big containers - these are about 24 inches tall and 24 inches across - filled with palms, hibiscus, and cannas if possible (would cut these back because they are far too tall to fit in the GH as is). That will be taking up most of the floorspace leaving a narrow aisle about 18 inches wide to squeeze a heater into.

I will have to figure out the best, least-expensive "lazy girl's" way to do things. I am at work usually 10+ hours a day so cannot open and close ventilation or drag plants in and out - it has to be fairly auto-pilot or it's set up to fail at my house!

    Bookmark   October 30, 2012 at 12:40PM
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Also, when I'm done, I expect my greenhouse to look like this picture, not my "before" picture above. :>

    Bookmark   October 30, 2012 at 12:58PM
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Oooh, I would like to live in your future greenhouse!

    Bookmark   October 31, 2012 at 8:00AM
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Okiedawn OK Zone 7

If you're going to dream greenhouse dreams, you might as well dream big. I can picture that future greenhouse overlooking your beautiful swimming pool, Mia!


    Bookmark   October 31, 2012 at 10:50AM
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Lisa_H OK(7)

Oh man!!! That is BEAUTIFUL!!!!

    Bookmark   October 31, 2012 at 11:43AM
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