What I have in mind is effectively an unheated structure strong enough to last in snow country, permanently covered with a white material to keep it shady enough for hardy landscape propagation and less prone to heat up in the winter. I'm thinking of this as a great way to keep deer & weed seeds out of my propagating bins, even as bottom-heated beds would give me a great place to keep them to encourage root growth as the tops remained cold.

Do you think these ideas are compatible? I don't know of any greenhouses of this nature, unless I've seen them and just didn't recognize them as such.

Thank you!

The only cold greenhouse I have ever heard of in in Atlanta and is used to grow cloud forest stuff mostly

SkirtGardener said: What I have in mind is effectively an unheated structure strong enough to last in snow country, permanently covered with a white material to keep it shady enough for hardy landscape propagation and less prone to heat up in the winter. I'm thinking of this as a great way to keep deer & weed seeds out of my propagating bins, even as bottom-heated beds would give me a great place to keep them to encourage root growth as the tops remained cold.

Do you think these ideas are compatible? I don't know of any greenhouses of this nature, unless I've seen them and just didn't recognize them as such.

Thank you!

If you haven't seen it, it probably isn't viable. Doesn't sound viable to me, either. An unheated greenhouse without excellent insulation will only stay a couple of degrees above the outside temperature after the sun goes down (and on cloudy days), and I don't see how bottom-heated beds will keep the plants (above ground) alive. The part above ground will freeze.

"An unheated greenhouse without excellent insulation will only stay a couple of degrees above the outside temperature after the sun goes down (and on cloudy days), and I don't see how bottom-heated beds will keep the plants (above ground) alive. The part above ground will freeze."

Um, that's the point. I want my landscape cuttings to be dormant and wake up naturally with the season... just with a bit of better protection and a bit of bottom heat to get the roots going earlier. This is not meant to be kept significantly warmer than outdoors at any time of the year.

I understand this idea is properly called a cold frame, only this is a much larger version of it. Structures like this are commonly used by commercial operations to keep their potted nursery plants in over the winter. I was just focusing more on the propagation of those plants rather than catering to selling off the grown out versions.

I guess I misunderstood. Sorry.

I tend to view everything from the lens of tropical plant because I am a tropical gardener. My greenhouse is specifically used to grow tropical plants that won't survive in my zone outdoors, or would survive but get knocked back.
The Atlanta cool house was designed to be a cloud forest greenhouse to grow cloud forest epiphytes and other plants, orchids, ferns etc that have to have a narrow range of cold to cool temperatures while still having extremely high humidity to thrive. Supposedly its the only greenhouse of its kind in the US

@SkirtGardener , I think what you are describing is not too unlike my "unheated high tunnel," except mine is mainly for growing tomato plants in the summer. (I do overwinter a few things in it that wouldn't otherwise survive.) I think what you have described would work perfectly for the type of propagation you are doing; just make sure you have a sturdy framework to support the snow load! You might actually want to consider a clear covering in the winter, so it will warm up enough to shed the snow on its own, and then add a shade covering for the warmer months. I'd love to see what you come up with and how it works out for you - good luck with your project!

@Weedwhacker , I think it sounds a lot like an unheated high tunnel... just built sturdy enough to reliably last through winters in use like summer-only structures may not be built to. (I've heard of too many high tunnels getting crumpled by a winter/spring storm.) I imagine a peaked roof would be best for shedding the snow load, and otherwise the whole sturdy structure needs to be anchored real well (against wind, etc).
I chose a white covering because it lets in light, but also tends to reflect a lot of it (for a bright-shadier result)... and low maintenance over the seasons is important for us. (We are over 50yr, and he is in a wheelchair; this has to be easily maintainable while it gets the job done.) So planning to put on and off shade cloths is not how I want/need to go. Mostly, I'm hoping it won't heat up too much during either winter or summer that I have to worry about (especially constantly) opening up vents to let the weeds/critters in through. I do imagine it will be a bit warmer than outdoors all year round.

Sometimes I've wondered if a permeable film wouldn't be best for this, treating the whole thing kind of like a giant row cover that lets air & moisture in and stays on all season... I'm just not so keen on the idea of drips constantly raining down on us as we walk (or roll) through it. Maybe impermeable on top and permeable on the sides would work, though? To maintain good circulation? Hmmm.

So - this is what I have


and also this smaller hoop house


The big one has pvc hoops for the roof structure, which I wouldn't recommend - although it's held up since early 2017 (with a little help at times for getting the snow cleared off it). The smaller one has a steel frame - it was originally an inexpensive 7x15 hoop house with a fitted cover that deteriorated after one season.

Things I've learned: anti-condensate film would be a huge plus, it is very drippy inside when things are closed up. (Once there is more ventilation in the warmer weather that isn't much of an issue.)

Also, it seems to be difficult to ventilate a hoop structure adequately; we have screen doors on each end, and on the big one there are a couple of screened side panels, but it would be a lot better with vents near the top. Also, without any vents near the top a lot of bugs tend to get trapped inside.

During the winter it will get quite warm when the sun is out, but at night the temperature is essentially the same as outside. However, you can add a secondary low tunnel inside, which will make quite a difference in terms of overwintering plants. If you haven't read "Four Season Harvest" by Eliot Coleman, I highly recommend it!

During the summer it will get even warmer... so you may very well want to utilize shade cloth to moderate that a bit. If you install "wiggle wire" to hold whatever covering you choose, that can also hold the shade cloth in place.

Depending on how much snow you get in the winter, you may need extra support posts; even in my 7x15 HH we add two 2x4" supports and if the snow really piles up on the structures I knock most of it off using a "shop broom" from the inside. Usually, though, the snow will slide off on its own; I haven't had to do much assisting with that for the past 2 winters. (The pvc pipes really aren't sufficient to support the snow load and I wouldn't recommend building a HH from those unless it will only be covered in the warmer weather.)

Adequate anchoring is absolutely necessary - we used the "screw-in" type of ground anchors and attached them to the frame with cable ties, and have had no issues in that regard.

Those are just some of my observations since I've been using the unheated high tunnels - maybe something there will be of use to you!