I grow many semps in containers, and they overwinter outdoors, in same zone as yours. I have them in plastic containers, some in wooden boxes. Many semps are hardy to z3. It is very possible that your nice container will crack, so I would not keep it outside (unless it is freeze-proof). The GH could get quite hot on sunny days, do you have any idea of how high temps get to be in winter, on sunny days? That could be worse than proper snow cover. Very fluctuating temps will cause thaw and freeze cycle very often. That usually kills many plants (not only semps). Make sure they are in well draining potting mix. Is yours made with sand?

Btw, why do you suspect that semps will not survive 'the winter treatment' you give to your bonsai?

I would just bury the pot in the ground to insulate it, in a well-drained area where they are unlikely to flood.

After reading the various comments, I'm thinking my best bet would be to, as tcstoehr and breton2 suggest, plant the entire thing, pot and all in the floor of the greenhouse - if the greenhouse floor were dirt. However, since it's cement, this would mean planting in a flat of dirt - some insulation, but enough? Oh, and Rina, it's not the plants I'm worried about surviving, it's the pot. The plants are supposed to be hardy to lower than my zone, but the bonsai pot probably couldn't take the pressure of the freezing soil. And I'm not really sure how hot it will get in there in this winter, our winters have varied so much the last few years. Last year, there were a couple of warm-ups and it maybe got into the 60's in there. Year before that, no winter warm-ups to my recollection, and things stayed pretty frozen in there even with the sun coming out. But good point about the fluctuating temps.

OK, so I'm left with few choices here, though this much is evident - shouldn't have planted in my bonsai pot. :) Stick to plastic, wood or cement for future if I'm using containers.

Think I'll wet it down, wait for the light freeze that's predicted tomorrow morning. It won't be enough to crack the pot, but it might be enough to hold the arrangement together and have it come out in one piece, if I first loosen the soil around the inside of the pot with a knife. Then I can plant the whole piece, minus pot, outside in one of the raised beds. Sound feasible? Next year, I'll see about a different container.

I meant bury it in the ground outside, not in the greenhouse..

You want to keep it in the pot? I'd do as suggested, find a place in the ground, bury it flush with the ground, then make sure the plants have a good cover of snow.

Of course, they grow best in the ground, but that's another thing entirely.

Thank you all. I had originally meant to build a real rock garden, but ran out of time and energy, there being more than one project in the works. I plugged in the plants I'd started here and there, ending with the sempervivums and not being sure where to put them where they wouldn't be overrun by everything else. That's when I hit upon the idea of the bonsai pots (there are actually two pots.) Now I'm rethinking the whole thing. Perhaps, finally, the answer is to simply put them out in the garden and start fresh next year. I might still be able to do a small rock garden in one corner of the perennial beds. So - in the ground they go!

It sounds like you found a good solution, but for others, we insulate clay pots with bubble wrap for the winter. It works very well.

Thanks, Ninkasi. It's done now; they're in the ground as of this morning. It was working well, the silly idea I had of trying to extract the planting as a single piece from the bonsai pot. And then I paused... to examine the root structure on the bottom. Disaster! Oh, well. I will construct something bigger and better next season - which will be fun! Gardening is as much the process as it is the end result, after all. Thanks for the bubble wrap idea - good to know.

- Alex

Alex

I did misunderstend, since your title said ...'winter care for sempervivums...but NP.

I ruined large sq bonsai pot by leaving it outdoors, first winter it was fine, next it cracked. Never tried the bubble wrap either, sounds like good idea.

What was a disaster with the roots? Your semps look very nice in photos...

The disaster was that the root system wasn't extensive enough to hold the mass together firmly, and it crumbled on me after I inverted the pot to get the plants out. Ha! If I had moved swiftly and re-inverted the block of soil and plants immediately, it would probably have worked as I'd planned. But no matter - I'll enjoy building something new next season. :)

- Alex