Tips/Suggestions for Winterizing on a Patio

[2manycooks]

Hi, I've been collecting a few tropical bonsai and succulents for a couple of years now with moderate success and now I'm interested in picking up some trees that are left outdoors to experience the winter and am looking for advice as to how I might keep said trees alive out in the cold. As all our situations are unique, I'm in zone 5a where we experience cold and extended winters, and to complicate things, my only available outdoor space is a 12x10 foot patio out on the roof of my condo building. I do not have a yard of my own. Assuming my interest in bonsai continues for many years, future-me will probably build a nice, expanded cold frame in a future home where I have the space. Present-me doesn't really have the time to build something nice or spacious, I'm just trying to experiment with combinations of species and winter protections that will keep a couple (2-3 max) trees alive through the winter.

I've heard of people using old beach/drink coolers as a quick method of insulating their trees during the winter. Has anyone (especially in zone 5a and colder) had experience with this particular method? Can you go into detail as to where you found success?

What other low-effort options would there be for simple cold-frame options out on a patio? I've thought about trying to make a quick-and easy box out of pink insulation foam from the home improvement stores but again not sure how much insulation that would provide.

Also, are there any species that are especially tolerant to cold? Since I have no experience winterizing here my plan would be to try keeping nursery stock pine/cedar/juniper alive for a winter before I even invest the time getting the tree cleaned up and re-potted into a bonsai container.

 

[jevanlewis]

One of the reasons cold frames work is because they are built on top of (or buried slightly into) the ground, which radiates heat into the cold frame. A cold frame on a patio that doesn't contact the ground might warm up in the day with sun, but it will quickly lose that heat overnight and basically cool to the air temperature, even with insulation. To mitigate heat loss, you need some sort of heat sink, but you may not get enough sun to heat the heat sink enough to maintain your desired overnight temperatures. You could artificially heat the cold frame with a heat mat or heater of some sort if you can get electricity to the patio. With cold frames, you also need to pay attention to not overheating the frame, which could force the trees to come out of dormancy early. Overall, I think cold frames not in contact with the ground could be tricky.

My vote would be to find species that can handle their roots getting as low as the outdoor air temperature or finding someone nearby with land who would let you winter your trees in a cold frame on their property.

 

[Jolly Green Ground Sloth]

I have the same situation, except I'm in 6b. I've tried a makeshift cold frame on my deck. Didn't work. Tried styrofoam coolers. Didn't work. Tried putting my pots into large planters or animal feeders and burying them in soil and/or mulch. Didn't work.

I think I just can't grow Japanese maples, bald cypresses, etc. until I move someplace with a yard or a private garage.

I'll stick to species that can handle the polar vortex on my deck. Thujas, larches, spruces, hybrid yews, boxwoods, mulberries, and corkscrew willows have all survived several winters. Local nursery stock junipers and Eastern redcedar have survived well, but my Shimpakus keep dying.

For potted trees, I think you have to look for species that are hardy to a zone or two below where you live.

 

[rockm]

@jevanlewis is correct. The principle of a cold frame is to trap ambient "heat" in the ground of underlying mass--heat is a relative thing here. You're not trying to keep your trees "warm" but prevent them from experiencing the extremes. Without an underlying mass that absorbs heat (sunlight) in the daytime, i.e. the ground--cold frames are mostly useless. Same for coolers--they can't do much of anything either since you can't trap heat that isn't there. Think of a roadway bridge. The freeze before the road surfaces on solid ground. That's because of air circulation underneath dissipates heat more quickly.

Anything that has air space under it won't work as a base for a cold frame, for the most part.

Using extremely cold hardy species (hardy below your Zone), finding someone who has a yard or a garage can help. You might also consider creating a large mass (pile) of mulch that can act as a heat sink and bury your trees' pot in it for the winter. Pile would have to be substantial--like four or five bags of mulch minimum to do any good. You have to keep the pots in the middle of the pile, not near the bottom to prevent exposure to extreme temps on the bottom --which would have open air under it.