I’m planning a 20 × 28 x 15 high sidewall greenhouse in Chicago, mainly for overwintering and recovery, not pushing growth. Doing it permitted and engineered, and trying to build it once rather than rebuild it every few winters.
A few guiding ideas:
Avoiding wood in permanent structural parts (rot worries me more than cost)
Prioritizing winter stability over peak light or heat
Leaning on passive systems instead of babysitting equipment all winter
The frame is a standard high sidewall hoop, but I’m giving a lot of thought to how it behaves in winter. I’m currently leaning toward SolaWrap instead of double-inflated poly — mainly to avoid blower dependency and reduce wind flapping and failure risk during cold snaps.
Thermal strategy is layered:
Black water tanks on the north and east walls
Swedish skirting
Interior thermal curtain over the large door
A second thermal curtain running across the whole bonsai area at roughly high-wall height (about 24 × 32 ft to overlap sides and corners), creating a calmer “bonsai zone” inside the house
Doors are simple:
One man door
One 6-foot opening, upgraded to bi-parting sliders
Greenhouse runs north–south due to site constraints.
I’d love to hear from anyone who’s:
overwintered bonsai through real winters
used layered thermal curtains or zoned interiors
tried non-inflated skins (or decided against them)
What worked, what didn’t, and what you’d do differently?
Thanks — always appreciate the depth of experience here
I am outside Spokane, WA and in the same USDA zone as Chicago, but I don’t think we are as consistently cold and windy as Chicago. I could not find solawrap in a quick internet search so here is what I do. My greenhouse is covered with 6 mil poly over a layer of roughly 3/8” thick bubble wrap. I also staple a layer of 0.3 mil on the inside of the frame when I rebuild this temporary greenhouse each fall. I keep the 11 x 25 x 8 foot greenhouse above freezing with a 1500 W box heater when the temps are above 5-10F and add some propane heat when it gets colder. Biggest issue was finding a roughly 5000 BTU/hr propane heater that would connect to a 20 lb tank. the frame is wood and after about 8 years I have a couple of pieces that need to be replaced.
Thanks for sharing this — it’s really helpful to hear what’s actually working long-term. Eight years out of a temporary structure is impressive, especially with wood framing and mixed heat sources.
The bubble wrap + poly combo clearly does a lot of work, and it’s reassuring to hear you can hold above freezing most of the time. Chicago’s wind is my biggest concern, so I’m leaning toward over-insulating for stability rather than chasing heat. Still, this is great perspective — multiple paths, different trade-offs. Appreciate you taking the time to share
Good luck with this week’s weather …in Chicago. (1/22/26) You could use it as a touchstone for a worst case. Here in Spokane Wa., it is currently 28F with no snow. Not a normal winter.
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Sounds like…If you are this serious, you should go with concrete, bricks, aluminum, and glass / stabilized plexiglass; with lp heat. Build a solarium. Maybe a evaporative cooler, too. Long term, permanent. It will increase resale value, too. That is my dream bonsai future. My wife is STILL planning on moving north. I don’t want to build and abandon it. I am just getting by right now…
I spent the WORST winter of my life in the Chicago area (1977, USN…). Froze my ‘jabuti’ off. Made Spokane, Wa. winters look easy. We (more than once) have had -20F with 4 feet of snow. Way eaiser here!
On the flipside, I have had WAY more bonsai die from heat and drought than in insufficient winter storage. Currently , summers are ranging up to 108F AND 6% humidity. 6 inch pots fry in 15 minutes, 36” pots in an hour in that kind of sun. Summer protection here is just as important. Above 90F, everything goes under shade.
