Overwintering bonsai in a chicago greenhouse

Design
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Hey everyone,

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 :evergreen_tree:

1 Like
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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.

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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

https://www.solawrapfilms.com/greenhouse-plastic-blog/solawrap-greenhouse-film-the-ultimate-greenhouse-cover-for-durability-efficiency-and-growth#:~:text=What%20is%20SolaWrap%20Greenhouse%20Film,Key%20Features%20of%20SolaWrap%20Film:

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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.

THEY say ‘bonsai isn’t easy’. Yup…

Bonsai On!

K

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Am in Chicago area too, and have lots of experience overwintering in this area outside without a greenhouse. If you have any questions, let me know.

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I am in the MPLS/St Paul area. colder than you generally and windy. I winter differently that what you are describing. I have an indoor insulated room that is cooled by bringing in outdoor air and disharging it by a louvre. this year that system works great. cold. the biggest issue you are going to have is variability in the winters. winters are all different now days. you need the versatility to provide additional cold when needed and additional heat when needed and a really good controls system. agree no wood. the closest I have to what you are describing is a 50 foot by 20 foot greenhouse with sub-slab hot water heat. it is thermopane glass and is doing well this winter despite -20 F at night sometimes. again it is all about the controls and what you did one winter may not work the next as we are having such variability in our weather these days. maybe also organize your trees in a manner that allows for temperature differentials in certain areas. Depends on what you are wintering in there. some trees tolerate freezing and like it in the winter. best solution is multiple wintering situations. I have 3 different ones.

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Any greenhouse or place that lets in a lot of light is subject to solar radiation and heat fluctuations.

If the goal is stability, you will need intake and exhaust fans as well as a heating device.

We all have seen the eves dripping when the outside temp is at zero, this solar radiation, while excellent for building heat it can be difficult to to control without fans blowing out heat build up and bringing in cool air. These will mostly be necessary as in the fall and spring when it is too cold to leave thing unprotected but you are striving to keep things under 40 F.

As someone mentioned, if your house is big enough a cold room is much easier to maintain and control.

Basically you take a room in the house, first floor preferably so you don’t have to lug huge trees upstairs, turn off the heat to that room cover the floors with traps, set up shelves or trays on the floor and use a 8”-12” intake fans from the grow industry to suck cold air in from outside via a open window.

Since two sides of the room will have heat from the rest of the home maintaining 30F - 40F wil be easy during the coldest months. This intake fan needs to be temp controlled so it cycles based on the room temp and it is the main component in keeping the room the same as outside conditions. The 2 heated walls are your friend as they keep the room from getting to cold and worse case you open the door to allow heat from the living space to go in and get you back to 35 or so.

I am in Northern Vermont about one hour south of Montreal so I am in similar conditions. The cold room is by far the easiest if you have the space in the home to pull it off, it will also be the least expensive.

I use a heated shed for my larger trees with a wind chill of -30 last weekend I was able to keep the shed at 23 degrees or higher with a 1500 watt greenhouse heater. Although that may seem low, no soil froze and using a meat thermometer lets me check temps in the pots/ soil and nothing was below 28 F, if I insulated it I would be better off but finding water proof insulation that is affordable is a lot to ask.

Why would you do a 15 foot ceiling? Harder to heat or maintain heat. Water barrels won’t do a lot once winter really sets in, you will need supplemental heat. Ventilation and air circulation are vital.

With no power of any kind you are more in the cold frame territory and you may want to block direct sunlight to prevent a heat build up. Lower the sides to 6 or 7 feet and use 2 feet of crushed stone as a floor to allow water to drain away and allow you to heel the trees into the gravel and benefit from the ground heat.

Just things to ponder, it is always hard to get someone’s full intentions online.

Good luck

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Today was a perfect example of what actually happens in the winter in a heated shed or greenhouse.

When I left for work it was -4 F outside, the heater was running and the shed was 29 F

Today was a very sunny day, so by noon the heater had shut off and both the intake and exhaust fans where running because the bonsai shed had heated up in the sun to the point that it triggered the fans set for 41 F

The struggle to keep the shed or greenhouse between 30-40 would seem impossible without intake and exhaust fans.

If you went with a much shorter structure, like 6 ‘ peak the water barrels and ground warm may just help you stabilize the temps if you block out most of the sun. If my shed had windows or a transparent roof it would have easily exceeded 60 degrees in there today, so these fluctuations are very difficult to control with a greenhouse which is designed to heat up.

I honestly think due to the wind and the cold you would be better off with a out building or pre fad shed or garage with windows on the North & east side to reduce the influence of the sun on the inside temps.

Good luck in whatever you choose to do but the struggle to maintain a 10 degree temp window is a very real challenge.

Best of luck in your new adventure.