Winter storage of my bonsai is exceedingly simple: everything on the benches until temps reach 25° F; everything on the ground below 25° down to 15°; everything covered with poly sheeting below 15° down to 10°; fervent prayer below 10°. Okay, only kidding about that last one – in addition to the prayer, I would mulch and cover everything.
So that’s down here in the Deep South, but many of my clients live in harsher climate zones. I’m often asked about winter protection for a variety of species, and I usually suggest an unheated garage or cold frame abutting a house. The unheated garage has its limitations, of course. The cold frame may or may not provide adequate protection if your winter temps will go below zero. And if you grow tender species, your problems get somewhat larger.
I have a long-term client who just turned 88 – isn’t that awesome! He’s been experimenting for the past few years with a heated winter storage container that he constructs in the fall and in which his tender bonsai remain until spring has arrived. I asked him to give me some details of his construction method, so I could pass the information along to those of you who live north of Zone 7.
The basic concept is simple: build a six-sided Styrofoam® container and equip with an oil heater (an electric heater should work as well), then maintain your winter temperature at 35°F. Building the container, of course, requires planning and effort. This post will give you some details on a reliable way to protect your tender bonsai through winter.
A critical step in prepping the Styrofoam panels is to glue on some tabs so they hold together. You cut small blocks and use 3M® spray glue to affix them to the panels, as in the photo at left (the 3M spray glue is a critical element; regular super glue does not work).
Here you can see the base has been put down and the first corner erected.
Here’s how the side panels fit together. You can see what the tabs were for.
The heater sits on insulating blocks.
And finally, a close-up of the heater, with Styrofoam panels on either side to keep the nearest trees from getting too much direct heat. The heater is equipped with a thermostat to maintain proper control.
The container is kept completely enclosed during the coldest times, and opened to let in light on good days. Watering is done as needed.
A final view with the container closed.
If you have any questions about how this container works, feel free to post them and we’ll get you some answers.