Preparing Bonsai and Pots for a Midwest Winter | Iker Bonsai Pots

Preparing Bonsai and Pots for a Midwest Winter

Here in the Midwest, winter is bearing down on us, and that means it’s time to start thinking about how to protect bonsai through the cold months ahead. Trees left outside will need some level of protection to survive our fluctuating conditions.

My Winter Setup

I use cold frames with the glass whitewashed and the lids propped open for air circulation. This setup lets me control the amount of rain and snow that reaches my trees. The goal isn’t to shield them from the cold itself, but to moderate rapid temperature changes. Around each pot, I pack mulch to help insulate and reduce freezing and thawing cycles.

Are My Pots Frost-Proof?

I get this question a lot — and the short answer is yes, with a few important notes. All of my work is made from vitrified stoneware, fired to the point that it can’t absorb moisture. This vitrification process is key to preventing spalling — the cracking or flaking that happens when moisture inside clay freezes.

Open clay bodies, such as terracotta or earthenware, can absorb up to 25% of their weight in water, making them vulnerable to freeze damage. Vitrified stoneware, by contrast, is non-porous and won’t absorb moisture — an essential quality for long-term outdoor use. Many imported low cost pots are made from earthenware. 

The Real Cause of Winter Breakage

When pots crack in winter, the culprit is almost always frozen water inside the pot, not the clay body itself. Water expands by about 4% when it freezes — enough to split a steel pipe, let alone a ceramic pot.

That’s why I focus on controlling moisture rather than avoiding cold. Protecting your bonsai from direct rain and managing water levels through winter storage are crucial. Even the best-draining soil will eventually reach saturation if exposed to too much winter moisture. And while we worry about the pot, the real concern is the tree — its roots still need air and balance, even in dormancy.

Pot Shape and Expansion

Pot shape can also influence how well it weathers winter. A pot that flares outward toward the top will experience less stress from ice expansion than one that narrows at the rim. It’s all about how the pot contains that expanding ice.

I’ve kept both styles outdoors for years without issue, but the key is still the same: manage waterlogging and freezing, and your pots will last virtually indefinitely outdoors.

Snow: Nature’s Blanket

There’s nothing more beautiful — or protective — than a fresh layer of snow on bonsai. Snow holds right around 32°F, insulating trees from harsh winds and deep freezes. Just be mindful: snow also means added moisture, so don’t let it build up to the point of waterlogging your pots.

Lessons From the Field

Last winter, I had a few trident maple pre-bonsai planted out in a field. It never occurred to me that deer might enjoy those tender branches — until they did. This year, I’ve built hardware cloth covers to keep them safe. Luckily, my visiting deer had an excellent eye for bonsai styling; the trimming it left behind wasn’t half bad.

A Season of Rest

Dormancy is an essential part of the natural rhythm — for both bonsai and the bonsai artist. Preparing your trees for winter doesn’t take much work, but preparing yourself for the slower season ahead might. Embrace the quiet months. It’s all part of the process.