Here in the Midwest, winter is bearing down on us. Bonsai left outside will need some level of protection to survive. I use my cold frames with the glass white washed for winter storage and keep the lids propped for good air circulation. This allows me to control the amount of rain and snow on my trees. I am not trying to protect them from the cold. The objective is to moderate fluctuations in temperature. I also pack mulch around the base of the trees and pots to insulate and control freezing and thawing.
I am often asked if my pots are frost-proof. My answer is yes, with a few caveats. All of my work is vitrified stoneware. It’s is fired to the point where it really can’t absorb moisture. Vitrification addresses the issue of spalling. Spalling occurs when the moisture inside the clay freezes causing the pot to crack or flake. We’ve all seen this on terracotta or earthenware pots. Open clay bodies can absorb up to 25% of its total weight in water. Vitrified stoneware will not absorb moisture.
The culprit for winter pot breakage is most often a pot full of frozen water. Frozen water expansion can split a steel pipe. Water expands by about 4% in volume when frozen. The best way to manage this is to protect your Bonsai from direct rain in the winter. You need to control the amount of moisture it receives. Even with proper soil a pot will reach a point of complete saturation in winter. More important than the pot are the tender roots of your Bonsai; the tree may be dormant but it still has moisture and air needs. Soil saturation from poor drainage in the summer is a bad thing, in the winter it can be devastating.
Pot shape can also have an impact on winter breakage. A pot that tapers out toward the top will have less expansion force applied than a pot that tapers in at the top. It’s a matter of containment of the expanding ice. I have had pots that taper both ways outside for years. As long as you control the amount of water-logging and freezing that takes place both shapes will last outdoors virtually indefinitely.
There is nothing more protective or beautiful than a layer of snow on your Bonsai. The snow is right at 32 degrees and actually protects your tree from harsh 0 degree winds and cold. But again, snow comes with cold and moisture so be careful not to allow your pots to waterlog.
Last year I had some trident maple pre-bonsai field planted. It never occurred to me that deer would enjoy the tender branches of a sapling maple. I learned this lesson the hard way. This winter I have built a hardware cloth protective cover over them. Fortunately I have a deer with a good eye for Bonsai styling because the resultant trimming he did on my maples wasn’t all that bad.
Dormancy is a vital part of the natural process for the Bonsai, and I suppose, the Bonsai Artist as well. Preparing yourself for winter can be more difficult than the few easy steps required to prepare your Bonsai.
Iker Bonsai Pots