Seasonal Care for Cold Weather…

Every year I try to wrap a blog post around the snarly issue of protecting our trees from cold. This year we’ll try a new approach, and take it in a few bite sized chunks, in a couple of blog posts.

Firstly, we need to bear in mind that the top of a tree or shrub is much more cold hardy than the roots. When we read about the ‘cold hardiness’ of a plant in a garden book, that designation is assuming the darn thing is in the ground, where a plant’s roots are supposed to be. The designation takes no heed of silly bonsaiists who will put it up on a bench.

Roots are much more tender than their tops-


A lot of people don’t know this, apparently even a few snow sculptures…

Nextly… on the ground is a good first place to put a bonsai in cold weather, generally when it is dropping to about 27 F / -2.8 C overnight. On the ground a pot might be as much as 7 F warmer than on a bench, three feet higher (according to our experiments here at Crataegus Bonsai). Which is a significantly warmer temperature, if you’re a root, and you’re used to being in the ground.

Once the thermometer drops lower, below 27 F / -2.8 C, many temperate trees and shrubs need more protection. You might need a greenhouse, poly tunnel, or coldframe, either for the whole winter or for the short severe cold snap that might last a few days. Many trees are OK outside on the ground down lower than this, such as some mountain pines and junipers, but beware the wind/cold combo…

Wind can be as damaging as cold, and both together are a real whiz-bang yikes thingy that can deposit a dead tree at the doorstep of spring. A frozen rootball with wind is seriously not good. The bonsai can desiccate, causing if not death often some branch dieback in the growing season. Keep your trees hydrated, and keep them frequently thawed out (even if they freeze occasionally, they shouldn’t remain that way).

You can work on many bonsai in the winter, including wiring and bending. If you’ve recently wired a tree, or done severe bending, however, they cannot be put out in wind and cold. Protect them.

Beware also of keeping temperate bonsai at too high a temperature overwinter, which may cause weakening the following year. Keeping bonsai over 50 F / 10 C creates some problems with chilling requirements and the ability to grow out in the spring nice and strong.

Next up on this miniseries…’Winter dormancy and chilling requirements’, which will get a bit nerdy…

(Please note: All temperature notations are approximations, and everything related to cold hardiness also depends on the species of tree, how late in the year it was growing, its health…all those things and others play into this discussion).

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  1. Greg says:

    How do you winter cascade bonsai, since you can not lay the pot flat on the ground without damaging the tree?

    • crataegus says:

      A tricky question, and that’s where a greenhouse or some sort of structure where you can have the tree elevated on a bench, and then control the temperature with a heater during cold spells, is best.

  2. Don Erickson says:

    Thanks Michael your blog information are always well timed and most helpful. Hope you survived the weather the past couple of days.

  3. Lance says:

    Great post. Depending on the country, air temp is usually measured 1.2-1.5 meters off the ground for the reasons you’re discussing, i.e. for the meteorolgists to get accurate air temperature readings not influenced by ground temperature also known as Earth, aka the heat sink..

  4. Teri Wall says:

    Question- in addition to wiring and bending can you do any trimming. I would like to do some work on a Shimpaku raft that I have. Also if I’m working on a tree in my basement and it’s about 60 degrees down there, that’s not going to bring the tree out of dormancy is it? Thank you,

    • crataegus says:

      Yes, trimming of the nature you suggest would be fine. Juniper is one of those trees that may be pruned or trimmed at any time of the year. Try to keep the time spent at 60 or above to a minimum when working on it, so if the work takes longer than a few hours, and you need to do more the next day, don’t leave your tree in the warm area, return it to the cold storage overnight. A few hours is ok for a juniper. Other kinds of trees can be popped out of dormancy with very little exposure to warm weather.

  5. Andy says:

    Would you ever bury the bonsai in the ground so that the edge of the pot sits just at the level of the top of the soil? Then perhaps mulch over that? Versus sitting the pot on top of the ground?

    • crataegus says:

      That’s a good next move if the temps are lower in your area. Any kind of mulch or burying the pot is going to protect the roots that much more from temperature extremes. But the issue with wind still applies. If you experience extended freezing, then you’ll likely want some sort of polyhouse or some other wind protection like a greenhouse or a cold frame, possibly heated occasionally, or a building dug into the side of a hill like a student of mine has in Montana. Take wind seriously. A lot of trees end up either dead or with dead branching here and there due to wind during freezing weather.

  6. bonsai eejit says:

    Reblogged this on Bonsai Eejit.

  7. Karen Trucke says:

    Does anyone know if “on the ground” would include a stone patio, i.e. is it the protection of the soil or is it just keeping the colder wind from under the pot/roots?

    • crataegus says:

      The bottom of the pot should be in contact with the soil. If air can get underneath the pot, in other words, if the pot is resting on its feet on concrete, it’s not as effective as if the pot was nestled into soil, mulch, or gravel.

  8. garyswiech says:

    Good timely article Michael. However for those of us who live in the far North as I do in Wisconsin, far more protection is needed for most bonsai to make it through our Winters. I just put my Bonsai in their Winter home in an enclosure I built in my garage, the walls of which are made of 2″ styrofoam. It’s going down to 10ºF tonight. I have an additional space heater that is needed when it gets down below -0ºF. 2 yrs ago I lost many old and treasured Bonsai of mine. All it takes is one night when the Temps
    drop and the heater thermostat isn’t set right. I also have to keep the humidity up also.

    • crataegus says:

      There are increasing degrees of protection, as you say. Any of those options with a structure may need an additional heat source for extended cold. A coldframe idea might be turned into something you can walk into, if you’ve got a slope somewhere to work with. Greenhouses may be equipped with fans and heaters to keep trees at a constant temperature. Airflow is very important in any structure that is closed for any length of time.

  9. Chris says:

    In Ohio I put all my junipers, elms and maples inside my unheated garage. It worked the first year but now I’ve aquired many more trees and am more nervous 😂😂

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