Winter Cold And Tree Roots

It pays to read. Not long ago I came across this paragraph in a book on tree maintenance, and thought it very succinct in describing what is going on invisibly inside the tree during times we might think it is fully dormant:

Cambial activity shuts down first at the top of the tree, then in the trunk, and lastly in the roots. Therefore a tree may well have an active root system well after visible growth or leaf drop has occurred. Also, the upper roots of trees in the ground are the first ones active in the springtime.

Makes you think, doesn’t it? Another complicating piece of the puzzle is that the top of the tree is much more cold hardy than the root zone. The tags we see on nursery containers that read ‘Hardy to_’ assume you will plant it in the ground, where tree roots are designed to be. Those cold ratings are for the top only; roots are more tender. Quite a cautionary tale for those growing bonsai. Roots are calibrated to be in the ground, not in a pot.

There are a couple of important lessons here. While light frosts in the fall will ease a tree into dormancy, unseasonably deep cold in the fall may kill the tree because the roots are still active. Spring is the usual time to be careful of hard frosts, but fall must be watched too. Furthermore, if we live in a mostly mild climate with infrequent arctic blasts we should be aware that bonsai may never go fully dormant, retaining active roots throughout winter.

Healthy roots of most hardy trees and shrubs are fine with light frosts in the upper to mid 20’s (F), which helps to ease them into winter dormancy, but lower than that they need better protection such as placement on the ground or in an unheated room, greenhouse, or coldframe.

Those bonsai spending their winter holidays out on the benches may need a careful weather watcher (that would be you!)

(Reference: Tree Maintenance, P.P. Pirone, 1988)

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  1. crust says:

    I have had this Pirone book in my library for years. I wonder what the time table or environmental cursors controls just how long the trunk and then the roots function. Once the top goes dormant, lets say drops its leaves, would the normal path given sustained low temps and shortening daylight, be easily recordable or would activity have to be descerned scientifically?This past fall I spoke to David DeGroot about how maintainable far northern species were for him and he stated that anytime you have weather that goes below 50 trees start experiencing dormancy and since he had sustained below 50 weather trees like American Larch were fine for him. Where I live dormancy is never a question as my trees sleep in cold storage facility half of the year. I don’t ever seem to have mortality due to general winter but rather early spring hard freezes–especially with repotted trees. As I type its below zero and my trees, which are all hardy species, are maintained at mid-twenties.

    • crataegus says:

      It is an interesting problem. I think we simply need to be very cautious of trees that have not had frequent light frosts and lower light before exposing to deeper cold. Dave is a great resource. Our climates are very similar, he lives and works near Tacoma and the conditions are similar here in Portland. What I’ve seen is that partial dormancy occurs with changes in daylight and lowering temps, but full dormancy does not happen here. The roots remain active most winters. Sounds like you’re in a much colder area, and you won’t have as much worry in the deep of winter. The scary things are the storms that come out of nowhere and are very unseasonable in severity, in the spring and fall.

  2. Michael Markoff, GSBS says:

    After killing a few (Quite a few) trees that, in retrospect could have been pretty good, I have adopted this rule of thumb: Consider bonsai potted plants to have a hardiness one zone less on each side. Therefore, a tree normally cited as hardiness 4-7 (eg; Upstate NY-No. Carolina) would be safe in zones 4-6, that is; mid-Sothern NY state-Virginia). Obviously some winter protection (mostly against desiccation and snow/ice damage to fragile bark, like Ponderosas) is of benefit.

    • crataegus says:

      That is a good idea. However I find that in pots unseasonably cold storms in otherwise mild times of the year or climates can still damage plants. Potted plants are really sitting ducks regarding cold—when it’s not cold, their roots will grow. And then it can get very cold. All we need is a 40 year storm to kill a tree we’ve worked on for 20.

  3. Michael Markoff, GSBS says:

    Make that zones 5-6 above.

  4. Fred Elford says:

    That explains it! i lost over 50 of my trees, some of my best plus a whole lot of those that – just maybe some day . . . . i lived in Calgary, Alberta at the time. we had a long autumn with very warm temperatures. The trees were out, Suddenly it went down to -25C and stayed there. Though i was able to get everything into the boxes in which they are covered with mulch – open so it can snow inside – they never came out. Well, there were a few that did and even leafed, but then died. i was moving at the time, so in a way it was a blessing. i now live in Windermere, BC – considered a ‘remote’ area – but am having to start all over with the trees. (The day after i arrived, a deer killed 6 of what i had left.) The trees are now in an enclosure and have no problems, though the deer have a regular trail that goes right past them and they have foraged as close as right under my bedroom window.

    However, I am now 71 and there is not much time. living on a small budget does sure make it a challenge – and then i have only ever had one class on what to do with them. There are more questions than i have answers available.

    But, thanks for the good info. I knew it was the cold that killed them, but knowing the mechanism is very interesting.

    Fred Elford

    • crataegus says:

      Sorry for your loss… that is a sad story. I had a yew once when I lived in upstate NY—still have it, actually—and a light snow covered everything that was green except for the bonsai benches where the snow blew off. Revealing my yew. Which was then restyled as a semi-cascade by a hungry deer. Improved it greatly.

  5. Graham says:

    Hi Michael, again an interesting topic of discussion.

    A timely one too, as Anton and I were reviewing this very subject recently.
    From my side of the fence (work wise – forest management) we are very aware of dormancy (is broken into quiescence and rest dormancy).
    Quiescent (earlier stages of dormancy) is a stage where conifers if well irrigated or moved to a greenhouse where a favorable growth regime is maintained (or for us if we have unseasonably dry/hot weather periods in late summer) it will become active again and break its bud and resume active shoot elongation (we get this in many of our conifer plantations, during some warm late summers = Lammas growth). In contrast, a conifer in the “rest” dormancy stage will not grow regardless of how favorable the environment is.
    Sounds like your climate only provide the quiescent dormancy whereas up here we get the full rest dormancy .

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