The weather has been unusually cool this year and especially wet.  And even though there will be more warmer weather to come before the cold of winter sets in, it is time to start thinking about winter protection for your outdoor bonsai.  On the website, there are two articles discussing winter protection for your trees.  In summary, they can be described as healing in, placing trees in protective areas or in unheated buildings (eg: garage) cold frames, and greenhouses. Some have left their trees on stands and the trees have done well during the mild winters that we have recently had. 

Well why do we need to do protect our trees?  We tell people that these trees are no different than regular trees and yet we take protective steps. Can’t our trees take the cold, freezing weather? The above ground portions of our trees can take the cold weather provided that the lower temperature changes have been quite gradual.  According to Brent Walston of Evergreen Garden Works, chemical changes take place in the above-ground portions of the trees resulting in a type of anti-freeze, the defense mechanism of the tree. Deep freezes after temperatures have been in the fifties and sixties can catch a tree off-guard before the tree anti-freeze defense mechanism is in place in the trunk and branches. Thus the freezing cold can cause the tree cells to freeze and rupture.  In other words, if a deep freeze catches us off guard, the above-ground parts of our trees can be injured; so, they require protection. If the freezing weather comes on gradually, the freeze defense mechanisms are produced, its own type of anti-freeze is generated and water from the cells goes into the spaces between the cells and the cells themselves are not damaged.

But what about the root system?  Regular trees roots are buried up in the soil and are protected from the extreme cold. Our bonsai tree roots are buried up also but not like regular in-ground trees.  The pots are shallow and thus the potting mix and the roots are very subject to freezing.  What happens to a tree (let’s say a juniper) when the foliage is exposed to the elements (wind, cold, etc) and the potting mix is frozen. The tree will be trying to transpire but the roots will be unable to send up needed fluids because the fluid in the potting mix is frozen. Then the tree dehydrates and may die. Also, it seems that the defense mechanism mentioned above does not apply to roots; so, your roots are very vulnerable.

You don’t have quite the same problem of dehydration with deciduous trees since there is no foliage but the root system needs to be protected from freezing to prevent root damage from ruptures.

In all cases, root systems need to be moist throughout the cold season even though the tree may not be transpiring.  Too wet and one can have a problem with root rot.  Ask me about it.

So, how are you going to protect your trees this winter? It is not too early to think about it although the thought of a long, cold winter is something we really don’t want to think about this early into the fall season. 

Be like a Boy Scout- Be Prepared.