Winter Dormancy

[papkey5]

I live in zone five and even though it’s nearly August I have started thinking about my set up for overwintering my trees. I found these articles and thought I would share my research.

this first one clearly explained to me the several methods of structure or structureless Overwintering. It includes common winter damage

https://rngr.net/publications/ctnm/volume-7/chapter4.pdf/at_download/file

the next one helped me understand light requirements for conifers.

Light Requirements for Dormant Evergreen Bonsai

 

[Paradox]

I overwinter my pines and junipers in a covered coldframe on the north side of my house for 4-5 years now.
None of them have died because of it and in fact the cold frame seems to be much better than overwintering in a shed because I dont have to worry about watering most of the time. If we get a period of rrain, Ill open the cold frame to let the trees get rain. Ill even let them get buried in snow. The only time I really have to cover them is if we get temperature drops below 25-30 and we havent had any snow to provide insulation.

The main thing is you need to protect from wind.
Wind+cold = dead tree.
Many people bury their trees in snow to protect from wind. Snow is an excellent insulator.

 

[Oerc201]

I overwinter my pines and junipers in a covered coldframe on the north side of my house for 4-5 years now.
None of them have died because of it and in fact the cold frame seems to be much better than overwintering in a shed because I dont have to worry about watering most of the time. If we get a period of rrain, Ill open the cold frame to let the trees get rain. Ill even let them get buried in snow. The only time I really have to cover them is if we get temperature drops below 25-30 and we havent had any snow to provide insulation.

The main thing is you need to protect from wind.
Wind+cold = dead tree.
Many people bury their trees in snow to protect from wind. Snow is an excellent insulator.

Awesome articles thanks for sharing

 

[JudyB]

A lot of this depends on what trees you have. If you have trees that are zone hardy at least where you are then you won't need as much protection, but really trees in pots are basically 1 zone less hardy. Anything less hardy than that will need some sort of structure to be safe.

 

[Leo in N E Illinois]

Some good advice above.

I skimmed the article, what I could get before my eye's glazed over (just teasing, but I am not in the mood to read for detail today.) the article is geared toward landscape nursery production. All good stuff, but not always directly applicable to "bonsai".

You are in zone 5. Judy is right, species matters. What species of tree, and what ecotype it is for species with wide native ranges really matters. If you only grow species that are native to environments colder than your own, you need to do very little at all for winter. Take the tree off the shelf, set it on the ground in an area that will be in shade for the winter. For example, in Logan, Utah, Lodgepole pine, Jack pine, bur oak, potentilla, mugo pine, black hills spruce, colorado blue spruce, engelmann spruce, are all native to areas that extend through zone 4 into some zone 3. These will be fine pretty much just sitting on the ground for the winter.

Obviously, Japanese black pine, crepe myrtle, Japanese maples and many many more species will require protection from temperature extremes. As these all require zone 6 or warmer to thrive.

Another thing to remember. The bonsai pot - when you get into fine ceramic bonsai pots, not all pots are reliably tolerant of freeze thaw cycling. High fired ceramics should be frost proof, but shape of the pot plays a significant role. The pot needs to have walls that slope outward. That way as the root ball freezes, and its water expands as it becomes ice, the root ball can "float" or expand up and out. If the walls of the pot are perfectly vertical, or incurve, as in "bag shape" pottery, the ceramic will lock the root ball in place, and the pressure of the expanding ice will fracture the pot, even if it is high fired ceramic.

So for young bonsai, early in training, still in plastic pots, just setting the tree on the ground is just fine, if the hardiness of the tree is not an issue. For mature bonsai, in fine ceramics, one must have a space that hovers between 0 C to 4 C, or 32 F to 40 F, cool enough to keep the tree dormant, and warm enough that the root ball won't freeze and break an expensive or hard to replace ceramic pot.