Cold Hardy Bonsai in Zone 3-4

@ViridianTanuki
19 yrs is a great age to begin bonsai. I started when I was about 14, and I am now 66 yrs. For various reasons, I've started over a number of times. Tree longest in my care currently is only about 15 years in my care. My record was keeping a pomegranate healthy and growing for some 42 years, then I forgot it when we had a cold snap. They don't like hard freezes to 10 F (about -23 C.). Anyway, my backyard has long been my proving ground for winter hardiness.

Your biggest issue will be to learn how to winter trees that are hardy in your temperature zone. I have some disagreement with the published "root hardiness" table in Mike Hagadorn's books. His published temps do not agree with my experience. (that is likely the source @HorseloverFat was referring to in his comments). But I have not done controlled experiments, so I am not going to call out specifics.

With material that is successful as landscape trees and shrubs or that are native to your area, key for winter hardiness is to use the element trees in the wild use. CONTACT WITH THE EARTH. The ground is heated by heat from the center of the earth seeping outwards. Local municipal build code departments will have standards for how deep water supply lines must be buried. This is a foot or so below the average frost depth for your area. In my municipality water lines are required to be roughly 5 feet below grade, about 1.6 meters. Below that depth the soil is usually above freezing even in an unusually cold winter. Cold frames where the earth is excavated to some depth, take advantage of the heat from the ground to provide enough heat to prevent roots from being killed.

I'm in zone 5b, for trees native to zones colder, like zone 4, for these species, like my spruces, Jack pines, crabapples, and others, I simply set the pots on the ground for the winter. I throw a little mulch over them, no deeper than the tops of the pots and forget them for the winter.

With more tender zone 4 to zone 6a trees, I will dig the pots in against the side of the house, and mulch deep enough to cover the pots, maybe the trunks a little too, but key will be wind breaks and winter shade. Sun scald on those clear sunny days where the air is well below freezing, but the sun is bright and warm, sun scald can be a problem with maples and other "forest" trees. Wintering them in a shaded spot helps with this.

Constructing a cold frame, but using plywood or other opaque material will help keep trees cold for the entire winter. You want to avoid freeze-thaw cycling which is a major problem with a glazed cold frame. Most temperate winter hardy trees have no problem with being in the dark for the winter. An opaque cold frame (technically a root cellar) will help with allowing the trees to freeze once, and then stay frozen until spring. Sited on the north side of a building, in the shade you should be able to avoid issues with trees coming out of dormancy before it is safe to put them outside.

SO any species native to your area or north of you, and higher in elevation than you is a good choice for bonsai.
Thank you for the tips! I did bury my shimpakus and chinensis up to the rim of the pot last winter and had no die back.

The seeds I will start in a couple months indoors to get an extended growing season before the next winter. I will bury the seedlings in their pots in the ground too.

I’ll look into a cold frame for next winter. Can conifers go without light in the winter? I can also put them in the attached, unheated garage.
 
I was being facetious. The Prairies are not known to grow much else. It would work on some level.
 
I had big plans to make some shelters for all my trees then I remembered I've had a potted larch my son started for a school project when he was in kindergarten. He's now 16 and the larch is fine, I never did any winter protection on that tree, the only damage it ever took during the winter was a moose chomped the top off one year.

Since it's my first winter with a bunch of sticks in pots I decide to just wing it and figure out how far I could push them trees. I grouped them together, packed soil around and in between every pot, threw a bunch of leaves over it all, left them out in the open and let them get snowed on. I don't have fabulous material and everything is local, if they die I'll know I need to provide better protection next winter. I'll learn what works through trial and error. Once I figure out what works on local trees I'll start pushing my limits on non native species and provide better protection.
Snow cover is one of the best if it lasts the whole season, it insulates against deeper cold temps and windburn. There is one downside. As the snow melts it turns to ice overnight and after several cycles of snow melting in daytime and pooling on top of snow at lower levels, then freezing overnight, ad infinitum, the snow at the bottom of the pile gets very thick with ice and that's stuck to the branches and can be heavier than the branches can hold up, so smaller branches can break and you can see it coming and be helpless to correct the situation. If the snow does more evaporating than just melting, that's better. Very low humidity is helpful. WeatherSpark shows low humidity for Calgary year-around.
 
For areas “Zone 5” and colder... finding out your species’ “Root Damaging Temperatures” is FAR more useful than hardiness zones.

;)
Being in zone 8, would it be safe to then say that our low temps are short lived and no hard freezes, so roots are prob not going to be damaged?

Excluding species that aren’t zone 8 hardy or extremely thin pots that only 1” deep
 
I don’t think zone 8 PRODUCES temps cold enough for damage... but just remember.. that it containers, your roots are experiencing DAMN NEAR the same temperatures as the air....

I’m unfamiliar with tropical root damaging temperatures.
 
Being in zone 8, would it be safe to then say that our low temps are short lived and no hard freezes, so roots are prob not going to be damaged?

Excluding species that aren’t zone 8 hardy or extremely thin pots that only 1” deep
Not a safe assumption. Zone 8 encompasses areas like Phoenix, Tucson, Austin, Baton Rouge, Jacksonville, Fla. Minimum temps in Zone 8 reach to 10 F-20 F. I used to live in Phoenix. It used to freeze a few times in the deep winter, late Jan. Feb in particular. I've been in Tucson when it hit 25 F. Those temps are deep enough to freeze a potfull of bonsai soil through if the pot is small enough (like under 20 lbs). I would not make the assumption you're safe from freezing in that zone. I would keep an eye on the forecast for cold snaps and be ready to move trees that could be vulnerable to temps below 32. Those temps probably won't affect species such as juniper and other temperate zone native trees, though.
 
Ooo, I thought it was a min of 20… damn, 10F, brr

I’m going to make a cold frame for my stuff that doesn’t want to be under 40. The rest of my material should be fine, but I’ll get it set up to wrap up in the case of sub freezing snaps
 
We are just one-upping each other, haha. That's pretty insane. How long is your growing season up there then?
The crazy thing was less than 2 weeks later we were 60F.

Growing season isn't long enough. This year last frost was in May and first frost in September.
 
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