@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.
. Never thought of kusamono but it could definitely work
