I don't know how I missed it but there's a little horse chestnut sapling in my garden. It's probably about 2 years old. How long could you reasonably grow it in a container before it became too big to keep?
I get them quite often, squirrels bury them. I just weed them up. They grow quite quickly. I would say grow it in the container until you think it looks too big. It may be hard to get out of the container by then! But why do you want to anyway? Or do you want to try and train it to be a bonsai? It won't grow as big in a container as in the ground.
If you cut off its tap root and make it rely on the other fluffy rootlings it should have, pick a nice shallow bowl. It should make 40-50 years as a bonsai. Natural growth in a big pot? We have self seeding hazelnuts, they get pot bound/breaking pots at 8-10 years. Horse chestnuts make bigger trees, so probably not as long as that.
Such a pretty thing, I didn't want to kill it. Same as it's easier to set up a charity to save an endangered pretty furry animal with sad eyes than something with multiple eyes and legs or no legs at all.
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I would say grow it in the container until you think it looks too big. It may be hard to get out of the container by then! But why do you want to anyway? Or do you want to try and train it to be a bonsai? It won't grow as big in a container as in the ground.
Definitely give it a go.
I am making a bonsai forrest planting myself after being inspired by Peter Chan.