Branch and root pruning on a 6' black pine

Hello Jonas,
I have no experience with black pine/evergreen bonsai.

If I cut a 6’ black pine down to the first branch as the new leader next end of winter, can I cut the roots too? Or is it best to cut the roots the following year?

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You can prune some of the branches when you repot, but not too many. I usually try to avoid pruning more than 50% of the branches at any given time, especially if I’m repotting the tree.

It’s also worth noting that many pines have started growing for the season. If the pine has started growing, I’d avoid aggressive rootwork or wait another year to repot.

Would it make sense to air layer the upper half of the tree since I’ll be reducing the tree and creating taper?
I don’t know how easy it is for black pine to air layer.

Also is “Thunderhead” a normal variety of Japanese black pine? Looking at nurseries that carry (Japanese) black pines here in Seattle region and I keep seeing that name.

Black Pine is not reliable for air layering. It can be done but I would not recommend the practice. The " thunderhead" cultivar is popular in the landscape trade for heavier pom pom like foliage, typically with longer internodes between tufts of foliage. I would look for smaller denser foliage for Bonsai purposes! One cultivar that may be available in Nursery stock is KOTOBUKI. If you are in the Seattle area a good source for Japanese Black Pine and other Bonsai material is Bonsai Northwest. They have field grown JBP grown specifically for Bonsai. Better nebari, trunk movement and lower branching to begin with.

Thanks for the information.
I’ll check with Bonsai Northwest once they open again from closure of covid-19.

Aaron, I would give serious thought to “rocking” the pot periodically to keep large roots from growing into the soil if you leave it on the ground in a pot. By periodically, I would check it weekly to see if any roots are trying to escape. The tree will not have invested to much energy in a new root(s) that it will set the tree back if it gets root pruned that way. Besides, you will look at the tree more often, which is never a bad thing.