Repotting from ground

[norwayman]

Hello from Reading, Pennsylvania. (zone 6) I have been collecting bonsai for over 10 years. I have a number of nursery plants (conifers including juniper) that I put into the ground 5 years ago and have trimmed the branches to keep them tight. The trunks are now the size I want. I understand that I can trim the roots while they are in the ground, and at a later time put them into large pots. I would trim the roots by using a straight shovel and cutting straight down around the tree. I know that I should put them into large pots in the early spring. My question is: What is the best time to trim the roots using the straight shovel? Should I do it a year ahead? Thanks

 

[Shibui]

Trimming roots with a shovel sounds like a great idea. The problem is we invariably cut too far out from the trunk which means a huge container when the tree is dug and then years of chasing roots back to fit a bonsai pot or cutting the roots shorter to fit the container which defeats the whole purpose of the root chop.
If you are aware of the final pot size and can cut roots close enough to the trunk then it's a great plan.
Roots should be cut in time to allow some new roots to start growing. How long will depend on your local climate.
A full year would be ideal. Mid Summer should be Ok to allow some new root growth before dormancy.
At a pinch, you can probably get away with cutting roots in Autumn for a Spring transplant. The new roots may not have emerged but the cut roots will have callussed and be ready to grow by Spring.

Note that many species can be successfully transplanted without any pre-treatment. Most deciduous can survive with roots chopped short at transplant.