@River's Edge in thinking more on what you've said about dividing the root ball and only doing 1/3 at a time. Would cutting off the bottom half of the root ball to get into a training pot be one whole repot season on it's own? Or is it safe to remove said bottom half and still perform the 1/3 bare root?
It depends on the condition of the root ball. Hemlock are very dependant on the finer fibrous roots typically near the surface.
When dealing with nursery raised Hemlock the root ball is significantly different and can vary depending on nursery practices.
If I understand your question correctly, I would interpret it as follows. Is it ok to remove 67% of the root ball in one go? The bottom half plus 1/3 of the remaining!
My recommendation when
adapting Hemlock for Bonsai purposes is to deal with
1/3 of the overall root ball at a time. Once the tree is adapted for bonsai use ( has developed the finer feeder roots to enable thriving in a Bonsai pot) then one can adapt more standard repotting practices. However they still need to be applied appropriately within the particular situation.
Using this approach one can typically accomplish a complete repot and change out of old soil in 18 months. 1/3 in the first session, 1/3 in second session and 1/3 in the third session. Typically I would recommend the following timing. First session just prior to bud break in the spring, second session early fall, third session prior to bud break in the following spring. Only proceed to the next session if recovery is evident after the previous session and the tree remains healthy.
With
established hemlocks ( those with developed bonsai root ball)I generally repot every two to three years. The amount of work done at that time depends on the soil condition and space remaining for roots to grow.
The amount done is approximately 1/2 but may involve bottom and edges, or central core and top depending on the stage the root ball is at and the condition of the tree.
This tree was developed in this manner and the progress time between pictures is four years from first work involving repots, styling, wiring etc.
First picture ( September 2014) is the collected tree which was placed in large nursery pot after collection with no root work done. Second picture is progress after four years. ( March 2018) I include this to dispel the notion that moving slowly in the beginning with repotting delays development.