Spiraea Repotting question

This is a question for folks who have tried Spiraea as bonsai.

I want to know how much of the root mass can be removed from a 'Spiraea japonica' in spring. The subject in question is a healthy 5 gallon shrub from a nursery. I am also planning on cutting back the shoots very hard in late winter.

Thanks,
- S

I killed a gold mound this year, I took about half the roots and half the foliage, I would go easy on the roots.

I repotted a spirea japonica in winter from large training pot to bonsai pot.

I was unsure also, and removed only about 1 third of the roots, and none of the foliage. It only has about 6 foliage branches as it has been training for some time.

The tree has some very nice spring growth emerging now.

The one thing i was careful to do was to keep as much of the very fine roots as the new pot would allow.
Hope this helps!

Greg