Hi paul
It is look like " To kaede" trident maple. It may be cut top off at branch in stead of grow as bonsai.
roots pruning : in your area , prune roots in eary spring is safe to do it. you don't have to prune all of it at once. you can prune falf this year, rest of roots you can prune next year, it is all depend on condition of the tree and timing. you have to have very sharp spade or irrigation shavel. sharp enugh to able to cut off your toe finger.(there are two type of irrigation shavel )
................ yama

Thanx Yama.

On the top. Do I prune like grape whereby by yearly growth is cut back to the main cordon or main branch?

thanx again

Paul

No,NO,No Paul
That maple is unusual shape and size for Japanese garden. you may have to prune it like bonsai. learn how to of "Hagari" check with bosai fourm. perhaps Mark Va can tell you how. Do your home work first. yama

Paul... bonsai techniques work, except that the cycles of bonsai may be influenced by the grower. eg: drought induces autumn = therefore leaning the water back brings leaf abscisson/dormancy. Shade after leaf removal triggers another set of hormones that are useful for growth.

Garden is the balancing of all plants together to an objective.
Bonsai is balancing a single tree to itself to an objective.

In the ground, pruning at the right time is at least 50% of the solution and getting a bag on those trees may not be quite aesthetic.

remember:
Spring pruning directs tree growth. (gives the illusion of strengthening the tree)
Late summer/early fall weakens trees reducing growth.

correlation: only remove poplars or weed trees at their weakest time, at near full leaf elongation & color change from bright green to full green. The tree is fully extended at that time and has not had time to manufacture sugars.

(which goes back to understanding color changes or leaving indicators such as a few 2 - 4 year pine needles to turn brown to know when the season changed for the tree, not ourselves. And keeping records of when plants show their indicators so we do not need to run after them and we are not blindly following someone elses calendar, climate zone or advice, when 'our' tree may be in a different level of development.)
edzard

Thanx guys

I will do my homework. I have the spot now I'll start looking for the tree.

paul