Answer: If you use edging strips, and they penetrate the earth only 5" deep, they shouldn't bother the roots of your maple tree. Most roots will be deeper than that and any that grow into it will either bend and go in another direction or will grow down and then under the strips. If you use landscape fabric and mulch, edging strips probably won't be necessary, and you can trim the area by setting the edge of your shovel at the edge of the mulch and stepping down on it for a smooth edge all around the bed. Hosta and Lenten Rose are good under Japanese maples; ajuga would be fabulous but high maintenance. Ajuga can creep at amazing rates so you'll need to keep it in check or it will take over an area in no time at all. I'd use hosta (it spreads, but at a slow rate) instead.
Best wishes with your landscape!
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