Answer: You're right--earthworms do amazing things for soil. If you have 5" of decayed matter, I bet you have lots of earthworms already. Gently pull aside the mulch in several places and see if you see any earthworms. They love organic material!
You may have seen worms advertized for worm bins--those boxes you keep in your basement to which you add your kitchen scraps. The worms in these bins are usually red wigglers, and they won't survive if you "transplant" them outdoors. Around here this time of year, nearly every corner store sells nightcrawlers for the local fisherman. I suppose you could purchase some of them and let them loose on a cool, moist, cloudy day. I don't know how they'd respond, but it might be worth a try.
How about growing some of those flowers in containers? I have lots of containers of flowers around my yard (my problem is dogs, not tree roots!) and that way, you can use whatever you want for soil--preferably a mix of potting soil and compost. (And you are wise not to till those tree roots. Don't be tempted to cover thim with a thick layer of soil, either--that can suffocate them.) I hope this helps!
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