It's possible that its Solidago flexicaulis, but the leaves in the 2nd picture have wider winged petioles than typical S. flexicaulis. Other plants that would produce this amount of root growth would be Verbesina and Leucanthemum.

A shasta daisy of sorts?

Ive researched it more, and those winged petioles are normal for the first few emerging basal leaves. Yes it's most definitely Solidago flexicaulis, Zig Zag Goldenrod. I had no idea the roots were so agressive. Maybe they are agressive to find sunlit areas to colonise under the tree canopy? I hope the small Zig Zag Goldenrod, that I planted last fall, is as healthy as yours

Solidago flexicaulis, Zig Zag Goldenrod

Thanks! I originally got the Zig Zag goldenrod because I understood it to be less aggressive; but then I remember last year one of my friends who is a gardener at a local butterfly garden suggested that I get another variety, as he thought Zig Zag was pretty spready as well.

Zig Zag is supposed to be non-agressive. At least nowhere near as agressive as Canada Goldenrod. The new shoots emerge far enough away from the mother plant that they look elegant and not like a huge crowded wall like Canada Goldenrod does. I grow the Blue Stem Goldenrod, Solidago caesia and it's beautiful and non-agressive. Another nice non-agressive goldenrod for part shade is Elm Leaved Goldenrod, Solidago ulmifolia.