lilly of the valley has a shallow root structure, and I often see them as underplantings. I have also seen japanese ferns used as underplantings- dig between the roots, tuck some (wet)soil-moist crystals at the bottom, and mulch afterwards. I used them EVERYWHERE at my old place, which is VERY shady- they bring color to places color won't live.

Ive got golden Moneywort rigth under a silver maple and it does fine.

Springcherry

But ... I just remembered, some trees do weird things to the soil around them.And I think one of the difficult trees is perhaps a locust?

Springcherry

There are some other suggestions for plants under trees in the discussion strand I linked below. Also, to my knowledge, locust trees don't create any chemical barriers to things growing under them.

Here is a link that might be useful: shade and drought tolerant plus summer color

Foxglove--they are native to forest conditions.