Q.What to plant under my mature (22 years) Sargent Crabapple tree
This tree pretty much shades anything under it except for the west side of the tree gets afternoon indirect light then direct light 4-7pm. I’m thinking of a smaller plant (2′ clearance around the bottom ) with color like astilbe maybe. Under 1/2 of this tree is almost all day shade/indirect sunlight.
Certified GKH Gardening Expert
The best thing you can apply under a tree is mulch out to the dripline. Trees have shallow feeder roots that shouldn't be disturbed by planting under it. Especially since you have a mature, 22 year old tree, I would just enjoy the shade.
However, if you are committed to planting, here is an article that tells how to plant without disturbing the roots too much: