Planting under trees is always a tricky situation. The tree's substantial root system will absorb most of the water and nutrients from the area. Also, you can't work the soil much around the tree. Many of the tree's feeder roots are in the top 6 inches of soil; if you work this soil too much and damage these roots, you'll risk damaging or even killing your tree. Instead of working the entire area, plant in pockets among the roots so you disturb them as little as possible. If you plan carefully, you can have a colorful shade garden from spring to fall. The key is to select the right plants, such as these. Corydalis (Corydalis elata) produces clusters of fragrant blue flowers in early spring. It may rebloom in autumn. Grows 15 inches tall.
Zones 5-8. Dwarf goatsbeard (Aruncus aethusifolius) bears feathery creamy white flowers in summer. Its finely cut foliage turns yellow in autumn. Grows 18 inches tall. Zones 4-8. Golden hakone grass (Hakonechloa macra 'Aureola') has arching bright yellow-green foliage that's attractive spring to autumn. Grows 14 inches tall. Zones 5-9. Jack-in-the-pulpit (Arisaema triphyllum) bears unusual blooms similar to calla lilies in spring. Grows 2 feet tall. Zones 4-9. Japanese painted fern (Athyrium nipponicum 'Pictum') offers interest from spring to autumn because of its lovely silvery-green foliage with red highlights. The stems are also reddish. Grows 12 inches tall. Zones 5-8; but often survives in colder regions. Lenten rose (Helleborus orientalis) produces pink, white, or green flowers in late winter and spring. Grows 18 inches tall. Zones 4-9. Masterwort (Astrantia major) bears unusual spiny blooms of pink, red, or white in summer. Grows 3 feet tall. Zones 4-7. Navelwort (Omphalodes cappadocica) produces charming blue or white flowers in early spring. Grows 10 inches tall. Zones 6-8.