Lots of roots and very dry sand. Tried to plant a shady perennial garden here for years. Tons of compost and potting soil. Always a losing battle to the tree roots and very dry sandy soil. This year gave up. Plowed and planted shade loving annuals and included many perennials but in planters where soil and moisture could be more easily controlled. Here are a few photos.
Perenial begonia is a nice choice for that shady spot. Overwinters and multiplies freely here in zone 8.
When nothing else works - impatiens!
I've never cared for coleus much. These turned out great.