Which flowers can be planted in the shade?
The University of Minnesota Extension Service article entitled, “Gardening in the Shade” provides lists of annuals, perennials and herbs that do well in the shade.
Gardening with Ed Hume offers advice about growing plants in shady areas under tall trees.
For resources specific to the Pacific Northwest, try the list of shade perennials from Paghat’s Garden, Great Plant Picks, King County’s searchable Native Plant Guide, and lists of native plants for mostly to fully shady sites Washington Native Plant Society. There are also numerous books available in the Miller Library. Here is a booklist.