Ask Mr. Smarty Plants

  The Wildflower Center has a really nice resource for this kind of plant shopping.  If you go to the "Recommended Species" link under "Explore Plants" - you can choose the recommended species that will do well in Oklahoma.   If you then narrow your selection by choosing  “Shade <2 hrs sunlight” - - That gives 29 possiblities.   Most of those are trees though.

For a space 5 ft deep and 10 ft wide - you might be able to have a mid-size shrub for the background. If you narrow your search to Shrubs – there are three results.  The one which looks a good choice is Rhus glabra (Smooth sumac), which blooms late spring to summer. The other two, Amelanchier arborea (Common serviceberry) and Cephalanthus occidentalis (Common buttonbush), are a bit tall and  the buttonbush prefers a wet habitat.

             
Rhus glabra                           Cephalanthus occidentalis             Amelanchier arborea     

  There are several attractive flowers that can be foreground.  Choosing “Herbs” as the aspect gives 11 results.  Good choices include  Aquilegia canadensis (Eastern red columbine), Lobelia cardinalis (Cardinal flower) and Phlox divaricata (Wild blue phlox).   

 The Columbine blooms from February to July with a brilliant red flower.  The Blue Phlox has a spring blooming season.  The Cardinal flower also has a red flower and has a longer bloom of May to October.  At 3-6 feet, the Cardinal Flower is also a bit taller than most herbs, so it might do for the background.  Coreopsis lanceolata (Lanceleaf coreopsis) is also slightly taller than the other herbs. 

             
Aquilegia canadensis              Phlox divaricata                    Lobelia cardinalis

Of course, just following the process described above, you can review your own results and have the plants you prefer!