How about some flag irises? These are planted with their rhyzomes sitting on top of the soil and this is essential to ensure they are baked by the sun as this helps with flowering the next year. I also plant some white cosmos annuals-Purity- the more the merrier, and which extends the season beyond Iris flowering period. This must be the easiest annual to grow ever. nice large seed, one per pot/module. Water and stand back! Plant out late may...lots of liovely foliage too to cover now manky Iris leaves.
Might you consider some sunken pots of Lily of the Valley for early interest and sunken pots of Crocosmia for later? Also some of the ferns are surprisingly toleant of light and competition with trees/shrubs.