Well, clearly, this is a thread in which I should participate! <g>
Becky, Iris is one classic flower that, in the wild, grows in wetlands. Of course, many varieties have been bred to prefer drier soil, but if you find a knowledgeable person at a good nursery or catalogue, they should be able to lead you to tubers that love the wet!
Also, I assume that you've considered alternatives such as adding mulch and topsoil to raise your beds a few inches to a foot above the damper soil below...this will often help prevent flooding that can cause the roots to "drown."
Here are some other perennials that one of my reference books include in "moist to wet soil" category. I don't even know what some of these are, but many are familiar. Check to see if they will grow in your zone:
hollyhock columbine wormwood goatsbeard sweet woodruff astilbe English daisy heart-leaved bergenia Siberian bugloss pink turtlehead Kamchatka bugbane fringed & common bleeding heart gas plant foxglove Caucasian leopard's bane dropwort gentian Barberton daisy Chilean avens sneezeweed Heliopsis Christmas rose Dame's rocket coral-bells rose mallow fragrant plantain-lily goldflower St. John's wort Japanese, yellow flag, and Siberian iris big blue lily-turf purple loosestrife pink plume poppy bee balm dwarf perpetual forget-me-not blue cupflower dwarf lily-turf Canada phlox garden phlox see this month's Martha Stewart's Living--yup, I read it! It's all about phlox
Jacob's ladder Himalayan primrose Japanese primrose polyanthus primrose blue lungwort pincushion-flower lavender mist meadow rue Virginia spiderwort Ledebour globeflower viola pansy
Happy gardening!
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