You might try one of the tall sedums -- Autumn Joy, Garnet Brocade, etc. Irises may work unless you have loads of rain and then the rhizomes might rot cause they need really good drainage. Have you considered a dry creek bed filled with stone/pea gravel and planted on either side?

That is a good idea! thanks,cherry

I had a similar spot on the west side of the house when I lived in zone 5, in a suburb of Chicago.

Autumn Joy worked well. Even more successful was the native daylily, Hemerocallis fulva. In the midwest, you often see it naturalized along expressways. It looks like it would work for your area. I had iris in my trouble spot, but it only lasted about a year.

Good luck.

Here is a link that might be useful: native daylily

We also have a steeply sloped side yard and erosion concerns. Here is what it looked like this time last year.
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First we brought in large boulders and created a path downhill, then last fall we planted a Katsura and Boulevard Cypress.

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Then we mulched heavily over the winter. This spring I've been planting Crimson Queen Barbary, Russian Sage, Blue Oatgrass, Artemesia, Daylillies, Hebe and some small trees. When I finish, we'll mulch again. The plants look pretty small, but hopefully in a few years, will fill out nicely. Here's what it looks like right now

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Should have added that the soil is rocks and clay. For each hole we dug, we ammended by removing the rocks and mixing in compost to improve drainage.