I'm looking for a pretty but "natural" looking ground cover for a sandy hill?

I live in Michigan and it's almost impossible to grow anything on the hill that I live on. It's pretty sandy and in the summer, it's covered with a thistle-like purple flowered weed. I rake it out in the fall, but each year it comes back. I planted some tulip bulbs on the hill for spring color, but its a pretty big area so I need a better groundcover. I want it to look pretty but natural, I have seen these flowers with multiple colored blossoms (pink, white, purple), about six petals to a flower. They have very lacy foliage and they grow to about 3' tall. I'm wondering what this plant is... someone told me "Cosmos" but I can't find it anywhere. Is it a perennial? Any suggestions on where to find this flower, or what to put in this trouble spot? Thanks!