As much as I love tree peonies, I do not think they make a good hedge. They look better as a lone specimen or a group of 3 with a good spacing between them.
Leslie, it sounds to me like you want to grow a straight row of peonies. Most lactifloras seem to have very good foliage throughout the summer. These will do best in full sun. The only problem with this is some of these are rather floppy peonies. Your best bet is to get those lactifloras with a single or anemone/Japanese forms. These will have good foliage and sturdy stems. Or better yet, look for those peonies that win the APS Best Landscape Awards. They win this Award because they make great landscape plants, that means you get a nice flowering bush with sturdy stems and nice foliage throughout the growing season.
