If you don't have any evergreen shrubs there, the bed will look really bare in winter/early spring. Eastern exposure is good for azaleas, for example. Just something you might want to consider.

As for your question, it depends a lot on how much sun/shade this spot gets. Small crepe myrtles will bloom July-August, but they need mostly sun. Clethra 'Hummingbird' blooms late and smells sweet. Solidago (goldenrod) blooms late, and there are both short and tall cultivars/species. Caryopteris (yet another shrub) has blue flowers in late summer-fall, and nice blue-green-grey foliage. Salvia coccinea is an annual but it self seeds easily in my yard, so I have red (with some white and some pink) flowers from now until frost. Many asters bloom late, and also come in a range of sizes and colors. Eupatorium 'Chocolate" has unusual dark red and green leaves, and white fluffy flowers in late summer. It does tend to spread easily by seed though. Finally, for fall you could plant for leaf color as well as flowers. Hope this is useful!