PA natives;

http://www.wildflower.org/collections/collection.php?collection=PA

Is that a full sun area then?

esh - Yes, it is full sun. I plan on planting an understory-type tree to the right of it, but that is west, so the shrubs will still get a good deal of sun.

jcalhoun - I'm familiar with wildflower.org and fairly up on my natives. I guess this is more of an aesthetics question I'm asking, like should I plant grasses or wildflowers to fill in the gaps.

Full sun flowers might include: Penstemon, clumping Solidago (for fall interest), Lobelias if it is moist enough, Rudbeckia or coneflower.

I think that it might be too hot for Heuchera - that would be true for Georgia.

As to grasses - I think that is a personal preference. I don't find a lot of them available in nurseries. But Little Bluestem (Schizachyrium scoparium) is certainly very nice if you can get it.

You might try some petunias or millionbelles for a bit of vibrant color. Would also help fill in around the fence. Both of them will take a lot of sun as well.

I prefer wildflowers to grasses but that's just me.

I don't know if this is native to your location in PA, But Northern (or Prairie) Dropseed is a gorgeous grass that would look beautiful there! The birds love the seeds, too.

I would fill in temporarily with annuals for a few years to se ehow big those shrubs get. They are going to grow a lot and that won't leave you much room for perennials or grasses, unless you expand the bed.

I planted two Viburnum Nudum of the same variety and they do not set berries. You will need a second one of a different cultivar for them to pollinate eat other and set the berries.

Rudbeckia subtomentosa grows to 4' or more and would fill in nicely, also has long bloom time. Joe Pye Weed is also a big beauty(5') for fall color. Hibiscus will also be spectacular. Keep all of these wet.