Oakleaf hydrangeas! Blue plaumbego might make it through your winters but I do not know for a fact if they do.

Lantana will do well with that amount of sun.
Wood ferns, begonias (they die for the winter but come back in the spring)
Oh, there's really lots of things that will work. I'm sure more people will chime in.

Azaleas, of course! In fact I thought it was illegal to plant anything other than azaleas under pine trees!

i have a large pine tree in my front yard and grass would not grow so i planted a sedum (can remember the name) and i grow very well under the tree as a ground cover

I have a similar bed under pine trees in my front yard. I have leatherleaf mahonia (Mahonia bealei) which blooms with yellow flowers in February, and then produces plump blue berries which the birds like to eat. In 7 years of growing these, I have only had two volunteer plants, so I don't think you have to worry about it seeding all over.

I also have several clumps of the perennial Indian pink (Spigelia marilandica). The spigelia blooms in the spring (mine is almost finished blooming now) with red tubular flowers. My plants do not go dormant until late fall. They take several years for the clumps to get large enough for the flowers to be really showy.

I also have some red flowering cedar sage (Salvia Roemeriana) in one spot. For me it seems to be a short lived perennial, but it will reseed some close to where it is planted. Near the cedar sage, I have some columbine as well. It also seems to act as a short lived perennial which will reseed some in the general area. Every fall I sprinkle some larkspur seeds under the trees as well. Mine seem to be a bit floppy in the dappled shade conditions, but they still bloom well.

I have some kind of tall clumping liriope which acts as an edging on one side (it came with the house). I don't know what kind it is - maybe Evergreen Giant?

I have also planted a dogwood tree, a japanese maple and two mexican plums to be understory trees. The dogwood and maple do need extra water when it is dry.

A woman walking her dog referred to my yard as having "the wild cottage garden" look. If your tastes run to more formal landscaping, then this type of planting will probably not appeal to you.

Kathy

I really like the wild woodland look, so the more understory trees/plants the better. I think it really comes down to my tentativeness with our first house -- I have quite a few "seemed like a good idea" plants out there that aren't dying but aren't really thriving. And at this rate it'll take them approximately 100 years or so to fill in and look "wild" instead of "straggly".

I will definitely jot down those names and look when I go out next.

How are you liking the mexican plums? Did you find them locally? I would love to know a good place to look!

I do like the mexican plums. When I grew them in full sun in the Dallas area, they grew with a very dense and more compact canopy. They have a more open (but not straggly) and spreading canopy in the dappled shade of the pines here.

Some good places to look for them would be Joshua's Native Plants or Buchanan's Native Plants, both in the Heights area of Houston. The Arbor Gate on FM 2920, just west of Tomball would also be likely to have them. Another good place to try would be The Pineywoods Nursery in Shenandoah (east of I-45) - Jason runs both the nursery and a landscape business, and his wife often helps out at the nursery. There are probably other nurseries around that would carry them, but these are some that I am familiar with.