You can build beds under the oaks with rocks to hold the soil. Be careful not to pile up soil against the trunks, I leave about a foot around them.

Some of the best things to plant are shallow rooted Iris, Liriope, and other ground covers. I also have large shrubs under the oaks and they work fine.

Scroll down for a picture of Los Patios here in San Antonio and what they plant under their oaks.

Here is a link that might be useful: Ground Cover under oaks.

I was wanting to do the same thing and found that there aren't that many flowers that will do well in shade. Some will grow and even flower, but you won't get a lot of flowers.

Keep in mind that any plants you put down will be competing with the trees for water/nutrients and you can't put more than a few inches of soil/mulch down without causing trouble for the tree. Also, remember that you'll still have to deal with the fallen leaves and acorns.

Good luck!

Around my oak I have cast iron plant, holly fern, turk's cap, coralberry, toad lilies, chocolate snakeroot, and my favorite, pigeonberry. The cast iron plant and holly ferns are the only ones that are evergreen for me. The others die to the ground, but that makes the fall clean up easier since I don't have to rake leaves out of many evergreens. I also cut the cast iron plant and holly fern completely to the ground every late winter cause that makes them look alot fresher, IMO. I was just careful to buy the new plants in 1gal or less pots so I wouldn't have to dig very big holes.

Great Ideas.

Thank you all

I have some nearly year-round stuff like Katie ruellia and the houseplant version of wandering Jew underneath my trees, but impatiens are the prettiest. They do require water and they do look horrible during the hottest part of the summer, but you could put a few here and there with your more evergreen plants and not be setting yourself up for too much work.

Is there anything I can plant now??
Something hardy??

Plants that are not affected by light freezes can go in the ground now. Snapdragons, Pansies, Liriope are all things I've planted in the last few weeks.

I'd plant any perennials that you're wanting now. It'll give them a chance to get established before the summer heat gets here. If you have a big freeze predicted, just water them well and cover them with a blanket or sheet. I've done that the last couple of years and only lost 2 autumn sage plants last winter, which I blame on the 95 hours below freezing more than anything else!