The ferns do not need you to build up the soil. I too have oaks and the ferns love living under them. I am not sure if your question is for astetics or the ferns needs.

I wasn't planning to add more than 2" if I built up at all. Also, I wasn't planning on any real digging.

I guess part of my question should have been, "How well will the ferns do if the area is under an inch or 2 of water once or twice a year?"

Amelia, I have ferns growing underneath killer maples and the ferns are doing fine. I had to dig up clay and surface roots to be be able to plant there and still built it up a few inches, but also used a lot of organic matter and even amended the baby ferns with some perlite and vermiculite when I first planted them. (I was really paranoid about moisture loss.) With the amendments, my soil stays very moist and I then topdress with manure in the spring. This helps keep them moist too.

Before I did this, I asked around what to do under the killer maples. I was told either to dig out the clay or make holes in the clay with a post hole digger or build up the area over the roots. And amend the clay like crazy. (I was also told NOTHING will grow there and chop down the tree!) Other areas of the yard for rugged stuff like ivy, I just make holes in the clay and amend it a little. (Ivy will grow anywhere on anything including clay so compacted it should be a tennis court.)

I'm an organic gardener and also use other stuff including cottonseed meal. Cottonseed meal attracts earthworms like crazy. I call it "Earthworm Gold". The worms are very happy here and stick around to help break up the soil, aerate the roots, and deliver usable nutrients.

I thought all ferns liked moist acid rich soil and were heavy feeders. If you know of ferns that don't need this, please let me know. I'd love to try ferns without any work...if you find some that will grow in hard clay, let me know about those too!

Good luck!

Ferns like lots of water and should do fine as long as you get ones that will tolerate your zone in winter. Soil will also be an issue although your soil should be acid enough under oaks. Ferns tend to like looser more water permeable soil. If the soil under your tree is hard I'd definately dig in some mulch. I have California live oaks in my back yard and their are naturally occuring ferns. I have had lots of success planting just about everything under them.