Question
I HAVE A HUGE OAK TREE IN THE FRONT YARD WHICH WON'T ALLOW ANYTHING TO GROW UNDER IT.  I WANT TO PUT A SMALL RETAINING WALL AROUND IT - 3 BLOCKS HIGH - AND PLANT BUSHES.  PLEASE DON'T SUGGEST AZALEAS!!!!!  THE AREA GETS 1 TO 2 HOURS OF MORNING SUN AND THE SAME IN THE PM. AGAIN, I AM IN ZONE 9 - ST. PETERSBURG, FLORIDA.  

Answer
Nope - I won't suggest Azaleas.  I would, however, suggest you not fill the base of the oak with topsoil -- or concrete, or gravel, or just about anything. It would be like having to go through a hot, Florida July day wearing a ski mask soaked in butter.  I wouldn抰 want to do it.  Dumping dirt on an existing tree is not a good idea; especially an Oak.  IF you have you're heart set on raising the grade around this majestic tree, be prepared that it may not like it's new living arrangements.

Yes, you've probably seen other installations with the bases of the trees planted this way.  They were either lucky, or they installed it that way to start with. I'd hate to get an e-mail asking what goes well in full-sun, where an Oak used to be.

It would be better to just spot or fill in a few heavy-shade tolerant shrubs & groundcovers - for zone 9, try Maidenhair ferns, Cinnamon Ferns, Caladium, Coral Bells, Mondo Grass, Hostas, Ivy, or Liriope.  Also, Virginia Creeper does well here, in our zone 7, but I'm not sure about Florida.  4 hours of total direct sun won't kill any of these suggested plants.

Since I don't know what specific type of oak it is, I would imagine there is quite a bit of leaf litter associated with it, too.  So, maybe something that can take raking (Liriope, Mondo Grass).  

Good gardening & good luck!  -Marc