How large is the circle, and how does it compare to the edge of the tree canopy?

I'll have to measure it if you need precise (I'm not home right now). I'd say that the circle is about 8' and the canopy isn't much more than 9 or 10'.

No, that's accurate enough. So you'd be adding from 0-21" over 8': that's awfully steep. And it also nearly covers the area under the canopy, so it would cover a major portion of the roots. I'm not a pro, but I believe that would likely kill your tree.

If one of the tree experts doesn't answer here, try the Trees forum. I'm sure they'll tell you to change your plans. Too much depth of soil added atop too large a percentage of the oak's roots.

Do you want to level or terrace the yard at all, or do you want to leave it all sloped? And is your intention to sell or stay there? My concern with leaving it sloped would be that a future owner might do the terracing, and if that's a possibility, you might as well do it now to the extent that it can be done because you will do it with care for the tree.

What you could maybe do is think ahead to when the tree is bigger and plan for a level area under it that relates to the size of the canopy in, say, 5 years or beyond. I'm not an expert on trees in general or live oaks in particular, but my guess is that if you make your edging and build it up as high as you eventually want the bed to be, and let the area gradually fill in as leaves fall and dirt goes downhill, that the tree would survive. The issue is generally that the roots need continued access to air, and the rootlets will work their way up into new soil as it builds up. Making the bed quite big now will ensure you do not cut roots, and the roots will grow around/under the wall as they progress outward.

As for material, I think what is available to you at local vendors would be the limiting factor.

KarinL

I don't think it would be practical to terrace the whole area. I have a corner lot. This part is the side yard, facing the street and it eventually mostly flattens out in the front yard. I have no plans to sell so I could live here quite a while. Obviously, my main concern is the health of the tree. Doing nothing is still an option. Thanks for the input.

You could put a low (10") rock retaining wall on the low side to retain the mulch. Thick mulch on the down hill side would not suffocate the roots. You should add an apron between the wall and the lawn to keep the grass out.

Yes, I was also going to mention specifically that things might work out better if you didn't think in terms of a circle, and also not in terms of just a patch for the tree, but of the contours of the yard as a whole. I think Steve's advice is an excellent direction.

KarinL

Have you considered changing your mulch material? Our live oak is probably 60-70 years old. I've mulched two-thirds of the yard with pine straw, basically allowing the tree to live as it would naturally. Live oak root systems are wide-spreading and shallow.

Pine straw is great stuff!

With such a steep slope as you have, use more than one "wall." Build multiple terraces for the area, following the contours of the land. If you worry that the walls are too conspicuous, choose stone or blocks that match the color of the earth or the color the mulch will fade to. Or choose a cascading plant suitable for your climate, and plant a few to hang over the walls here and there.

Check out the link below for some interesting tree circle information.

Here is a link that might be useful: The Astonishing Truth Behind Tree Circles