*Rule of thumb*, (as I was told) is 4" at a time! If it's composted soil, it will settle down, over winter
so you may be able to add more the following year! I suspect, that you can gradually increase the depth
over the years but not too much or the roots can smother! If you do it gradually, it will allow the roots
to adjust and grow upwards if they need to, I hope!

Sounds like it will be a fabulous look!

janice

My experience is that oaks will tolerate quite a bit of additional soil (more than most trees), especially if the additional soil is sandy. I have a friend that added 2 ft. of sandy top soil in one area of his wooded lawn without losing any of his oaks. I would just leave it away from the tree trunk a few inches so the soil isn't right against the bark. Once the hostas are leafed out in the spring, you'll never see that the additional soil isn't right against the tree trunk.

Thanks HeyJ and DonR,

It sounds like with staying below 4" depth that the only place I have to worry about is the 4 foot round circle (which starts about 8' away from the tree. It is actually less than 8" deep, maybe more like 6" deep inside this as-yet unplanted circle.

I can easily remove soil inside the circle OR add sand to the soil blend I put there last weekend. I hadn't considered adding sand.... would this be a good idea with clay soil for hosta? At the time of planting, I will also be adding vermicompost.

If I wasn't moving a large, established hosta (either a S&S or Blue Mammoth) to this spot, I wouldn't need to get so much depth here, and I hate to dig down too much since simply removing the grass in the area revealed some of the surface roots of the oak. Once I dig up the hosta, if I find that the roots DON'T go down deeper than 4" I won't have a problem, but it seems unlikely that such a mature hosta (both are about 5' across) would have shallow roots.

Also, this spot is pretty much full shade, so would I be better off putting the Blue Mammoth there or the Sum and Substance? They are both the same age/size and have been in their present location at least 8 years.

I am sorry to be so hesitant about all this, and I appreciate SO MUCH all the instructions and encouragement I have received from everyone here... it is just that I raised these two big hosta from pretty small divisions to the size they are now and I keep reminding myself that they will really appreciate the better location/soil that I am moving them to!

BettyLu

Personally, I'd put the 'Blue Mammoth' in your shady spot. The blue ones seem to tolerate
shade well and the leaves will actually be somewhat larger, while multiplying will be less!

Adding sand to clay can be kinda tricky, as I've come to understand--it's a great recipe for 'concrete'!

I'd go more for some pea gravel (if you think you need it) mixed in and maybe some composted soil on top!
That really has helped my clay to be more till-able, in subsequent years! I don't add the gravel, myself--just the
composted stuff! Doing that this week in my own garden beds!

For the small amount you're adding, I don't think it matters if it's sand or clay.