The message
from Iain Douglas contains these words:

I've an oak tree about 25' tall growning at the side of my
garden. I want to raise the level of the garden about 18" on that side.
Will the oak survive if I cover the lower 18" of bark or do I need to
protect it ?

You shouldn't earth-up the bole of an oak tree or it will probably get
disease in. (Theoretically, you shouldn't earth-up any tree, but some
don't mind too much: willows especially.)

--
Rusty
Open the creaking gate to make a horrid.squeak, then lower the foobar.
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