Mother Earth News had an article many years ago (20+) where they used
tires to terrace a hillside. I ran across your question in searching for
more information about how this was done. I would think that the tires
would be held together (bolts possibly) and I know that they were
staggered such that they had a “reveal” that would hold vegetation. The
next row was offset (side to side) as well as back into the hill and dirt
was packed into the tire/open spaces that were created by this offset.
Then plants were planted in the open spaces. Looked much like a pattern
of window boxes or planters going up the hill. The tires would hold heat
in the spring, which may be helpful but would do the same in the summer
and may cause problems with tender plants. Don’t know how your gophers
would react to this method. I believe that there are plants that they do
not like and will avoid. As one person said, check with your county
extension agent. Also seeing that you live in CA it may be a problem
dealing with your neighbors when you mention tires and landscaping. I am
still looking for more information on terracing with tires and will
forward any good information I find.

Alan