If you can use sand. Mulch will make the slate unstable to walk on when it rots. Tamp the sand down and if you like mix in some mortar, dry, then set the slate and then water. If you don't interefere or cut the roots there should be no problem. It would be nice if you could run a border around the patio to keep the slate in place. Some steel edging perhaps.

Here is a link that might be useful: Propagating Perennials

Thanks, Bullthistle. I'm concerned that the slate slabs will suffocate the feeder roots, which is why I chose mulch. If I used mortar and water, wouldn't that make cement? Is it OK to cement my tree roots?

Thanks, Florrie

How close to the surface are the roots? If possible 4-6 inches of sand would provide a good base for the slate. Water will get through the joints so you should be okay.

I'm in an oak, hickory, pine "wilderness" in New York and I grow tons of plants.