I am not sure--you say a "solid rock floor"? Does that mean that where you want to plant them there is a shelf of solid bedrock and no soil? Or is the rock under where you want these trees more like a very rocky soil?

If you will be planting where there is now no soil, and the surface is solid unbroken rock, I would think you would need something like three feet of good soil. If you are in an area where there is not a big risk of prolonged droughts, maybe 28 or 30 inches would be OK.

If what you have underneath is just very, very rocky soil or a underlayment of broken rock, maybe 24 inches would be enough.

--Spruce

This are is a solid flat slab of rock. I have put top soil about 24 in. deep. I will go deeper if necessary? It's not in full sun, so that might help some I guess.
Thanks

I have to admit that I don't know if there is any strict rule about the depth of soil you need. I am guessing. But this quess is based on some experience. At the university where I used to teach, they planted trees on the top of a parking garage--on a poured cement surface. Where the trees were planted the depth was about three feet, tapering down to about 18" to 2' between the trees. the trees were planted over support pillars. But these trees had a sprinkling system.

In my timberland management experience I have had trees growing on soil with bedrock three feet underneath, and they did fine. When I see less than that, the trees usually do not do so well, but my observations are informal non-scientific ones.

If your soil is a kind that holds moisture well, is mulched well, and is watered during droughts, you may be fine with the two feet. Honestly, I would be a little nervous and would probably want to pay to have more soil brought in.

--Spruce