Sounds like you're planning on burying the soil /root system of your pines. That's not a good idea. Also, think of the mess when the needles fall all over the rocks. Pine needles are good for the soil.

Do you think the ferns underneath is a no go all together? Or do I just need a new method?

What if I only made a small/limited size wall around the base? The roots would surely still be exposed as you go away from the tree since they are sticking up everywhere!

Thanks

In sandy soil you might have better luck with bracken fern. Ostrich fern is usually found in moist areas like swamps and low woods.