Robert

Hi, Here are a couple of things I don't think were mentioned about rocks for "root over" planting.
First of all, make sure it's a "hard" rock. Someone mentioned Limestone, but limestone will erode over time. Sandstone also will erode, and actually "melt"......so go for something truly hard.
Also, choose a rock which is big enough. I've seen some nice trees over a rock where the roots have almost completely covered the rock. When the job was done the proportion of the rock to tree was fine, but in time the tree grew but the rock didn't. This is especially true if you're doing tridents...which grow VERY fast. You should also choose a rock of a color which will contrast with the roots...that is, if the roots are light colored, choose a dark colored rock. It'll be more noticeable. I've seen tridents which have tan roots on a tan rock, and it just didn't look very effective.
Hope this helps.

Robert......in Sta.Cruz

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Robert.........in Sta.Cruz