A friend of mine does a lot of landscaping work for some Waikiki hotels.  One of them had several trees in planters along the street, and one day the manager told him to replace the river rocks with mulch.  He asked why, as they had spent a lot of money on the rocks, and was told that the pimps on that side of the street were throwing the rocks at the prostitutes on the other side of the street to reduce competition.  That particular situation may not be a consideration in your case.