Tips for removing soil from periwinkle roots?

A while ago I removed a large patch of periwinkle from my yard, starting at the edge and physically peeling the whole thing up like turf. While efficient, this has left a lot of dirt stuck in the roots of the plant. My local garden waste disposal site accepts periwinkle but does not accept sod like this. Any tips for removing this dirt without also leaving behind tons of periwinkle roots/rhizomes would be much appreciated.