What Product Do I Use Around My Backyard For Fleas?

I have the dogs on Frontline Plus but live next to an empty lot that has cats roaming free.It is fenced in so I have no way to spray but it runs along side of my backyard fence and everytime my dogs love to lay out there,they come in with more fleas.My yard is all dry dirt and cement,do fleas like dirt and if so what spray or powder do I buy to put along the fence.I Hate Fleas!!!