Japanese Knotweed

I have an old yard and a neighbour pointed out a troublesome weed to me about a year ago. He called it Japanese Knotweed. I have tried to kill this with Roundup, by covering the area with stones after spraying with Roundup, by cutting all shoots away, and all to no avail.How can I kill this Japanese Knotweed plant. I think that it is now spreading and appearing around the yard?Tangman