Double dig, get that poison as far down as you can, or loads top soil and reseed or turf, turfing on top of that may not work. big mistake, but weve all done something stupid, i had a slope i had put potatoes in, they were coming on brilliantly, path along the top, i used weed killer in watering can, just drizzled it, , , it rained at night, cpl weeks later, tattie schaws started yellowing all down slope not a tattie left, couldnt grow in it for ages, learned very quickly.