I have an allotment and can keep pretty much on top of my weeds apart from one area where all my rasberry canes are growing. The couch grass just keep on coming! I have dug some out but am worried about damaging my canes, it just keeps coming back, should I just cover it all with large amounts of manure, the canes look ok but seem to be slower growing this year.The canes are autumn bliss and about 5 years old. Thanks