My grandmother has had a bed of canna lillies for 40 years. It is about 20 feet long, 5 feet wide up against the house. They have always been very hardy and beautiful. Last year, a handyman put down a layer of dark mulch in the canna bed to help with weeds. It looked nice, but this year, none are growing for the first time. Have they been choked out? what can we do to revive them? They have never really needed any care. They have even survived a few mowings over the years. Any suggestions?