I don't know where you are in the States, so can't work out your USDA Zone, but in the UK, we just wait for the foliage to die back on its own as winter approaches. If, though, its been six weeks since yours flowered, you can cut the foliage down if you want to. Perhaps feeding them with a general fertiliser will make a difference to flowering next year, but they don't flower as much nor for as long if they don't get enough water when in growth.