I think the plant height wouldn't include the flower stem, and being under eaves it could have been drawn up a bit more. Being next to the house will give it some cold protection too, heat is absorbed by walls and radiated back. Zones can be flexible according to micro climates, and zones reflect minimum temperatures rather than actual climatic conditions. The food you give it could be encouraging all the extra flowering stems.
I wouldn't cut the main stem, it might be a food source for the young growth. I think what comes out the ground should go back in the form of dieback, that could also apply to the plant dying back and providing food for the young growths. Watch it and see, if the main stem eventually starts to die back then that would be the time to give it the chop. Otherwise, while it's looking good leave it and enjoy, you are lucky to be able to grow such a plant.
