Should outdoor chrysanthemums be cut down after flowering in the fall? If so, to what height. My plant is still flowering and is about 3 feet high. Also, should the plant be hilled up for the winter. It was originally a houseplant.
It doesn’t matter whether or not you prune your Chrysanthemums now, or in the spring. If you like a tidy garden, you may prune them in late fall to a height of a few inches which is enough to trap the snow and provide some winter protection. If you like to leave your perennials for the enjoyment of the birds and that interesting winter effect, wait until spring and prune right to the ground. Or, wait a bit and prune to where you see green starting.
You said the plant was originally a houseplant. There are some Chrysanthemums that are not perennial and not hardy in Toronto. It’s sometimes hard to tell. Also, was your plant in the ground all season, or did you just put it in the garden for the fall? The only reason I query, is even if they are hardy, if they were kept in a pot, then put into the garden after flowering, they may not have time to settle in before winter comes — some say they need at least six weeks before a killing frost. I always plant my potted hardy plants in the garden come October, but I must admit, they don’t often survive. However, it’s always worth trying. You could try hilling, or covering with lots of leaves, but I don’t think it will make too much of a difference. The issue is whether or not the roots have become established.