I agree with purpleinopp; instead of using a bucket to protect a marginally hardy plant, I'd use an empty, overturned and weighted plastic plant pot with drainage holes. The holes will allow excess condensation and sun-produced heat to escape. Drastic changes in day/night temps and excess moisture build-up can be harmful in some cases.
I cut off the top third to one-half of my mums and toss the cuttings on top of what's left. They return year after year using this method.
