Answer: Perennials can be difficult to overwinter in containers because the roots are not as insulated as they would be when planted in the ground. Using large containers, heaping mulch around them or sinking them to the rim in the ground, or keeping them in a sheltered location (out of the wind in particular) or in a cold frame can all help. The plants must be watered as needed to keep the soil from going bone dry, but they must not be overwatered or there is a risk of rot or fungal infection. In many cases it is a "trial and error" process for finding what works best for you.
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