Answer: Unfortunately this plant just is not winter hardy in your area. There is nothing you can do except possibly try to dig them up and bring them indoors. However, in my experience this usually does not work because it is such a shock to the plants. You would need to pot them, bring them into a cool but bright location such as a greenhouse kept at about 45 degrees and keep the soil just barely damp, never quite dried out. It is not easy to provide these conditions at home -- and the plants still have to contend with the disruption and root loss of being dug and potted. I'm really sorry I can't be more encouraging. If you like the look of these you might consider replacing them each spring or possibly growing an annual with a similar look such as purple millet. This would be a less expensive option.
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