You are a little too far north for plumbagos to be perennials in most years. If they are not in a protected location and mulched well I don't think they will be dependable survivors. They will be killed by temps in the mid to low teens, maybe even a little higher. I would consider container culture or sink containers in the ground so they can be dug easily for wintering in a protected spot.
Hope this helps with your gardening question!
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