Answer: Some perennials are more winter hardy than others and those that are considered half-hardy to your gardening region can sometimes be coaxed to survive winter weather if you protect the roots and crowns of the plants with several inches of mulch. I suspect that the perennials you lost over the winter just needed a layer of protective mulch. Wait until frost kills the tops of the plants then cut away the dead plant parts and pile up some dry leaves or some straw to insulate the roots. Rake away the mulching material in the early spring and your perennials should begin to grow for you.
You didn't mention whether or not you amended your planting bed prior to planting. If your soil does not drain well, the perennials can develop root rot. Before planting new perennials, dig in several inches of compost or aged manure. Compost will help loosen the soil, help it hold just the right amount of moisture and make it easy for the roots of your plants to become established.
Hope this helps!
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