Root Hardy

• Soil and water, present as snow, are both denser than air and act as insulators. Sudden drops in air temperature may kill the tops of plants but the soil around roots falls gradually, protecting roots. Bulbs of lilies, tubers of peonies and biennial hollyhock roots survive underground.
  
  

Crown Hardy

• Other plants, like daylilies, hostas and chrysanthemums, are crown hardy. Deciduous tops die back but their crowns sit at the soil line or just below. Crown hardy plants often survive with snow cover or leaf mulches that protect them against drying winds, sudden cold or warmth from the sun.
  
  

Woody Plants

• Woody perennial plants grow thick skins of dermal tissue called bark. In addition to woody trunks and stems, evergreens grow thick, fleshy leaves. Although severe cold may kill some branches, the bark of trees and shrubs insulates them like a jacket against the cold.