Dictionary:

winterkill

  (wĭn'tər-kĭl')

To kill (plants, for example) by exposing to extremely cold winter weather.

To die from exposure to cold winter weather. Used especially of plants.

Death, as of plants, resulting from exposure to winter weather.

The dieback of twigs in winter because the wood was not sufficiently hardened to withstand severe weather or because the plant is not hardy enough for the site.