University of Vermont Extension System
Department of Plant and Soil Science                                                                                                        COH31

Herbaceous Perennials—Container Overwintering and Hardiness
Dr. Leonard Perry, Extension Professor

Types of coverings


Soil/pot temperatures

Cultivars and species, controlled freezing (CF) studies

Date of freezing

Plants are not hardened by early fall, some may be hardened sufficiently by Nov. (depends on fall acclimation variables), most maximum hardiness by Dec. or Jan.; plant may die at 32ºF in Sept but survive 7ºF or less Jan.

Freezing duration

Cycling temperatures

Acclimation (fall before covered or moved inside, winter prior to freezing)

Rate of thawing

In general the more rapidly plants are brought from colder below freezing temperatures to above freezing after a severe freezing episode, the less damage and better regrowth; keeping plants at colder temperatures for longer seems to cause more injury than the rapid increase in temperature; depending on pot size, above freezing temperature, and time below freezing, pots may take 3 days to thaw at 4ºC (40ºF)\

Established (older) vs new plants

Often new, more vigorous plants (divisions, plugs) are more hardy than rootbound older plants, but this seems to vary with species, and studies continue

Fertility effects

More information on these studies, and updates, can be found at Perry’s Perennial Pages ( under the Research and Professional Industry sections. More on this topic can be found described in the book Herbaceous Perennial Production, available from NRAES (www.

Return to Perry's Perennial Pages

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the United States Department of Agriculture. Lawrence Forcier, Director, UVM Extension System, Burlington, Vermont. University of Vermont Extension System and U.S. Department of Agriculture cooperating, offer education and employment to everyone, without regard to race, color, national origin, sex, religion, age, disability, political beliefs, and marital or familial status.

COH 31, updated 7/04