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

• zones 3-4: poly/straw sandwich, 2-3 layers microfoam/fleece, 2 layer hoophouse heated or fleece
• zones 5-6: one layer microfoam/fleece, 2 layer hoophouse unheated, one layer hoophouse heated, heated coldframes
• zones 7-8: evergreen boughs/straw or similar, one layer remay or similar, coldframe

• Cover as late as possible before snow (mid-Nov north, mid-Dec south), although this is not as crucial as uncovering as soon as possible in spring (early April north, early March south)
• Cut back to 2-4" prior to covering; use mouse bait if potential/known problem; use fungicides sparingly

• Roots of more tender perennials injured in high 20s
• Soil: much residual heat, just 2" of snow/mulch can moderate especially in fall and spring cycling, by 5-10ºF
• USDA zone 4, 2" depth, 32ºF around Christmas, below first of year, mid to high 20s, seldom to low 20s then only for few days
• Pots: often follow air temperatures closely; if small or space around, within a day; if large or tight, several days; benefit from residual ground heat if not on fabric; even 2" snow or mulch moderates as with soil

• Will vary with many factors, but can compare relative hardiness under similar conditions (see lists)
• If a plant is not hardy at a CF temperature, it is likely not hardy in the field at that temperature
• If a plant is hardy at a CF temperature, it may not be hardy in the field at that temperature due to variables

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.

• Longer the duration at a low temperature, the more likely the harmful effect
• May see more effect from 2hrs, 24hrs, 48 hrs or more at low temperatures than higher; depends on species

• Often cycling between –3/+3ºC (26/38ºF) results in more hardiness than holding at constant -3ºC
• For hardy perennials, cycling may have no benefit.
• For less hardy, more than one cycle may be detrimental.
• Injury is likely with less hardy when cycling occurs late winter or early spring.
• More injury may occur at wide cycling such as –8/+8ºC (18/47ºF)
• Duration at each temperature up to 3 days is not as crucial as the temperature

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

• Outdoor exposure to low temperatures and cycling, prior to covering, is currently being studied
• Fall cool prior to controlled non-freezing may harden less hardy; more hardy may benefit from less cool and warmer non-freezing temperatures.
• Holding below freezing (-2ºC, 28ºF) for more than 2 days prior to lower temperatures, may be harmful

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)\

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

• Increased nitrogen (N) levels at least to 300ppm applied weekly from 20-10-20, and until mid Oct., result in increased growth before and after freezing
• Slow release fertilizer may result in increased growth and hardiness, with studies underway
• Increased fertility levels and liquid fertilization into the fall, in general has little or no effect on hardiness, and in fact may make plants more hardy (unpublished studies, Quebec)
• This differs from woody plants due to the quite different nature of hardiness of perennials

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