Perry's Perennial Pages

Hardiness of Herbaceous Perennials
Dr. Leonard Perry, Extension Professor
University of Vermont

Whether or not a perennial survives over winter, whether in the ground in landscapes or in containers in a nursery or overwintering structure, is dependent on more than just its genetic hardiness expressed as a hardiness zone rating.  There are at least nine factors that influence plant hardiness that we have been studying over the last couple of decades at the University of Vermont.  An understanding of these will help you understand how perennials may survive winter when exposed to various conditions.
1. Acclimation is the first factor affecting plant hardiness, resulting from the environmental cues (mainly temperature) prior to the onset of and leading up to prolonged and deep winter cold.  Plants must be properly acclimated in order to survive winter cold.  Even if they are “hardy” genetically, they wont survive if not properly acclimated.
What cold, yet non-freezing, temperatures and for how long are needed for proper acclimation aren’t really known and will likely vary with perennial, but generally around 40 degrees is used as a target.  In our studies we’ve seen colder soil temperatures than this in the fall (but not freezing) may benefit winter survival of some less hardy species, but have no benefit for more hardy species. 
The amount of time soils are below freezing (28 degrees F or above), just prior to colder soil temperatures affects their survival and acclimation.  For the few species we’ve tested, over 2 days at this temperature prior to colder temperatures reduced regrowth and survival.  Whether one or two days didn’t seem to matter, just the longer times did.
Whether perennials are acclimated to cold outdoors in fall, or in a cool (non-freezing) structure didn’t seem to matter in our studies.  Cool, minimal cycling of temperatures up and down indoors seemed sufficient for hardening as did wider air temperature fluctuations outside, even below freezing.  The exception would be extremes, such as 10 to 20 degrees F air temperature, or below 28 degrees F for a prolonged period that would of course lower soil temperatures, particularly in containers. 
How quickly container or soil temperatures change depends on several factors.  Small containers may follow air temperatures daily, while large containers may take a couple days.  A row or two of guard pots around the edges will give several degrees protection in fall and winter, as will tight spacing of pots.  An extreme drop in temperature one night may not be a detrimental as a lesser drop in temperature but one that remains low for several days.  If in the ground, even a couple inches of organic mulch is sufficient in cold climates to trap ground heat and provide 5 to 10 degrees soil temperature protection.
2.  Date of freezing relates to acclimation as already mentioned.  Sedum, for instance, that may be hardy in midwinter to soil temperatures approaching 0 degrees F usually wont survive soil temperatures below 32 degrees F in early fall. In one of our studies, ‘Biokovo’ geranium had 50% loss in September when soils were 32 degrees, while in January 50% loss didn’t occur until soils reached -10 degrees—a level very unlikely in most nurseries unless totally exposed containers in the colder climates.   In northern climates some perennials may be hardened, at least partially under cold conditions, by November, with maximum hardiness by December and January. 
3. Duration of freezing, as you might expect, affects hardiness with the longer times at colder temperatures the most harmful.  This, as most factors, will of course depend on species.  In one of our studies, there was less regrowth from a couple of species (a dianthus and geranium) with one or 2 days at each low soil temperature, compared to just one-half to 2 hours at each of the same temperatures.  For these and several other species and cultivars, there weren’t any differences in regrowth after either one-half or two hours at each cold soil temperature. 
4. Deacclimation, or losing hardiness, relates to acclimation. Herbaceous perennials differ from woody plants in having only one stage of hardiness, compared to two for woody plants, and that being a weak hardiness that easily can be broken.  Our recent studies are focusing on just what temperatures, and for how long, and when during the winter are needed to deacclimate perennials.  Then we need to look at once a perennial is deacclimated, how much cold is needed to make it hardened again.  Warm temperatures in winter on sunny days are a common occurrence in overwintering structures, even in bare soil, so understanding these factors will help predict outcomes of such events.
In our first study (with a sedum and salvia),        warm air temperatures (55 to 65 degrees) for as little as 5 days in midwinter are enough to deacclimate plants that had been held just above freezing.  Bringing the plants back to 40 degrees at night didn’t help improve survival of these species once deacclimated.  Whether plants were held with soil temperatures of 34 or 40 degrees didn’t seem to matter, both survived well when subsequently exposed to sub-freezing temperatures.  The effects of deacclimation were more pronounced at colder subsequent temperatures, soil temperatures of 16 degrees and below damaging sedum and 12 degrees and below damaging the salvia.
5. Cycling soil temperatures, those going high and low in cycles, and their affect on acclimation and hardiness have been a focus of several of our studies and relate to the previous factor of deacclimation.  While in our recent studies we’ve begun looking at cycling temperatures above freezing, in previous studies we looked at temperatures cycling above then below freezing.  For hardy species we tested, cycling soil temperatures such as between 26 and 38 degrees F, with a couple hours at each before shifting, had no effect.  In fact, such cycling might result in more hardy plants that those just left at 38 degrees.  For less hardy species (such as zone 5 rated and above), only one such cycle may cause damage.  Such injury is more likely in late winter or early spring. 
More damaging is a wide cycling of media temperatures, such as we tested between 18 and 47 degrees.  At such extremes there were no differences between one, two or three cycles above and below freezing.  From all our studies on cycling, it appears the duration at each temperature is not as crucial as the temperatures the soil temperature reaches and fluctuates between.
6. Rate of thawing, once plants have been frozen, is a factor pots in a nursery or structure may be exposed to on sunny days and particularly in spring when uncovered.  With the five cultivars we studied (foamflower and geranium), a rapid thaw was best.  A slow thaw from low soil temperatures to 28 degrees, then holding there for various periods prior to returning to 40 degrees, resulted in injury and less regrowth subsequently. 
7. Plant age is a factor affecting hardiness, or perhaps more appropriately plant vigor.  In the five cultivars (among 3 genera) we studied, we compared recently potted plugs, established plants, and rootbound older plants.  Those that were more vigorous survived at lower temperatures than those that were rootbound, not growing actively, and somewhat stressed.  Amount of differences will vary with species depending on their natural growth habit.  As most growers appreciate, those species that really need to spread and that are restricted when held in pots too long will lose vigor and eventually die out even without extreme cold.
8.  Fertility is another factor that differs between perennials and woody plants, and that relates to the previous factor of plant vigor.  While recommendations for woody plants are not to fertilize late in the season, to avoid stimulating woody growth that wont harden properly, our studies over the years have corroborated what we’ve heard from some growers that fertility late in the season can actually help plants overwinter better.  In our studies, increased nitrogen levels and for periods well into the fall increased growth prior to, and after, freezing as you might expect.  Plus, there was no effect on survival and regrowth no matter the level of nitrogen or fertility duration.  Best regrowth has been from the use of controlled release fertilizers, either top-dressed or incorporated.
9.  Soil moisture, or watering, has been mentioned to us by several growers as a factor in their overwintering programs.  In our studies, running two regimes on several perennials during their growth, we saw no effect on overwintering.  In our studies we kept a wet group around 40% soil moisture content and one group about 10%, then watered all well prior to freezing.  What we didn’t examine was if plants were dry going into freezing, which may have an effect. Effects we did see were of course on growth, those with sufficient soil moisture having more growth.
All of our studies on hardiness factors so far indicate that for best overwintering survival, given winter protection of some form if plants are in containers and not the field where factors are less easily controlled:  give proper acclimation in fall, have vigorous well-fertilized plants going into winter, and limit extremes in temperatures both high and low.  For the latter, this means wide swings above and below freezing, and more than a few days at warm temperatures once plants are hardened.  Keep up with our future studies and results online ( 

This article first published in the OFA bulletin.  Appreciation is expressed to the following for their support in making these studies possible:  New England Grows, New England Greenhouse Conference, Perennial Plant Association, New Hampshire Horticulture Endowment, Vt. Nursery and Landscape Assn., Vt. Agr. Expmt. Station and affiliated graduate research assistants; and to many perennial growers with special thanks to Sunny Border, Creek Hill, North Creek, and Dunvegan nurseries.

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