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 (perrysperennials.info).
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.