Of course, the farther you live from the equator, the longer your summer days will be. On June 21st, the days in Homer, Alaska are almost 19 hours
long, while in Houston Texas there are only 14 hours of daylight. By October 21st, the days have dropped to just 9 ¾ hrs in Homer, while they are
still 11 ¼ hrs in Houston. Plants native to specific latitudes have tuned their photoperiodic calendars to the place over the eons through natural
selection: individuals that miscalculate the arrival of winter are quickly dispatched in favor of those that respond to the shortening days
appropriately. We used to cultivate a rare species in the aster family called Alabama warbonnet (Jamesianthus alabamensis). It is a fall-blooming
species, and flowering is triggered once the days shorten to a genetically determined length (this type if plant is called a short day bloomer). By
August 5th, the days in Mobile are only 13 ½ hours long, which begins the process of flower bud formation at the tips of the tall stems and insures
the plant will have plenty of time to grow and bloom before the weather turns cold in December. Farther north in Massachusetts, the day is still
14 ¼ hours long and doesn't shorten to 13 ¼ hours until the end of the month. Consequently, when we have tried to grow Alabama warbonnet, it
does not even begin to flower until late October and is more often than not cut down by frost before the first few blooms open. It is not just
flowering that is effected by this confused internal clock; the dormancy process is also initiated by the shortening days well before the first frosts
sparkle on the leaves.
You often hear that the only things we can count on are
death and taxes, but as fall approaches and the nights
are dipping down to the lower half of the brass
frog-shaped thermometer I tacked to the porch post, I
can add winter to the list. While folks in the tropics
swat mosquitoes and sip margaritas on the screened
porch, we northerners are stacking wood, putting on
storm windows, and changing the oil in the old snow
blower. The perennial plants around me also stocking up
and preparing for inevitable cold. Many get their first
hint of winter's approach in midsummer, when the days
start to shorten. As we shoot fireworks and roast
weenies on the hibachi, the nights start to increase in
length a minute or two per day. In a world of
uncertainty, you can also count on this, and seeing as
plants are already highly light sensitive things, it is not
surprising that the photoperiod or length of the day is
an excellent calendar.
Plant Hardiness - from draft of
Understanding Perennials
Cold tolerance takes a lot of work to maintain, and plants
- even extremely winter-hardy species - loose all cold
tolerance during the warm days of summer and then only
slowly build it back up again in fall and winter. There is
no sense wasting precious resources maintaining cold
tolerance when it is not necessary, and even when it is
cold, the idea is to stay just a few steps ahead of the chill.
Perennial plants go through three stages of winter
dormancy: acclimation, mid-winter hardiness, and
deacclimation. During the summer, even very
winter-hardy species that can withstand - 50o F in
January may be killed by just a touch of frost. Decreasing
daylength and then decreasing temperatures (especially
night temperature) spur the production of ABA and
ethylene to ready the perennial for winter. Stem growth
slows and carbohydrates are redirected to roots as leaves
begin to wither. Mature tissues are more resistant to
frosts than young, tender ones, so this is a good first step.
Excessive fertilization and watering in late summer raises
auxin levels that counteract the effects of the dormancy
hormones and delay the onset of early acclimation,
making your perennials more vulnerable to early frosts as
well as more extreme cold further on.
During early acclimation, the plants can weather light frosts, and these
light frosts as well as temperatures below 40o F trigger the further cold
tolerance to develop. In this later stage of acclimation, temperatures
just above freezing, rather than daylength are the important trigger.
Ice is of course the big danger to plant cells. Since water expands when
it freezes, ice crystals can blow cells apart. Cold-mediated increases in
ABA induce plant cells to lose some water and increase concentrations
of sugars and other cryoprotectant proteins and alcohols. Like
antifreeze in your car, these lower the freezing point of water to 20 o F
or even 10 o F. This corresponds to USDA hardiness zones 8 and 7, and
there are many perennials that are hardy to these temperatures. After
completing this acclimation process, they are able to withstand hard
freezes without harm. If we experience a period of cold in fall followed
by 2-4 weeks of unseasonably warm temperatures, plants can actually
lose some of their cold tolerance. This happened in much of the
Northeast during the winter of 2006-2007. We had record warmth in
December and January. I remember going on a family walk in early
January with the air temperature approaching 70 degrees. In late
January, the temperature abruptly fell and February was very cold with
disastrous effects on many typically hardy perennials.

For perennials to withstand colder temperatures without the benefit of a thick insulating layer of snow, they have to control the formation of
ice crystals in their tissues. Very cold-hardy plants (those hardy to USDA zone 3 or - 40 o F) are able to prevent ice crystal formation in their
cells. You have probably heard of planes seeding clouds to produce rain. One technique is to spray silver iodide crystals into clouds. The
crystals act as nuclei or nucleation points on which ice crystals begin to grow, and it is these ice crystals that fall as rain once they melt at lower
levels of the atmosphere. Ice crystals in cells need nucleation points to begin forming, too, so very cold hardy plants prevent them forming by
removing potential nuclei from the cellular and intracellular fluids. Such fluids become in effect supercooled and can withstand temperatures
as low as - 40 o F without damage. Below this temperature, ice crystals will form without nucleation points, and this explain why USDA zone 3 is
the lower winter hardiness limit for even many very cold-hardy species. The list of perennials that can survive below - 40 o F without snow
cover is relatively small, but a few high alpine and tundra species can weather temperatures around - 60 o F (USDA zone 1) with a combination
of supercooling and cellular dehydration that removes much of the water from cells that the ice needs to grow. Their cellular membranes are
also fairly flexible and resilient so they can resist thrust of ice crystals. So, winter hardiness is a combination of first the proper recognition of
daylength cues and response to cool temperatures to slow and stop growth and build up food stores. Lower water content decrease and
cryoprotectants increase to shield the plants from the sub-freezing temperatures that follow, and very cold-hardy species also prevent ice
crystal nucleation and growth. A Crinum erubescens (swamp lily) from South America can withstand temperatures as low as about 10 o F by
lowering the water content and increasing antifreeze levels, but it lacks the genes to allow supercooling. You can grow some crinum lilies in
the warmer parts of USDA zone 6 where the ground does not freeze too deeply, but unless the genus undergoes so major evolution to develop
the ability to prevent ice nucleation, it is very unlikely that there will ever be a crinum that will be hardy to zone 3. However, if you are a
breeder searching for the cold-hardiest crinum, you would naturally search for plants growing high in the mountains at the limit of the species
cold limits. Natural selection will have eliminated all but the most cold-hardy plants that produce the highest levels of antifreeze or a
particularly effective combination of chemicals. Since as I mentioned earlier, cold tolerance requires a lot of resources, plants from a particular
region are only as winter hardy as they have to be, so when you are looking for cold-hardy cultivars, look for those native to the coldest part of
the range. Likewise, if you try a particular cultivar and it fails to overwinter even though the species is purportedly hardy in your zone, it may
have originated from a warmer region. Before giving up on the species, try another selection from a more northerly place or higher elevation.
One of my favorite grasses is Hair-awn Muhly (Muhlenbergia capillaris), which has become justifiably popular as a landscape specimen in its
native Southeast. I tried unsuccessfully to get plants from a North Carolina source to overwinter for us in Massachusetts as they lack the full
cold-hardiness necessary to survive zone 5. It was not until I obtained some seed from a rare Connecticut population that I was able to grow this
plant as a perennial. Wire-awn muhly is gradually evolving increased winter-hardiness as it moves north, though -15 o F seems to be about its
current limit. Whether it will be able to evolve even greater cold tolerance may be a moot point, since global warming is extending its comfort
zone father north by the decade.
Potentilla robbinsiana - an extremely cold-tolerant alpine from the
summit of Mt Washington in NH
Carex laxiculus in late fall
Muhlenbergia cappilaris in fall
The heat from the earth's molten core radiates up through the
bedrock and actually keeps the temperature of the soil substantially
higher than the air temperature, especially if it is covered by an
insulating mulch or blanket of snow. As a consequence, roots and
other below-ground structures are not nearly as cold-hardy as
evergreen leaves, stems and buds. Since most perennials retreat
belowground for the winter, root-hardiness rather than stem
hardiness is the critical factor determining their ability to survive
cold. The ubiquitous groundcover Japanese pachysandra
(Pachysandra terminalis) is rated as hardy to - 30 o F, but its roots
will die if exposed to merely 15o F. For nursery growers, root
hardiness is a major concern, as the roots are far more exposed to
cold in a container than they are in the ground. Growers use
insulating blankets, minimally heated greenhouses and other tools to
keep pot temperatures from falling below 15 or 20o F. If you garden
in the ground, you can still use root temperatures to your advantage
when trying to overwinter marginally hardy species. We all know
that thick mulches can help retain ground heat and protect
sensitive roots and bulbs. Folks with reliable snow cover of more
than six inches all winter can often overwinter plants rated a zone
or two warmer they should because snow is an excellent insulator.
Planting tender species along the foundation of your house where heat leakage from inside keeps the soil just a bit warmer can make all the
difference. I once worked at a state research facility that heated all its buildings from one central boiler that distributed steam via
underground pipes as needed. The horticulturist was able to overwinter even subtropical plants like bananas in zone 6 when he planted them
atop the buried pipes. This didn't say much about the energy efficiency of this centralized system, but it did raise eyebrows on garden tours.
Since cell dehydration is an important part of the acclimation process, waterlogged soils can lead to winterkill. This is especially true for some
taprooted or fleshy rooted perennials as well as bulbs and species native to desert locations. Excessive water in the soil leads to higher than
optimal water content of root and stem cells and subsequent freeze damage. Rock gardeners know the value of a very gritty, free-draining soil
when it comes to overwintering water-sensitive species. On the other hand, persistent drought in the late summer and early fall can adversely
effect the hardiness of other perennials by interfering with carbohydrate buildup in the roots.
Evergreen plants face additional challenges in the winter. When the leaves of Japanese pachysandra or any other evergreen perennial are
exposed to the sun when the ground is still frozen, the leaves heat up and lose water through transpiration which the frozen roots re unable to
replace. The leaves and stems are often winter burned (effectively dried out to the point of damage or death). In my area, it is usually not until
late February or March, when the sun has regained some of its strength but the ground remains frozen that winter burn occurs. The best way
to avoid winter burn if you live in an area that is prone to it is to plant evergreen perennials and groundcovers where they will be shaded
from the southern and western sun in late winter. Since the sun is still fairly low on the horizon at this time of year, a hedge of evergreen trees
or a nearby building will often cast shade on an area in February that will be sunny by the time the growing season begins and the sun is
higher in the sky in spring. A deep layer of mulch applied in the autumn will keep the ground from freezing as deeply. Additionally, a light
mulch of hay or a blanket of a spun-bonded row cover like Remay can also be effective if applied over the plants temporarily for shading
during the critical 2-6 weeks in which winter burn occurs. Just be sure to remove the shading as soon as the ground thaws. Some folks swear
by anti-desiccants or anti-transpirants (oil or wax sprays that coat the leaves and lessen transpiration related water loss), though their efficacy
remains in question. Many evergreen perennials can survive one bad year in three, but the loss of their leaves (and the carbohydrate and
nutrient reserves they contain) will weaken the plant and may effect flowering and long term survival.
While the acclimation process and maximum winter lows are important pieces of the winter hardiness puzzle, deacclimation in spring is just
as critical to understand. With the onset of cold weather in late fall, high levels of ABA will prevent fully acclimatized, dormant perennials
from resuming growth even if the weather warms. Once the plant has been exposed to a few weeks of sub-freezing temperatures, a spell of
unseasonably mild weather will not break dormancy. Only after a genetically predetermined length of cold (anywhere from 1-5 months below
45o F) do levels of ABA drop sufficiently to allow growth to resume. Christmas rose (Helleborus niger) or Snowdrops (Galanthus spp) need
very little chilling before ABA levels drop and they bloom at the slightest hint of mild weather in midwinter. If cold returns, the plants will
slow or stop growing again and resume when temperatures stay above freezing. There are very few plants that risk flowering during the
winter, and the ones that do are usually from mild maritime or Mediterranean climates with predictably mild winters (this is the case with
Galanthus). Christmas rose, on the other hand, is an alpine species from the mountains of Europe. In its native haunts, it is reliably covered in
deep snows all winter that do not melt until spring, so it has little need for additional hormonally induced dormancy.
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