[A note to the reader. This is not a scholarly treatment of all of the peer reviewed material on this topic.  There are no footnotes or listed sources.  This is a product of my more than 35 years of horticultural field experience and gardening along with what I’ve gleaned from reading several technical peer reviewed articles on the subject.  Such material is difficult to read and can be off putting and intimidating to even the educated layperson.  This posting is my attempt at interpreting the research and reviews that I read in a way I think is understandable without overwhelming the reader with bi0-chemistry and the technical esoterica scientists must consider in their pursuit of understanding.  Any faults are mine.]

I’ve been thinking about plants and their response to cold having watched the deciduous trees drop their leaves, dug tender plants out of the garden and moved pots around to where they would be adequately protected from freezing temperatures.  We all know what happens to water when it freezes, going from a liquid state to a solid one, its molecules forming crystalline structures, expanded and rigid, responsible for burst water pipes and snowflakes.  Water possesses some amazing qualities, as a solid, kind of counter-intuitively, it becomes less dense floating rather than sinking even taking on some insulative qualities and stopping the convective flow of heat that is normal in liquid water.  At the instant of freezing water releases a small but measurable amount of heat. What happens inside plants when temperatures drop below freezing? How does the plant keep from bursting its own cell walls like the water in pipes within an unheated crawl space or wall cavity of a building?  You’ve seen what happens to plants like Coleus and the sodden black heaps they become upon thawing out.

Acclimation

Obviously plants have worked out some strategies to prevent or limit damage, after all, many temperate species have evolved to prosper on alpine mountain sites, the tundra and high desert situations where plants must not only contend with minimal water but often temperatures that can vary widely in just a few hours from well below freezing to cambium tempting highs when the low angle sun warms trunks and stems.  On the other hand are those true tropical plants that begin to call it quits when temperatures near 40, well above freezing. Plants, as organisms, unlike animals, are unable to get up and move to another preferable site, or to change their immediate environments to ameliorate their growing conditions, (except over time as a community of organisms like a mixed forest.)  Plants respond to their photoperiod, to the intensity of the light that they receive, available moisture, the availability of nutrients or lack there of, to attack by insect pests, storm damage and pruning…the same species and clones will ‘respond’ individually, though within a genetically defined range to the conditions on different sites…and they certainly respond to temperature.  Plants even respond to wind and gravity.  Under windy conditions woody plants grow vertically more slowly building up girth and strengthening cell walls to better withstand it.  Climbing clasping vines grow upward and are able to sense the presence of supporting structures nearby that they can attach to.  Ask one of the researchers who have worked to learn to grow plants in space without gravity if it makes any difference….  Plants are ‘adaptable’.  They ‘grow’ in response to all of this and the changing of seasons.  Growth and adaptation are incredibly intricate and complex biochemical processes. When enduring the many ‘stressors’ such as these listed above, a plant’s genes will ‘express’ themselves differently across the range of options they allow.   A plant does not simply grow or die.

In nature, or in your garden, all of these variables change along a continuum.  Nature is not a lab where all other variables are ‘controlled’ so that we can measure a plant’s response to cold, or photoperiod or….but that is how science is done in their quest for understanding.  Change too  many variables at one time and it is difficult to tell what ‘causes’ a given response or chain of responses.  But that is what we have.  Scientists, very often crop scientists, trying to figure out how we can improve production under deteriorating condition, or on marginal land, as population and development continue pushing production on to less desirable land, are who get the funding.  Science is expensive.  So, scientists have been studying the effects of cold on plant growth, the research I found was primarily concerned with cereal grains, fruits and brassicas.  It is not always safe to extrapolate too broadly, but science can move our understanding in a positive direction.  The changes within plants when exposed to stressors is very particular to the plant, but there are processes that more generally hold over many different genera.  Scientists have begun to measure more of these changes that include both physiological and metabolic effects.  We don’t have to be scientists for this to be of value to us.  A broad understanding of these kinds of responses will help us as gardeners improve our practice while at the same time deepen our wonder at the miracle of life and growth.

All plants have optimum ranges of conditions within which they best perform.  (I discussed in earlier postings how heat, above their optimum range can effect their internal processes and how over time, given their particular genetics, this has worked to alter both their internal/metabolic process and changed their morphology). In temperate zones, such as ours, as we move into the fall, average and daily high and low  temperatures begin to drop, while the photoperiod and light intensity decreases. Our annual summer ‘drought’ begins to yield to rain as we move through our mediterranean cycle.  Decreasing temperatures and photoperiod ‘signal’ to the plant to begin preparation for coming winter.   Falling temperatures, when they move below the optimum range for a given plant, begin to slow growth by reducing the efficiency of chloroplasts to utilize the solar energy they receive, in effect wasting it, and, when cool enough, between 0C-15C, or 32F-59F, can even begin to change some specie’s morphology, and the ‘expression’ of its genetics, altering the proteins, carbohydrates and other metabolites a plant produces.

This ‘chilling’ range of temperatures probably varies between species and specific populations.  There has not been enough research to pin this down to species beyond the few being examined now.  It is safe to say that as conditions cool a plant is  altered.  It maybe imperceptible to the unaided eye but it is changed.  Cooling temperatures prepare a plant for the coming cold, acclimating it to cold, making it more able to withstand the more severe freezing temperatures to follow in winter.

While the articles I read did not address this specifically, it would be logical to assume that any plant we grow ‘here’, in Portland, that evolved in a region with a warmer/hotter growing season, with fewer ‘chilling nights’ will be different than one grown under its natural conditions.  The proteins and various metabolites it would have produced through ‘summer’, the ‘kind’ of growth it would have produced, will be qualitatively different than here. The same plant grown here, undergoing the pattern of chilling here in the fall, is not the plant that would be heading into winter where it came from.  Having said that, the chilling and other factors I’ve mentioned are working to prepare a plant for the freezes to come, given its state/condition coming into the Fall season.

Similarly, non-native plants from other regions with typically wetter summer growing seasons than ours will be different coming into the Fall here, if they are drought stressed by our dry summer and receiving to little supplemental water.  Our native plants here often initiate root growth in the fall with the coming of the rains.  They are genetically adapted to this cycle.  I’m going to go out on a limb and suggest that many exotic non-native summer wet plants, especially those from cooler temperate climates, may get ‘confused’ by this unfamiliar mix of signals, responding with more top growth to the now available soil moisture.  Such new Fall growth will be ‘soft’ going on into winter, with inadequate time to harden and acclimate.

Scientists have found that the tested plants respond quickly to cooling temperatures, changing the metabolites they are producing.  Proteins are the building blocks of organisms so, in a sense cooling temperatures change the materials with which the plants have available to grow or regenerate tissues.  A few hours of chilling here and there won’t make a lot of difference in a given plants ability to withstand freezing weather.  It needs to be accumulative over time.  How much is difficult to say.  We should also keep in mind that freezing and below temperatures do not have the same effect as do chilling temperatures.  Freezing, ‘bypasses’, this intermediate stage.  This process does not occur as if a switch were flipped.  It is not simply off or on.  There are many, hundreds, of proteins, carbohydrates and other metabolites that change in response to chilling temperatures.  Not all have even been identified let alone had their ‘function’ determined within all of this.  To further complicate this, some of the metabolites that have been identified and understood serve multiple purposes.  For example they may also be produced by other stressors like drought.  Science tends to be ‘reductive’ breaking processes down into individual discrete parts, but many organic processes are highly complex so that while a particular metabolite might be found to be associated with chilling, there are many more that are likely necessary, but incomplete by themselves.  Metabolites, in short, likely work in concert with each other within a given plant.  This ‘matrix’ of material the plant has to utilize, is then ‘directed’ by its genes in patterns that are different than they are under optimal growing conditions.  Tissues begin to change.

Survival Strategies

Again, the science today is incomplete, but among the physiological changes scientists have been examining are internal to leaves themselves.  These changes may include the leaf epidermis, its outer ‘skin’ and thickening of the cell walls, resulting in a more rigid, tougher, structure, than is ‘normal’.  Such changes help protect the cells from rupture and consequent collapse.  Such structural changes take time to form and don’t result after just a few hours of chilling.

Scientists are researching precisely how plants sense these falling temperatures and then ‘transmit’ instructions to their cells directing change.  There is an interest in figuring out how to manipulate cells of plants, that are not cold hardy, so that crops can be grown under conditions impossible before.  I think that it should be remembered that these ‘new’ plants will be qualitatively different than their precursors with ramifications for their nutritive quality maybe even in terms of allergens as well.  They will be foods that we’ve never had before.

Many evergreens have evolved their leaf structures, both broad-leaved and conifers, to be tolerant of significant cold, heat and drought. Evergreen leaves allow plants to photosynthesize when conditions allow, the availability of water being the controlling factor within a broad temperature range.  An evergreen plant is always ready to photosynthesize when moisture is available, spring, summer, fall, or winter. They are not dependent on the confluence of all of the factors in spring and summer.  Their leaf structures can have the thick walled/rigid structure that will help them with freezing conditions. When temperatures are amenable the plant can photosynthesize without first having to make a large expenditure of resources to first ‘leaf out’.  Their leaves are already there and ready.  Conifers with their thick cuticles and epidermis, are even more cold adapted than are the larger leaves of many broad-leafed evergreens like Rhododendron.  These and many other broad leaved evergreens are less cold adapted because of their larger surface area and the relative thickness of their cuticles and epidermis.  (There is a relationship between leaf size and cold, but there are many other factors.  Larger leaves are often associated with tropical to warm temperate plants though there are such cold deciduous trees tolerant into zn7.  Dropping into lower zones quickly thins out even these while large leaved evergreens become fewer and fewer as you cool below zn8. Conversely, this does not hold true for small leaf plants, though they may dominate colder climates, they also occur in warmer.) In sustained freezing, where the root zone is frozen rendering water unavailable, many broad-leaved evergreens are more susceptible to desiccation by wind than most conifers.  Climates such as Oregon’s high desert are difficult ones to grow such plants on as they are prone to winter desiccation (It should be noted that plants like Rhododendrons evolved in summer wet areas and have relatively shallow root systems rendering them less tolerant of frozen soils.)  There are also conifers like Coast Redwood that are less tolerant of such climates.  These plants, while they may outwardly possess the more gross physical characteristics of very cold tolerant plants, don’t possess the genetic adaptability to make the internal changes to survive in such climates. Looking specifically at Coast Redwoods growing near the central and northern California coastlines, with their heavy maritime influence and the frequency of summer fog, these conifers are unable to adapt to the extremes of colder/dryer northern and interior climates.  As a group however, conifers are predominantly cool temperate and warmer plants with around 90% of the world species limited to areas that get no colder than zn6, at or just below OF.  The coldest regions, the cold temperate and arctic have relatively few species of conifers.

Another problem with making such hardiness assumptions is that some of the features associated with it are similar to those made by plants that are tolerant of summer drought.  Sclerophyll, are plants with ‘hard’ leaves that crowd the stems on short internodes, most common in chaparral country found in the world’s mediterranean regions.  Similar hard ‘structures’ are often common among many freeze tolerant evergreens.  Many true alpines share similar structure and survive temperature extremes of cold and the dry baking heat of summer across rock and scree.  We need to keep in mind that this is not an exclusive characteristic unique to cold regions.  For example there are the Grevillea and Eucalypts of Australia, many southern hemisphere Proteaceae and Ericaceous plants of South Africa that possess similar structures, along with California Chaparral plants that we are more familiar with, that are tolerant of our summer dry conditions, but may be less so of the sustained temperatures below freezing we experience.

Hard leaf structures are also characteristic of Palms, but of the recognized 2,600 species there are only a handful that are cold hardy for us, a list that may grow as others are tested, but unlikely to increase much.  One might think that Palms, as monocots without cambium tissue wrapping its trunk, just below their bark, might be more resistant to cold, but no.  Most of them are of tropical and sub-tropical origin and do no possess the ability to acclimate as do temperate palms.  Over the years I’ve had many people quiz me about the palms I grow and they express shock that you can grow palms here.  I hasten to reign in their burst of enthusiasm, telling them that no, only a relative few palms come from warm to cool temperate regions, the vast majority will never be hardy here.  They do a similar double take with Bananas.  I’m sure most of you have experienced something similar…most people aren’t gardeners and their lack of plant knowledge is stunning!

Cold deciduous plants, native to temperate climes, drop their leaves in fall as part of the acclamation process and are best adapted to regions with sustained freezing winter temperatures and wet spring/summer growing seasons.  When a cold deciduous plant retains its leaves, still green, into the winter, it is a sign that the plant has not fully acclimated and if a sudden downward spike in temperature occurs it could be damaging.  Leaves can be frosted/killed before they abscise if temperatures have been too mild.  Very cold tolerant plants may be slow to acclimate when grown in milder winter zones than they evolved with.  This may not be a big problem though as these plants will be less likely to ‘need’ the full range of their potential hardiness.  The leaves of deciduous plants tend to be thinner and softer than those on evergreen plants and because of this are more susceptible to freeze damage ‘while’ they remain on the plant.  Evergreen leaves, both thicker and more rigid are better able to withstand the freezing of water in their extracellular spaces.  Having said this the woody structures that remain behind after leaf drop on deciduous woody plants are sturdier and even more resistant to damage than any leaves.  Minerals and dissolved compounds sugars and proteins tend to reduce the temperature at which it will freeze.

Herbaceous perennials, like Hostas, begin to shut down their metabolism and photosynthetic functions in the Fall.  This may be through a combination of signals they receive among them chilling and shortening day length, photoperiod.  Top growth of plants like these often begin to collapse before the arrival of freezing temperatures through a process probably similar to the fall abscission of leaves in which a layer grows between the upper leaf structure and the growth buds embedded in the rhizome, itself protected to some degree by being within the soil.  Hostas, like many other herbaceous perennials, are able to tolerate a level of ground freezing without damaging their all important rhizomes, bulb structures or crowns.  Obviously they undergo internal adaptations that allow themselves to either tolerate the freezing of water within their structures or are able to hold within themselves ‘supercooled’ water and survive winter. Again, any plant’s ability to do this can be linked to their genetics.  The likelihood of a plant surviving winter in your location will be linked to how similar your conditions are to those it has evolved responses for during its own evolution.  Plants will have limited ability to survive colder conditions than they evolved with.  It is therefore very important to understand both where your plant came from and what the conditions are on your site.  This is true of any plant whatever its above ground structure maybe.

Occasionally plants will surprise us and be adaptable to colder temperatures than it may have experienced in its native region.  This is possible likely in part to other characteristics the plant may have and the fact that cold tolerance is an amalgam of responses within a plant, not a narrow function limited only to cold tolerance.  Remember, a plant’s tolerance to any stressors is a whole expression of itself and cold tolerance may be shaped by its tolerance for drought, etc.  For example, Musa basjoo, the so called Hardy Japanese Banana, is able to grow back each spring from its rhizome despite being frozen by temperatures below 0F, with mulch, even though it is native to Okinawa and the Ryukyu Islands where freezing does not naturally occur.  Keep in mind that for now, given our limited understanding, such cases are anomalies.

This brings us to tropical and sub-tropical plants (which include many of our commonly grown flowering ‘annuals’ and vegetable plants) and their response to chilling and freezing.  When discussing protecting bananas and other tropicals/sub-tropicals with people, I have talked about them not having a response to freezing.  Evolutionarily if there has never been a ‘need’ for something, an organism/plant will not evolve its genetic code in such a way as to have a response to the ‘missing’ stressor.  If plants, having evolved in a frost free region, are exposed to freezing, having never developed a response to it, they will die…probably catastrophically with extensive failure/ruptures to its cell membranes.  Tropicals have the additional limitation of rarely if ever even experiencing ‘chilling’ temperatures.  Temperate plants, remember, alter their metabolism and growth when experiencing ‘chilling’ temperatures.  Tropicals may even have limited capacity to do this and because of this begin to ‘fail’, shutdown or die at temperatures well above freezing, remember that chilling occurs, between 32F and 59F!  So, when your daily highs drop below 60F even though your lows remain above 32F your tropical/sub-tropical plant’s health will be compromised.  Most of us who use these plants have probably noticed how, while they may still remain standing and look relatively good into our cooling fall, they have essentially stopped growing. They are not changing their metabolisms.  They are still trying to keep on as if it were summer.  Keeping such a plant in this range over time will weaken it.

We have two choices if we want to save these plants to put them into a heated greenhouse and continue growing them on or to place them into a cool/dark space and induce a kind of drought dormancy.  Remember that these plants are equatorial plants, they evolved with relatively consistent growing conditions year round.  Even day length remains relatively steady because of their low latitude.  Temperatures may never dip into even the chilling range, sub-tropicals of course will.  Typically seasonality in these regions are defined by precipitation.  There are wet and dry seasons.  They have evolved a varying ability to tolerate drought.  This will be determined by their genetics.  Some tropicals will actually tolerate very little as they may have evolved in tropical maritime bands where rain, fog, humidity and leaf condensation combine to maintain moist conditions year round.  If this is the case, your efforts to winter these over will need to recognize and reflect these conditions.

Wintering over and freeze tolerance, for any particular plant, will ultimately be limited by its genetics.  Your tropicals will never survive a typical zn 7 or 8 winter without appropriate action being taken to protect them.  Even temperate plants, grown at their genetic limits for cold, will not always survive.  In Portland, even if we perfectly understand our micro-climate, have studiously followed temperature histories and can safely say, for example, that ‘my’ zone is a 8a, we will still have to be attentive when growing zn 8a plants.  Each year we will have to ask when winter arrives, are my plants suitably acclimated? Have they had a long enough period of chilling to have adjusted both their metabolic processes and to have shifted their internal structures to allow them to survive a typical ‘zone 8a’ winter?  Is the plant fully dormant?  This is one of the most difficult things to gauge as gardeners in the Portland area with our maritime influences, latitude, low elevation and proximity to the gorge that works as a kind of ‘relief valve’ for the intermountain air mass east of the Cascades.  Further south in Salem and Eugene, weather tends to be more steady and therefore, predictable.  Ours can have relatively wide and quick pendulum swings in the fall and winter…swings that can interrupt the chilling and acclimation process of plants.  Acclimation does not simply turn on or off at a particular temperature. It may begin at a certain point but its ‘quality’ and completeness depends on both the temperature and the time spent there.  The early, for us, cold snaps, can be destructive to plants as can be those January ‘thaws’ followed by another abrupt cold snap.  Our winters can tease our plants as temperatures dance up and down from freezing, to chilling to the upper 50’s in winter.  This is what makes hardiness so difficult to determine here.  It is not a fixed limit.  Concerned new gardeners are always asking if a given plant is hardy here???

This brings up a related temperature issue.  What was the previous summer like?Were your plants able to grow with appropriate vigor? Did they struggle with stress and grew weakly? Were your plants ‘pushed’ with nitrogen producing a lot of soft growth? Were they still actively growing, pushing vegetatively well into the fall?  How your plants grew in the summer will effect their cold hardiness.  If the production of proteins, carbohydrates and other metabolites are effected by temperatures, then plants that come from regions with hot summer days and warm nights will be physically different if they must endure cool nights and overall cooler temperatures during their summer growth period.  Some gardeners will talk about ‘ripening’.  Many temperate plants from warmer regions may bloom much later here.  This should be an indicator that these plants have not been able to go fully through their growth cycle and will likely be compromised in terms of winter hardiness.  They will not be the same plant going into winter.  Some of us may choose to grow these for their foliage effects even if they never or rarely bloom for us, for the ‘feel’ that they bring to our gardens.  We will need to remain vigilant.

Similarly, if plants from normally moist summers are grown here with inadequate summer irrigation they can stall and struggle and then, with the onset of our fall rains, be pushed into new vegetative growth, which may be left with too little time to adequately harden off, the whole plant having a delayed response to shortening days and cooling temperatures that should be shifting the plant into acclimation.

I won’t go into in any detail here, but I want to mention another climate issue.  Temperature does not work independent of other factors.  For many, if not most plants, whether a winter is wet or dry can be a huge factor.  Cold dry soils, moist well aerated organic soils and moist/wet clay mineral soils will all contain different bacteria, fungi and other microbials.  Our soils here can contain winter fungal populations that can cause root rot in many plants especially those we might choose from winter dry climates.  It is not absolute cold that will end plants in this situation.  Temperature effects many different limiting factors.

A good example of these plants would be members of genus Agave and many of the Bromeliad family all of which are also CAM plants (see).  Include with these the Cacti. While these vary in cold tolerance, these are all desert/drought tolerant plants and have a relatively low tolerance for wet soils.  Agave and Bromelidaceae function best in low water situations as CAM plants and when they get water they prefer warm to hot temperatures.  This is true for many Cacti as well, but some, like northern Great Basin and the Intermountain West native Opuntia are somewhat more tolerant as long as they aren’t growing in heavy soils.  In fact all plants like these need quickly draining/gritty soil otherwise your plants will perish before they reach their minimum temps.

One last and potentially important factor affecting acclimation is something completely within our control: pruning.  Pruning, among other things, stimulates vegetative growth and it does this by interrupting the release of hormones from buds at branch tips especially the apical terminal buds on a tree’s leader.  Heavy pruning of shrubs, such as hard rejuvenation pruning, in which the whole plant is cut down, stops the release of hormones from the terminals which normally works to suppress the initiation of new growth below them.  Hard pruning stimulates a strong growth response.  Doing such pruning late in the summer season will encourage strong regrowth.  The danger is that having passed the heat of summer and with Fall fast approaching the new growth will remain ‘soft’ going into winter and with such soft tissues suffer extensive damage to the new growth. It will also delay the process of acclimation putting the whole plant at risk.

Freezing in Plants

All temperate plants are subject to some amount of freezing temperatures, to -40F and colder, yet the water they contain, in and between their cells and in their water conducting xylem tissue, often has to go well below freezing for ice to form.

Northern temperate plants have the ability to retain their water in a ‘super cooled’ state, well below the crystal forming norm of 32F.  This is only possible when water is ‘pure’ containing nothing the crystals can begin their growth on. There is a process called nucleation which describes the initial formation of a crystal around a tiny bit of dust, certain bacteria (INA or Ice Nucleating Bacteria) or other often organic impurity, water molecules can cling to.  (Nucleation also occurs in clouds collecting/condensing water vapor/moisture into raindrops.  In winter clouds can contain supercooled water that will freeze/crystalize around tiny particles forming snowflakes. Remember that techniques have been developed to ‘seed’ clouds, which provides the nuclei for snowflakes to form on, causing them to snow over mountains that they would otherwise pass over their moisture withheld.) If their are no ‘nuclei’ there is no freezing.  It is believed that during the acclimation process the population of these bacteria, one of the main sources of nuclei within the plant’s structure, drop over time.  During winter warm spells their population tends to increase.  Then when the temperature again drops these same bacteria again begin to decline, but their is a lag period making the plants more vulnerable to freeze damage providing more nuclei for internal ice formation.

The supercooled water in the plants can ultimately reach a point where it will freeze even without nuclei, but these can be very cold.  Supercooled water, perhaps counterintuitively, requires a relatively large volume for this to happen, though the amount declines with the temperature.  Water is limited to relatively tiny spaces inside and between cells and narrow xylem vessels reducing the volume and likelihood of supercooled water freezing. Freezing can occur on the outside of the plant, where nuclei are far more common, first forming crystals that can utilize openings like leaf stoma, the calyx end of a fruit or wounds.  When crystals come into direct contact with the internal plant water, especially super cooled water, ice forms and spreads unless it is disrupted by internal structures such as internodes, tissue barriers and other plant created barriers.  This explains part of the advantage of winter deciduous plants having  dropped their leaves there are no stoma for crystals to utilize to enter woody tissues.  The bark and lignin of their branches and trunks are even more ‘impermeable’.  Evergreens, while having physiological advantage over the thinner leaves of deciduous plants are still less sturdy and resistant to penetration by ice, making them overall less hardy to more extreme cold.  Once inside the plant the freezing tends to spread via water between cells and doesn’t directly harm the plant.  As freezing continues liquid water is drawn out of adjacent cells dehydrating them (Ice drops the surrounding ‘vapor pressure’ which tends to draw water vapor out of the cells still containing liquid water, through their surrounding membranes, in the process also increasing the proportion of dissolved solutes left behind in the cells, which works to lower the freezing temperature of the cells, protecting them from critical damage.  To a point this dehydration of cells protects them from fatal ice injury.

The previous acclimation/chilling process is important to any plant’s ability to tolerate this freezing.  Remember that as a plant acclimates it changes the metabolites, proteins, carbohydrates, etc., from those it produces during its more active growth stages.  At least some of these compounds then function as an ‘antifreeze’ controlling where the crystals form and collect.  Depending on the plant they will have more or less tolerance for this supercooling, freezing, dehydration process.  The genes of temperate plants signal cells to toughen cell walls making them more resistant to damage by ice crystals.  It is these crystals that cause the serious damage by rupturing the cell walls and causing them to lose a fatal amount of cellular water.  This is why particularly stiff foliaged evergreen plants don’t show the extent of their damage until growth initiates in the spring and the cells, unable to rehydrate due to the extensive damage, die.  Different plants have varying abilities to come back from freeze damage.  It is a matter of degree and genetics.

I think it is important to point out here that injury and cuts to plants just before or during freezing weather can be a bad idea for both herbaceous and possibly, woody, plants.  Fresh wounds provide an open avenue not only for rot organisms to enter a plant, but perhaps even more deleterious is that it greatly increases the opportunity for ice crystals to penetrate into the plant and cause extensive freeze damage.  In woody plants this damage is more likely to be limited to near the wound site.  In plants like Bananas with their relatively huge open cell structure it would greatly increase the odds of freezing stems down and in cases where the meristematic tissue is damaged at the top of the rhizome, through late season division or digging, these plants would be completely unable to compartmentalize damage, meaning that they would be unable to seal off wounds to infection, fungi and serious freeze damage.

Chilling and freezing temperatures trigger different process within plants.  The ultimate effects of freezing is different within any given plant and varies depending on how acclimated it is and how a plant’s freezing response is effected/delayed by warm spells during the winter, no matter how acclimated the plant had been previous to the initial onset of winter temperatures.

Protecting Your Plants

It is important to keep all of this in mind when considering how you plan to protect tender plants from winter freezing. I have four different groups of plants I pay attention to when cold approaches: 1, my few tropicals that have no ability to respond to cold; 2, my sub-tropicals that prefer a mean minimum temperature of 50F, and have a minimal tolerance for cool and occasional quick light freezes, these include Cuphea, Aeonium, Rhodocoma, Chondropetalum and more; 3, plants that, herbaceous or evergreen, are helped by a heavy mulch around their crowns; 4, and those, mostly succulents, that need protection from winter wet, but are otherwise cold tolerant.  I will discuss each of these briefly in terms of how I treat them.  I have discussed Palms elsewhere on my blog.

1. My Neoregelias, tender Musa, Ensete, Calocasia and some of my more tender succulents, Brugmansia, Fuchsia boliviana and my Heliconia shiedeana, among others, spend the winter in my basement where temperatures range between 50F and 60F, inching up above that when outdoor temperatures are consistently warm. My H. shiedeana is zn 8a in terms of survival, but because it takes two years of top-growth to flower I bring it indoors in hope of preserving the top from which the flower buds form.  Part of the space is lit by a big grow light with fluorescents on a rack.  Another area, where I keep the Bananas, I try to keep light levels down to help slow growth (See my posting on Wintering over Abyssinian Bananas).  This space gets a little on the warm side as a holding/non-growing area because it is inside of my house.  These will stay inside until spring when freeze danger has passed.  Spending too much time below 40F is something to be avoided.  Sustained temperatures below 40F have been fatal for plants like Ensete.  I don’t know if it encourages bacterial or fungal growth, but these can end up dead, stinking and rotten…not unlike a carcass.
   

   Here’s the lean-to roof, permanent, with my very temporary ‘walls’ on the south and west sides, both open here because of the sun and ‘warm’ day. The palm is a Trachycarpus maritianus which is marginal here especially since it has not reached its mature trunk diameter. The Beech in the left foreground has been in a pot for 30 years, grown from seed, and is hardy here without protection, even in the pot, though in really cold stretches where highs remain below freezing I’ll wheel it over under a roof.

2. These I used to always put in the basement during colder winter spells especially when daily highs are to remain below freezing, otherwise I would shuttle them back and forth to outside in sheltered places.  Now some of these go into my new and still evolving protected deck space where I keep temperatures above freezing while opening it during the day so the space doesn’t get too warm encouraging growth and reversing the acclimation process.  I want them to stay cold resistant.  I will also put newly divided/potted plants like Iris x pacifica in here to protect the vulnerable crowns and other odds and ends some I’m just trying to protect from roving ‘critters’.
   

   Melianthus major, one of my favorite textural foliage plant is a zn8a from South Africa. I have lost several of these in the past, but now consistently mulch it heavily with leaves. I think part of the problem was my ‘heavy’ soil so organic matter helps as well.

3. Melianthus, Cestrum ‘Orange Peel’, Erythrinia, Fuchsia, Canna, Hedychium, Alpinium, Musella and some of my Begonia, I mulch all of these heavily with leaves in place, in the ground.  Their crowns are where important meristematic tissue is located and can be sensitive to cold.  Mulching these give a few degrees of protection.  Most of these grow back annually from this tissue here, their tops often frozen down.  Keeping the crown healthy insures the plant’s vigorous return.
   

   My front porch which doubles as a holding area in winter containing most of the plants that you see and several others. The roof covers this area to two feet beyond the post on the knee wall. We’re contemplating adding an awning with temporary sides to enclose it more completely,…but that’s a few dollars away.

4. This group, comprised of my hardier, potted, succulents like Agave, Aloes, Dyckeia, Hechtia, Fasicularia and Puya spend most of the winter on my front porch under the roof where they can stay dry and receive much of the long angled winter sun.  These conditions more closely mimic those found on their native sites. Overhead cover of any sort reduces the loss of heat by radiation.  Plants open to the sky, especially under clear calm conditions, loose more heat which helps explain the formation of frost on unprotected plants.  Without such protection leaf surfaces can be colder than the surrounding air.   I have a potted Mediterranean Fan Palm that shares the space, even a couple of Citrus, which I can move into the basement for brief periods if I’m really worried.  Keep in mind that these are typically zn8 plants. (I do have a few of these experimentally in the ground to test them out.)  My other less cold hardy succulents spend much or all of the winter in the basement.

 

There are rules of thumb to follow, especially when dealing with plants that are new to you.  There are exceptions to how you can winter over plants, as I’ve already mentioned.  When I ‘experiment’ I like to have back up plants I treat more conventionally so that I don’t lose all of them should the decision prove to be less than solid. There are several varieties of the Common Taro, Calocasia esculenta, a typical tropical plant, several of which I have grown.  Many guides suggest that these can be treated as if they are zn 8 plants and can be left in the ground for the winter.  I know people who do this.  In fact I have dug some of these in the Fall, like the dark stemmed metallic sheened ‘Fontanesii’, leaving a few errant tubers and had them return in the Spring.  Keep in mind that this was in well drained humusy soil.  My usual result has been to have the tubers rot out.  Additionally these are sensitive to soil temperatures and can be very slow to initiate growth in the spring…which is another reason I bring them in.  It gives them a ‘jump start’.

Assessing the Hardiness of Your Plants

How hardy is a given plant?  Which strategies does it employ in its own defense?  What ultimately are its limits to cold?  Can I do anything to improve the growing conditions that will improve the overall health and cold hardiness of my plant?  How will I know when and how to protect a given plant?  Some gardeners main priority is to ensure the survival of their plants and act conservatively and will respond well before the hard freeze that they know will kill.  They install them in a warmed space, monitor them for bug infestations and tend them indoors.  Some simply don’t worry and expect to replace those they want in the spring and try something new.  Others whether out of frugality or simple curiosity are more interested in their ultimate limits and want to minimize their expense by not doing what isn’t necessary.  There are all kinds of gardeners out there.  So, we read, we ask friends, we visit other gardens and above all else we observe our own plants and speculate about how to improve our practice.  No one has written the book for your particular garden.  Your plants.  We have to ask how credible is the information that is out there.  How much can we assume applies given all of the differences that inevitably exist between the author’s experience and our own?

With some plants there is a problem of conflicting hardiness ratings one source claiming zn 7 while another may say zn 9.  What if they’re both right???  I’ve been discussing how the particulars of how a plant has grown is important to its hardiness.  Part of this may stem from the fact that different growers may have gotten their stock plants from various sources with different provenances. There can be significant variation in hardiness depending on the source material.  In other situations, especially when a plant is new to the industry, we don’t really know its limits.  Growers experience may reflect only growing it where they were sure that it is ‘safe’.  In this situation some growers will tend to be conservative and assign it a ‘warmer’ rating.  At best zone ratings are a short hand and, as such, are helpful.  What is more helpful is more specific information about the conditions across a plant’s natural range. Knowing that a plant came from the cool moist Lake district in Chile or the summer dry Cape province of South Africa gives you much more information as a gardener when you decide whether you should try to grow it, where exactly you should site it and what protective actions you should take if any.

Plants have altered their outward physical appearance over time in response to stressors like cold.  These can be misleading to the gardener because these outward structures may be shared by plants that grow across many different zones.  Generalizations won’t be particularly helpful in practice.  Desfontainia spinosa, comes to us from Chile but looks like an Ilex with its Holly like leaves, until it blooms.  It is described as a zn8a plant and as being intolerant of drought.  Treating it like ilex aquifolium, because it looks like a small version of it, will be fatal. Conifers, with their distinctive leaf structure are readily identifiable as hardy to our region with 65 species native along North America’s Pacific Slope. Portland’s Hoyt Arboretum contain some 360+ species of Conifer, including the rare and rather tender Wollemi Pine. On the west coast of North America conifers range northerly up just shy of the tundra and as high as and defining alpine timberline.  Each individual species can be very different. But even in this broad group of plants are members like Norfolk Island Pine that will not survive one of our typical winters outdoors without protection, something that gets harder to do as they gain stature…and though we might complain, ours is a relatively mild temperate climate.

Looking at a plant does not immediately give away it’s limits to cold though the more broad your knowledge of the many thousands of plant genera and species the easier it is to at least ‘ballpark’ a given plant’s limits to cold, not that that is safe to do.  It is certainly not, ‘Ya’ know one, ya’ know’em all!”  Northwest gardeners know Mahonia aquifolium zn5, the ubiquitous and native Oregon Grape.  It is a durable performer here.  But how many of us know its limits? How much further north, how much colder can it tolerate?  Mahonia repens zn5, another tough performer that survives even drier and colder conditions, along with Mahonia nervosa zn6, a denizen of the understory of much of our local and coastal coniferous forests, all sharing very similar leaf structure, may give us the confidence to make assumptions about other members of the same genus when we come across them.  To assume that they have the same limits may prove erroneous.  Several species and popular hybrids come from Japan and China which have wetter summer climates.  One of these M. lomarifolia, is less hardy, a zn8 plant. while M. japonica is a zn6 and M. bealei zn6 The same genus contains plants from southern California, the desert SW and into the southern half of the Rocky Mts, though identifiable to many as Mahonia by their foliage and flowers, have much smaller leaves characteristic of desert plants that must be more thrifty in their use of water. Mahonia fremontii is cold tolerant well into zn5 pushing its range much beyond ours to the east of the Cascades and the mountains of Colorado and Utah.  Mahonia nevinii, an endemic to southern California, is never the less cold hardy across M. fremonti’s range. One can’t safely assume that two related plants of the same genus, that share very closely much of the same DNA, are interchangeable in terms of their site requirements.

There are other plants, like Hibiscus, that much of the public, including many gardeners, have come to associate with the tropics and if they choose to dabble, will purchase these and either use them as if they were annuals and disposable or will figure out how to winter them over.  So, many people are startled to discover that there are hybrid Hibiscus available that are reliable into zn4, -30F! (For a short introduction of the species and hardy hybrids see this link). Interestingly species used in these hardy North American hybrids are summer moist plants often from wet/marsh areas, east of the Rockies. There are other species.  I was startled one day to find one growing in Ventana Canyon in the Santa Catalina Mountains next to hot dry Tucson, AZ.  The many species of Hibiscus, while sharing much of their genome and outward physical characteristics, range broadly across the world growing under conditions that would be prohibitive to others.  Genetics determine much more than appearance.

These days plant hunters go out in search of new plants with less expectation of finding previously unknown species, though I’m sure that they are always hopeful, then they did a couple generations ago.  There are no longer areas unknown to humans, but many that are still not fully explored by watchful, knowledgeable and expectant people.  Plant hunters are often looking in regions and at elevations for plants of known species growing at their limits or with other hither to unknown characteristics.  They bring back material to grow new forms and hardier clones.  Stock material is ‘shared’ in the plant industry, often from very few collected clones.  In some instances nursery plants may all derive from one imported clone with all of its ‘limitations’.  As gardeners we become familiar with these clones and their limits and may be surprised when new collections are made available to us that have proven to be much hardier.  Collectors bring back new material that can stretch the limits of what was previously on the market.  These may be clones that are asexually propagated or grown from seed collected from a ‘stable’ population growing remotely at a high elevation and are able to pass on this more extreme cold hardiness to their progeny.  This is why, particularly when growing plants at or near their limits, that the gardener understands the plants provenance…where it came from.  It can make all of the difference.

Scientists are working on unravelling the puzzle of how plants sense temperature and ‘communicate’ it to cells signaling them to alter their production and how cells may re-express themselves.  For many their focus is a more complete understanding of cold hardiness and how plants can be manipulated, genetically, to keep them viable and productive under colder conditions.  It is unlikely that this process will be defined for plants that are not of major economic interest as it requires extensive lab work.  Where does this leave us?

For the foreseeable future we will have to rely on our knowledge of the plants that we grow while paying attention to the progress of cold as we move through fall and winter.  We’ll have to understand the limits of our plants and gauge their condition when we consider how we will protect them.  We’ll have to be careful that our protection efforts are not decreasing their hardiness and setting them up for failure.  I’ve seen many gardeners protect plants in the ground by covering them with cloches and I always wonder if these raise daytime temperatures too much and so prevent them from fully acclimating.  Such ‘structures’ provide very little insulation allowing night time temperatures to drop to outside temps.  On the other hand I have covered plants, the sides open, to try to keep those that prefer drier conditions in fact drier.  In some cases given the temperature swings we typically experience, we might choose to keep a plant in a container rather than plant it in the ground especially if it is one that may prefer drier winter conditions. A plant that is unable to fully acclimate won’t by as cold hardy as it would with long enough and consistent enough chilling.  Under such conditions a plant will be susceptible to damage at temperatures somewhere above its ‘limit’.  The next time someone asks you if a plant is hardy here, consider how you answer and remember that hardiness is a moving ‘target’.