Does size/age affect hardiness of woody plants?

Over the past few years I've read many postings that seem to contradict each other regarding this question. Some say that just because a tree has survived a few winters doesn't mean that it won't be severly damaged or killed outright once more normal winter conditions return. Others say that as plants become more established, they are more likely to withstand a harsh winter, perhaps suffering only slight dieback. As a poor sap who gardens in z4, this is something that causes me concern. Like many of you, I've collected numerous marginally hardy trees and shrubs over the years, many of which are starting to achieve respectable size. The trend over the last few years has been for lower snowfall amounts and warmer temperatures. This winter has been extremely mild, with coldest temperature thus far being -10F, which means we're basically having a z6 winter. Any thoughts on this?

I think both statements are true. Don't count on having mild winters forever, but as your marginally hardy trees grow larger and become better established, they can be expected to exhibit SLIGHTLY more tolerance of cold snaps than they did when they were very small. Whether that's enough to get them through a really severe winter is unpredictable, but it might get them through a moderate winter with less damage.

By the way, you might understandably have a little zone envy, but Eau Claire is a beautiful part of the country, so don't be too depressed that you can't grow bananas and coconuts there!

Guy S.

Thanks for asking, Maackia. I've been sort of wondering about the same thing.

I have picked up on these seemingly contradictory statements myself.

To each situation is brought a new set of variables. I believe that both statements are correct. They just depend on species as well as other factors not noted above.

One variable that I find to be critical mass as pertains to future survival of a species in my zone is snowfall. Snow has insulating properties and if temps return to those more consistant with our region, we are probably going to need good snow cover regardless of whether a species is hardy or marginally hardy. It has been my experience winter droughts can be more devastating on plants than summer droughts. The last few years, we've had decent accumulation... before that we were in a drought for all practical purposes. No snow cover combined with -20º - 35º can = death and that's about what happened over here to hundreds of trees and shrubs that were indigenous to this area. In this particular situation, more mature deciduous trees that had considerably more developed root systems capable of withstanding drought were more likely to survive where juveniles were not. On the flipside of the coin, more mature species of say Ilex did not survive because there was considerably more area susceptible to desiccation wherein which juvenile evergreens made it with little dieback. It's not all that difficult to wrap a small Holly or Rhodo in burlap, or to hang strands of Christmas lights on them, and it isn't all that difficult to spritz juveniles with Wilt-Pruf but how the heck do you do that to mature specimens? In consideration of this, when it snows we have been directing the flow of our snowblower to all plants in range. We also remove the snow from patios and walkways and dump it around the bases of as many newer plantings as possible just in case we don't get enough accumulation to protect the plantings. Just my 02¢.

Editing to correct quote and add a link to Wilt-Pruf for basic info-
http://www.wiltpruf.com/

This message was edited Jan 14, 2006 9:36 PM

I think plants are imparted a degree of "potential cold-hardiness" that never changes and is encoded in genetics. An example is cloned material, which will have the same potential cold-hardiness as the parent material. Though the potential cold-hardiness never changes, phenology and cultural conditions impact the actual cold-hardiness.

Phenology: Roots of the same plant withstand cold to varying degrees. In an imaginary tree, the finest and youngest root parts might begin to succumb to cold at (let's pick a number) 28* F., while larger, intermediate size roots might tolerate temperatures several degrees colder. Finally the oldest and heaviest, highly lignified roots might tolerate temperatures many degrees, perhaps 15 or more degrees colder. As the tree ages and the mass of older roots increases, so does the probability the the trees rootage will not be killed at temperatures just under the 28* chosen as an example. At the same time, the younger tree, with a high % of less lignified roots is more likely to be damaged or killed by cold.

Culture: Drought stressed trees are likely to be more cold hardy than well hydrated trees and trees that have grown with good vitality prior to entering dormancy will have more organic solutes in cell tissues to act as antifreeze & will also be more resistant to cold. The cultural impact of photo-period, prior exposure to chill or prolonged warm spells in mid winter can also impact o/a cold hardiness and are considerations, as are other cultural conditions that were not mentioned but impact physiology.

Plants cannot "adapt" to cold by being exposed to cold, even after 100 years of growth. Their potential cold-hardiness remains the same but the actual hardiness changes. There is the tendency to be moved toward a greater resistance to cold with age, but that tendency can be trumped at any time by culture.

Al (Still holding dual-citizenship) ;o)

Love the antifreeze analogy!

quote]I think plants are imparted a degree of "potential cold-hardiness" that never changes and is encoded in genetics.[/quote] You touched upon some of the reasons why I prefer to purchase my plants from nurseries that grow local genotype. A tree may have a native range extending from Missouri on up to Canada. The genotype from MO will generally not "adapt" as well here in Northern IL as a genotype from WI for me.

Nature/Nurture issues abounding!

I certainly agree that provenance affects us much more than even those familiar with its effects often realize. In some species, even 100 miles change in latitude is enough to make noticeable differences in vitality, fruiting, leaf color, leaf abscission, and on and on. Dr D. Chapman is head honcho at Dow Gardens, a stones throw from me & I'm fortunate enough to be able to speak to him occasionally. He is passionate about genotype diversity & often works his way around to lengthy discussion on the subject of provenance. He also often mentions that the genetic material for more than 90% of the woody plants in our landscapes come from less than 500 plants. He uses this as a springboard to propose the use of more plants of local provenance, which of course increases diversity.

Al

Good thread, Maackia. Glad to see hard WI winters don't affect gray cell hardiness. It is sunny here this morning, with just a spit of snow left on the grass. I think I'll plant some more viburnums today.

Al/tapla has summarized most points I would make. An individual plant is imbued genetically to tolerate only so much; our best hopes and wishes don't change that. That plant's maturity in the landscape provided (length of time growing there) provides increasing amounts of roots/branches/leaves/trunk to support life, and ward off insults from climate and pathogens. This is why little plants that are repeatedly browsed by deer, burned by fire, dried by drought, or chewed by insects go ahead and perish where older plants (with more reserves or defenses) can survive.

So.....a very recent B&B transplant of say, Southern magnolia (Magnolia grandiflora) will be far more likely to suffer winter dessication damage, woody tissue damage, or even death, than an exact replica (clone) that has been growing longer (more roots, etc.) in the same place. Sound like the voice of experience? It is. 1994, to be exact; rural Fayette County, KY felt -28F and very nice plants dropped like bricks.

Similarly, Southern magnolia plants commonly derived from far southern sources performed far more poorly than Southern magnolia plants derived from proven performers growing far north of their "native" range (like from Cincinnati's Spring Grove Cemetery, or the late Bob Simpson's place in Vincennes, IN). Example: severe winters 1975-76 and 1976-77, which extirpated many Southern magnolias in central KY (as well as many other broadleaf evergreens that had migrated north due to extended duration of mild winters, like Burford hollies). The plants seemed tough, but hadn't been really tested (like some football teams come the playoffs). As Al mentioned, each of these plants had a certain genetic capacity for cold hardiness. Once that was exceeded, plant death occurred.

The antifreeze analogy is a good one. WARNING: CHEMISTRY LESSON COMING. CLOSE EYES AND SKIP TO NEXT PARAGRAPH IF EXPOSURE COULD BE OFFENSIVE. The exact biology may also include some plants' ability to translocate water molecules out of individual cells and into the space between cells, so that when severe cold occurs, and the water molecules change phase to solid ice (with concomitant expansion), cell walls are not burst but compressed instead and cell death doesn't occur. Plants that cannot do this, the cells explode and we all are familiar with what that looks like (in keeping with the examples above, the brown mush of early magnolia blooms, or the bark sloughed off of formerly turgid stems).

People are softened and persuaded by that which they don't have, or what is the "pretty shiny thing" of the moment. It's why development keeps moving back to ocean shorelines, the steep slopes below volcanoes or along rivers, active seismic zones, and so many other geologically and climatically (historically) hazardous places. Call it risk/reward.

Making the choice to have "...collected numerous marginally hardy trees and shrubs over the years..." means that one is set up to replicate one of those History or Discovery Channel programs like Engineering Disasters MMVI! Everything looks OK, even spectacular, until the right combination of forces align and the whole thing comes apart. So, as Guy and others have said repeatedly, invest in these only if you can afford to lose them all.

Eat, drink, and be merry, for tomorrow the Viburnum davidii may die.

John

This is turning into the type of deep thread that we used to enjoy regularly on GW!

I would like to add something about hydration. Al said that:

"Drought stressed trees are likely to be more cold hardy than well hydrated trees and trees that have grown with good vitality prior to entering dormancy will have more organic solutes in cell tissues to act as antifreeze & will also be more resistant to cold"

which might sound like a contradiction, but it's not. The key is that slightly stressed trees might tend to enter dormancy sooner, and thus reach their midwinter hardiness peak sooner. Lush, juvenile growth that continues growing into fall due to surplus moisture is very vulnerable to early winter damage. However, trees that have had ample summer moisture (to manufacture the sugars necessary for "anti-freeze") followed by a normal hardening period in early fall tend to fare best of all.

Another factor to consider is winter transpiration, which can be especially tough on new trees not well rooted (per John's experience), evergreens which continue photosynthesis on warm days through winter, and marginally hardy trees growing in soils which freeze deeply and preclude water absorption by roots.

The removal of cellular water to intercellular space that John discussed might be illustrated best by hardy Opuntia species (prickly pear cactus). They can loose turgidity to the point that the pads (stems) actually collapse and lie on the ground, only to resurrect themselves the following spring as though they've taken a massive dose of Levitra!

Guy S.

OK, Guy. Now that might just be a little painful to all the Opuntia partners out there.

Now: let's get all the broadleaf Ericaceae lovers out there to talk about why the leaves "roll up" when it's really cold............if that's allowed on this forum.

Doofus editing of repeated posting.

This message was edited Jan 16, 2006 9:40 AM

This is all very interesting. I'm sure it's what Al Gore had in mind when he invented the internet. ;)

I think I've been able to follow the gist of what's been said so far (well, at least until V V had to enter chemistry into the mix). Correct me if I'm wrong, but every plant will have an absolute minimum that if subject to for any length of time will kill it, regardless of size, age, or overall condition of plant. For younger plants, this absolute minimum will be less than for a larger established plant. As it becomes established (Al's Phenology description), it wil generally become more likley to survive colder temps. All of this, of course, will be influenced by a variety of cultural factors.

Thanks to everyone for offering your thoughtful and lucid explanations. While I can afford to lose any individual plant (well, maybe not my 20' Yellowwood), my concern is that an abnormally cold winter (it's got to happen sooner or later) will be a complete disaster for my landscape. Of course, there's not much I can do but hope for the best and keep them as healthy as possible. I'm apparently not too disturbed by this scary scenario, as my intentions are to plant a bunch of Abies and Magnolias this spring. :)

Now, Guy, about that Levitra analogy...

"Correct me if I'm wrong, but every plant will have an absolute minimum that if subject to for any length of time will kill it, regardless of size, age, or overall condition of plant."

Almost. It has a potential absolute minimum. The actual lower limit will vary from the potential lower limit because of the influences of age and culture. Actually, duration of exposure to cold is one of the cultural influences.

The chemistry is easy. As water moves out of cells, it leaves behind the stuff that was in it, the solutes. The higher concentration of solutes dissolved in what water remains in cells lowers the freezing temperature of the bound (within cells) water, thus the antifreeze analogy. The water that moves out of cells is now free water (between cells) and can freeze solid without damaging the cells. When the water inside of cells freezes (after the temperature drops below what the tree can tolerate), ice crystals pierce cell walls and plasma is torn from the walls, killing cells.

Al

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MY Opuntia didn't germinate last year - maybe I should invest in a Bunsen burner. Newfoundland has different climatic zones - I am in a tolerable 5a near the coast........ and part of nearby St. John's is probably a good 6. What interests me in this thread: what is the effect of Short Growing Season? Winter temperatures used to hit minus 15-20 C with some regularity........ now it's once in a while. However, the cold nights and cold soil last well into June, and last season I had a record late Spring frost on June 27 (a nasty nick). On the other end......... I had dahlias .......... not too battered or nicked onOctober 15 when I lifted the clumps. My observation: the trees/shrubs need a longer period of warm even hot weather here in the SE Nfld. to do well. Similarly, summers are short(er) in central Newfoundland....... but temperatures are hotter in summer and colder in winter than in my back yard.
Bill in SE Newfoundland