Cold and plant hardiness

It's been cold, seriously cold, and while restating the obvious, it perhaps prompted a reader to ask about the effect of extreme wind chills upon plants.  OK, this is an easy one; wind chill doesn't affect plants.  Only the absolute temperature matters, not how the wind makes the cold feel to us and other animals. Recently our absolute temperature hit -17 F (-27 C), and this is very near the rock bottom hardiness for zone 5 plants. In TPP's 35 years here in northern Lincolnland, the coldest it has been is -19 F. This is the absolute limit for freezing-avoiders, a topic discussed before. Until spring you really don't know what damage has been done to what plants.  Some tender woody plants will die back, and many will sprout from the base, so be patient before cutting things back.  Our Vitex (chaste tree) will certainly be in this category.  Over the past couple of years all of it survived the winters and it had gotten 2.5+ m tall.  My Mother, a native southerner, had a pet mimosa tree in upstate New York that survived because it was planted where the ample snow fall always drifted providing it with insulation.  Granted the mimosa never grew much taller than 4 feet, but she like it anyways and in bloom it often stopped traffic.  So snow cover makes a big difference, as does mulching.  This year the deep cold came with snow, so low growing things and herbaceous perennials will probably be largely undamaged.  In a few cases TPP is worried because either he or the nurserymen were cheating by planting marginally hardy plants (TPP) or over stating the hardiness of some plants (a lot of plants labelled zone 5 on nursery tags are not so rated anywhere else).  OK, so you gamble.  But it will be another 4 months before the data can be collected about what was damaged and what was not.  Another problem associated with this is that winter kill is more often about dehydration, especially of evergreens, both conifers and other types, than it is about cold.  If you stopped watering newly planted evergreens when the weather got cooler, and drier, then you put them at risk.  Again, mulching helps greatly.  Now stay tuned because next TPP will explain about frost cracks in trees and why it happens and why it doesn't.

Why don't trees freeze?

The Phactor gets questions, and what could be more appropriate than asking why don't trees freeze and die on a morning where the temperature is -13C and my system is still trying to get from tropical to temperate, so arctic is still beyond me. In actual fact some trees do freeze, but they do not die. The biology of some woody plants, those whose ranges extend the farthest into the high latitudes, allows their cells' water to freeze without harming the cells. These are called "freezing tolerators" and once frozen they can withstand almost any temperature. One such shrub in the Phactor's collection is the Siberian cypress (Microbiota) and it is hardy to Zone 2, -45C. It doesn't have to worry about such severe cold here in Lincolnland, but it can suffer from the opposite, our summer heat.
Other trees don't freeze; they have the ability to deep super-cool the water in their cells and avoid freezing, although that gets more difficult as the temperatures get colder. The ability to deep super cool has its limits at right around -40 (F or C!), where the two temperature scales actually equal each other. This means that freeze avoiding species can disperse and grow further north but then when they finally encounter a really cold temperature, they will freeze and die. The first map shows the distribution of a freeze tolerating species, the red-osier dogwood, which ranges north right up to tundra. The second set of maps shows the distributions of 4 species of freezing avoiders (A-D, American beech, white ash, sugar maple, and northern red oak). The heavy line running just north of the Canadian border is either the border drawn by Google maps (not a funny joke in Costa Rica right now) or the -40 C minimum temperature isoterm, which by definition connects all the points where the minimum expected temperature is -40 C. Other freezing avoiders are less able to deal with colder temperatures. Here in Lincolnland the minimum expected temperature occurs about every 10-15 years (-29 C), so a less cold tolerant tree or shrub might dodge the bullet for quite awhile before the climatic extremes demonstrate that my sweet bay magnolia is growing too far to north. Obviously global warming will shift such climatic zones and the distributions of organisms along with it. 54 million years ago when the climate was much warmer tropical forests extended to 50 N latitude! Imagine southern magnolias growing in Green Bay, Wisconsin!