Wind chill and plant cold hardiness

A reader asks an interesting question.  How does wind chill affect plants?  If a plant is cold hardy to say 10 F [22 degrees F below freezing for those of you who use rational C units], and the wind chill is -20 degrees, will the additional cold hurt or kill my plant?  OK, the short answer, no. The wind chill doesn't affect plants, only the absolute temperature. The wind chill factor is how much colder the temperature seems to us warm-blooded animals because of the wind, but it is still the same temperature.  Remember the wind chill factor is the number of degrees that is subtracted from the actual temperature; it is often reported so that you don't know if it is the apparent temperature that is -20 degrees or if you must subtract 20 degrees from the actual temperature.  But to the plant it is simply 10 F; not being warm-blooded plants don't get colder because of the wind.  Here's a couple of refreshers on cold hardiness from TPP's archives:
Why don't trees freeze? and It's the extremes. This winter, '14-'15, with the jet stream positioned where it is, our snow is coming from fronts moving up from the south west, and then the cold air pushes back south and a blast of frigid air follows.  So this winter has seen several nights where the temperature has reached 32-39 degrees F below freezing. Zone 5 plants will be fine, but any plant not fully hardy in that zone may get damaged or killed. So TPP has a bad feeling about the Helianthemum replacements. They were under a good snow cover for the first couple of cold blasts, but the last blast caught them bare, naked of snow. Ah, well. What kind of gardener are you if you don't push the envelope a little says the man with an Ashe Magnolia to plant come spring. 

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.

Brrrrr! Temperature and wind chil

It's bloody cold outside, and a recent trip to the tropics makes it seem even colder.  The temperature this morning was 1 F (-17 C) and a moderately brisk breeze contributed a -15 F wind chill factor so the apparent temperature is -14 F (-26 C).  It makes TPP's sinuses hurt just thinking about this.  Weather persons in the USA quite often screw up the wind chill reporting the factor as the apparent temperature, which today are fairly close numbers.  The apparent temperature is the temperature it feels like when the wind chill factor is subtracted from the air temperature.  They say something like "the temperature is XX and the wind chill is YY", and you have no idea what they are talking about, was that the wind chill factor or the apparent temperature once the WCF was subtracted?  In all likelihood they have no idea themselves, but it's annoying when it comes from someone whose job it is to report the weather that they don't make that clear especially when they portray themselves as a meteorologist.  However, this is quite cold no matter what.  Once in Chi-town the temperature was near zero (F) and the wind was strong enough to generate something like a -50 degree wind chill.  Walking just two blocks was torture. Temperatures like this make it a great pleasure to walk into the local coffee shoppe this AM with its steamy roasted coffee humidity and aroma. Roasted coffee just has an appealing smell, and you also get to wrap both your hands around the cup.  Nice.  A colleague was sitting there with a pile of evolution exams to grade and he has no intention of going anywhere else for quite awhile.  Such a good idea.