How Windchill Affects Roses

Yesterday, we woke up in Minneapolis to a temperature of -17 F, with a windchill of -36 F . Every TV weather-person in town was wide-eyedly proclaiming the dangerous windchill temperatures, to the point, I'm sure, of scaring the average person, who had to go out of the house to work or school, half out of their wits. In fact, every school district cancelled school for the second time this winter, due to the windchill temperature.  Our most sophisticated local (and national) meteorologist, Paul Douglas, posed in his blog yesterday his rhetorical question of whether they cancel school for windchill temps in Canada?  It's no wonder people start believing that the temperature really is -36 in Minnesota, when -17 is quite bad enough.

Question: What temperature do you think our roses were "feeling" yesterday morning?

Simple Answer: -17 F.  Neither roses nor any other plant experience any temperature other than the actual ambient temperature.  And -17 is plenty cold, thank you very much.

Here's a good quote from a National Weather Service article about windchill:

"Wind chill is the term used to describe the rate of heat loss on the human body resulting from the combined effect of low temperature and wind.  As winds increase, heat is carried away from the body at a faster rate, driving down both the skin temperature and eventually the internal body temperature.  While exposure to low wind chills can be life threatening to both humans and animals alike, the only effect that wind chill has on inanimate objects, such as vehicles, is that it shortens the time that it takes the object to cool to the actual air temperature (it cannot cool the object down below that temperature)."

Here's that whole article

http://www.crh.noaa.gov/ddc/?n=windchill

Here's another good quote from a Kansas State University article:

Plants Don’t Care if the Wind Chill Tanks

"When wind chill temperatures plummet, gardeners chafe about their landscape and fruit plants' odds for survival.  Some gardeners worry too much.... Cold can be a killer if people are growing marginally hardy plants or if air temperatures drop well below what's usual where they live.  Hard freezes are particularly destructive when plants aren't fully dormant.  But cold and wind chill aren't the same thing.  Wind chill only affects warm-blooded animals -- including people.  It's an indexed, scientific measure of how wind speed and air temperature combine to impact animal heat loss.... We know, for example, that our heat-loss rate will speed up as the air temperature drops.  The faster the wind is blowing, however, the more dramatic that heat loss is going to be .... Wind chill has no meaning for plants.  Unlike warm-blooded animals, they don't try to maintain a particular body temperature year-round".

And here's that whole article:

http://www.ksre.ksu.edu/news/story/wind_chill121709.aspx

Of course, we know that  roses feel the winter cold and die back according to the level of protection afforded them.  And winter-winds do, of course, have an effect on that die-back, desiccating the canes, but the important thing to understand is that wind does not make a plant "feel" colder than the actual temperature, even though it shortens the time it takes for the plant to reach that temperature.

Here's an example: Suppose that the ambient temperature is 35 F and the wind is blowing 30 MPH. According to the chart in the NWS article (above), the wind chill is 22 F.  So are your roses freezing?  Or, better yet, are the puddles in your garden freezing?  Of course not, because the freezing point of water is 32 F.  However, if you go out in your garden without a hat and jacket, you will feel like it is 22, not 35, because of the combined effects of the cold temperature and the high wind on your flesh.

Another example of the effect of wind chill on the human body is to go back to yesterday's -17/-36 situation.  If I go out for a walk, as I usually do in these temperatures, any exposed skin (like my nose) will be frostbitten in about five minutes because of the -36 wind chill.  That is a serious problem, especially for children waiting for school buses or people who have to work outdoors.  Children are notoriously bad at covering-up in cold weather and, thus, it is probably wise to cancel schools (in Canada too).  And, while my plants certainly are not feeling the wind chill, the old Minnesota Rose Gardener certainly is, so he has to dress accordingly in his thinsulate-lined parka and antarctic mask.

The temperature was about -2 and the wind-chill about -15 when this picture was taken.

Cold enough for ya?  It sure was for me!

So, bundle up and make sure your roses get the right winter-protection each winter.  See my blogs "Winter Protecting Your Roses":

http://theminnesotarosegardener.blogspot.com/2013/09/winter-protecting-your-roses.html

and "How Winter Affects Roses":

http://theminnesotarosegardener.blogspot.com/2013/12/how-winter-affects-roses.html


Jack Falker
January 24, 2014