Tuesday, January 6, 2015

Don't "Sweat" the Wind-Chill on Your Roses!

Tonight is likely to be the coldest night of winter 2015, in the Twin Cities.  Temperatures are predicted to fall to -13 F (-25 C) at Minneapolis-St. Paul Airport, which, in the context of winter history in Minnesota, is  no big deal (It's 0 F  [-18 C]  right now at 9 PM). That will put us in the upper reaches of USDA Zone 5 for the year, i.e. approaching Zone 6, when the USDA still has us listed in Zone 4 (incorrect in my opinion).

Author's note: The actual low temperature recorded on the night mentioned above (January 6, 2015) was -11 F, which should be the extreme minimum temperature (EMT) for 2015.  This puts the Twin Cities (MSP) just one degree shy of a USDA Zone 6 winter in 2015. 

But to listen to the apoplectic, Twin Cities TV weather folks creating "shock and awe" among their listeners, the real news is that wind-chills are going to reach a "dangerous" -30 to -35 F (-34 to -37 C) by tomorrow morning.  By doing this every day in the winter, they have created the idea among a very large number of people that temperatures are much colder than they really are.  In point of fact, wind-chill is only relevant to the cooling of the exposed flesh of warm-blooded animals (with no fur like us, unless you grow a beard). It has nothing to do with the temperature of cars or houses or, most importantly, plants, which in our case means ROSES! (For a full explanation of the effects of windchill on roses, see the quotes below from my wind-chill blog of last January).

Looking at it another way, if you're foolish enough to run around outside in your birthday-suit tomorrow morning in Minneapolis you're going to freeze your "you-know-what" off in the "relative" -30 to -35 F wind-chill, because of the effect of the cold temperatures on your exposed flesh, plus the wind which doesn't allow your body to warm itself. But if you dress warmly with a coat, hat, gloves, ear-protection and maybe a scarf over your nose, you have nothing to worry about, except the real ambient temperature of -13, which is cold enough, without trying to make it sound worse.

Here a couple of images of one of my Buck Earthsong beds taken last year at this time, when we had a lot more snow than we have now, which was a good thing then:

And here are several quotes from my article "How Windchill Affects Roses" from last year at this time:

First, from a National Weather Service article:

"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)."

And 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 another:

"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.

And this one is important:

Here's an example: Suppose that the ambient temperature is 35 F and the wind is blowing 30 MPH. According to an NWS  chart, 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."

And here's that whole article from last January:


Let me know if you have any questions.  I'll be safely bundled up tomorrow morning when I go outside.

Jack Falker

1 comment:

  1. Oh WOW, i have never seen snow in real life so i had a great time watching the photos. I love winters but -25 degrees is indeed too cold to handle!