Friday, January 24, 2014

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://jack-rosarian.blogspot.com/2013/09/winter-protecting-your-roses.html

and "How Winter Affects Roses":

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

Jack Falker
January 24, 2014










8 comments:

  1. Hi Jack--Another interesting and informative post. I have a question for you. If the wind does dessicate the roses (and we know it does), does that in turn make them more susceptible to the wind chill factor. I agree that 32° is 32° regardless of wind chill. However, if the roses aren't hydrated and there is a 40 mph wind, wouldn't their internal structure be damaged as a result? The roses in my front bed are in the open. The wind comes fiercely down off of the Cascade Mountains as there are power lines going straight up and over them several blocks from my house. Now, whether it is wind chill or not, these roses that are the same hardiness as those in my back beds that are protected by several different fences including my own from this wind, have a more difficult time surviving the winter. As a result, I now use more protection on the roses in the front. I understand that even on my own property there are numerous micro-climates. Next to my home is 10° warmer than in the middle of my backyard. Next to the northern fence, it is colder than the southern fence where the wind is blocked. (I've measured these temperatures.) It would seem that even non-warm blooded living things would be affected by the chill regardless of the temperature. Their bark, etc., would be dried out. In a non-direct, but consequential way and based on 29 years of observation, I think windchill does more seriously effect the roses in my front flower bed. What do you think? Sue

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    2. Sue... Of course, you're right, cold wind hastens desiccation but it doesn't make the plants any colder, temperature wise, which is the implication of the windchill factor that the weather people bandy about all the time. As for micro-climates, I have them too. Right now it's -18.5 in the back of my house and -12.5 in front but that doesn't have anything to do with the wind. Right now the windchill is something like -39, which is how it would feel to my warm body, if I went out without covering up. In that sense, windchill is about relative warmth rather than relative cold. Read those attachments again; they're pretty good. p.s. I'd love to feel some cold Cascade air right now, vs the polar vortex :)

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    3. Regarding the above comments, it's worth repeating one of the paragraphs in the article:
      "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."

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  2. Jack, Thank you for the information. Very informative.
    Tom

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  3. Great post, Jack! Thanks for sharing...

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  4. Thanks, Jack. I understand that wind only hastens the time it takes for a rose to reach a cold temperature (or warm temperature, too, for that matter). I really wouldn't want to wish any of the cold Cascade air on you as it often comes from the Arctic, too. I see people doing their countdowns for spring and think I may start! Sue

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  5. This is great Jack! It is difficult to find good, pertinent content to share on the social networks with my customers this time of year. Consider this shared.

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