Wednesday, January 30, 2019

The Impact of Wind Chill on Plants

Photo by Donna Duffy
Excerpted from Wind Chill Doesn’t Really Matter to a Plant, Mark Longstroth, Michigan State University Extension

In a typical Colorado winter, it isn’t uncommon to experience several days with wind chill between -10 to -20 degrees Fahrenheit. But what exactly is wind chill? Wind chill measures a combination of wind speed and temperature. In calm conditions, there is a fine layer of air called the boundary layer that insulates us from the cold. As the wind blows, it blows away this boundary layer and the cold wind can carry away heat from our bodies faster because there is no air insulating us. The faster the wind blows the more heat it can carry away. 

Photo courtesy
Wind chill impacts humans more than it impacts our outdoor plants. Plant temperature is usually close to air temperature. If the wind blows hard it cannot cool down the plant any colder than the air temperature. Conversely, on a sunny day, if the plant warms in the sun it may get much warmer than the air if the conditions are calm. If conditions are windy, then the plant will only get a little warmer than the air as heat is carried away more quickly.

Under cold conditions when much of the water in a plant is frozen, a strong, dry wind will carry away moisture and dehydrate the plant. For plants that retain their leaves or needles in the winter, desiccation is a greater problem when temperatures are above freezing and it is windy. 
Under calm conditions it can get much colder after the sun goes down. Without the wind to stir up the air, cold air collects close to the surface and flows into cold areas.

Snow is also a factor impacting your plant health in the winter. After the sun goes down, snow chills the air above it and without a wind to stir it up and mix with the warmer air above, a very cold layer develops just above the snow. Often we see the worst winter injury close to the ground just above the insulating snow. 

The bottom line about wind chill is summed up by Linda Chalker-Scott, Washington State University, on the Garden Professors Blog:
It's air temperature that matters - that's what will cause water to freeze. What wind chill can do is increase the rate of cooling. Wind chill does not actually make the temperature of the plant any cooler than the air temperature. Wind chill can cause damage by speeding up the cooling, causing ice to form faster than it would under normal conditions.

So when the wind chill is ferocious, bundle yourself up and know that your plants aren’t feeling the same effect as your human body.