When a plant is assigned a zone or temperature tolerance, is it the actual temperature of the air or is wind chill temperature counted? In other words, if I have a plant that tolerates temps to 20 degrees and the temp today is 28, but the wind chill is 15, is my plant still hardy?
Here is a link to the USDA Plant Hardiness Zone Map. This map is meant to help determine which plants grow well in a certain area and is based “on the average annual minimum winter temperature. To find an area’s hardiness zone, all one has to do is enter a zip code.
Apparently, wind chill does not affect deciduous plants. If your evergreen plants are sheltered from the effects of the wind, then they may be protected from moisture loss. Below is additional information about plants and wind chill.
From Portland, OR area garden writer Ketzel Levine:
“Wind chill has little effect on deciduous plants, so if we’re talking forsythia and blueberry, it’s not an issue. Of course it does becomes an
issue if the subject is broadleaf evergreens (boxwood, hollies, osmanthus, etc.) and to a lesser degree, needled evergreens. Sub-freezing winds rob leaves of moisture, a serious problem if a plant continues to photosynthesize through winter. But the USDA zones do not take wind chill into consideration, and if I were you, I wouldn’t either. If you’re not a risk taker, stick with Z6 plants; otherwise, let the cold winds blow.”
From Rob Gough, Montana State University Extension agent:
“Do plants feel wind chill like we do? It sure feels a lot colder to us when the wind is blowing. It makes 30° feel like 0°. Meteorologists express this feeling by the term ‘wind chill’ and we often hear wind chill advisories on the local radio station. Do plants feel wind chill? Does a wind chill of 25° below make the plant react as though it were that cold?
The answer is in the word ‘feel.’ The term ‘wind chill’ was developed to express how the combination of wind speed and temperature ‘feel’ on exposed skin. The skin has nerves which transmit that feeling to the brain and we say, ‘Boy, it sure feels a lot colder than 30 degrees with all this wind.’
But the plant doesn’t feel. It has no nerves to transmit that impulse. So to use the term ‘wind chill’ in relation to plants is meaningless. But that’s not to say that wind and cold do not affect plants. Last time I told you how cold influences plants. Wind plays an important role too. Wind can increase the evaporation of soil moisture, thus speeding drying and making water harder for the plant to come by. Wind also speeds evaporation of moisture from the plant surface. Even without leaves, deciduous plants can lose moisture through their young bark. The faster the wind, the faster moisture is lost.
You may notice the windward side of your evergreens are scorched, or burned. The needles are brown or reddish on that side. That’s windburn and it’s simply desiccation of tissue caused by the wind. Again, like cold damage, wind damage is also the result of drying out of the tissues. If you want to know more about winter damage, contact your county extension office.”