Field of Science

Cold and plant hardiness

It's been cold, seriously cold, and while restating the obvious, it perhaps prompted a reader to ask about the effect of extreme wind chills upon plants.  OK, this is an easy one; wind chill doesn't affect plants.  Only the absolute temperature matters, not how the wind makes the cold feel to us and other animals. Recently our absolute temperature hit -17 F (-27 C), and this is very near the rock bottom hardiness for zone 5 plants. In TPP's 35 years here in northern Lincolnland, the coldest it has been is -19 F. This is the absolute limit for freezing-avoiders, a topic discussed before. Until spring you really don't know what damage has been done to what plants.  Some tender woody plants will die back, and many will sprout from the base, so be patient before cutting things back.  Our Vitex (chaste tree) will certainly be in this category.  Over the past couple of years all of it survived the winters and it had gotten 2.5+ m tall.  My Mother, a native southerner, had a pet mimosa tree in upstate New York that survived because it was planted where the ample snow fall always drifted providing it with insulation.  Granted the mimosa never grew much taller than 4 feet, but she like it anyways and in bloom it often stopped traffic.  So snow cover makes a big difference, as does mulching.  This year the deep cold came with snow, so low growing things and herbaceous perennials will probably be largely undamaged.  In a few cases TPP is worried because either he or the nurserymen were cheating by planting marginally hardy plants (TPP) or over stating the hardiness of some plants (a lot of plants labelled zone 5 on nursery tags are not so rated anywhere else).  OK, so you gamble.  But it will be another 4 months before the data can be collected about what was damaged and what was not.  Another problem associated with this is that winter kill is more often about dehydration, especially of evergreens, both conifers and other types, than it is about cold.  If you stopped watering newly planted evergreens when the weather got cooler, and drier, then you put them at risk.  Again, mulching helps greatly.  Now stay tuned because next TPP will explain about frost cracks in trees and why it happens and why it doesn't.

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