For Release February 1, 2004

Winter Hardiness in Plants Not an Easy Thing to Explain

by Chuck Otte, Geary County Extension Agent

There are annual plants, plants that die at the end of the season and have to start from seed the next spring, and there are perennial plants, plants that survive and come back from some rootstock, crown or above ground woody part. Some plants that we grow in our fields and gardens are actually perennials if grown in a non freezing environment. Grain sorghum, peppers and snapdragons are all examples of this. These plants never evolved adaptations to help them survive freezing weather.

What makes a perennial winter hardy or cold tolerant? Basically, these plants have to be able to go dormant, which may or may not mean losing all their foliage. It usually means that the water content of the plant, both water within the cells and between the cells, has to be reduced so that the remaining sugars and other microscopic solids impart a sort of antifreeze in the plant. If ice crystals form to the extent that cells are ruptured, their will be damage.

Different plants have varying degrees of winter hardiness. This is where the plant hardiness zones were developed. Different locations are placed in twenty different zones based on average annual minimum temperature. Geary County is in zone 5b meaning that in an average winter we can expect temperatures of ten to fifteen degrees below zero. Averages are nice but averages can be deceiving.

We may go through several years in a row where we don't see temperatures below zero. That would be equivalent to a zone 7a. So we plant some landscape plants that are hardy to zone 7 and we have several mild winters and they do fine. Then we have a winter like December of 1989 and it hits 23 below zero. All of a sudden we are into a zone 4b. If our landscape plants are 5b plants, they'll probably survive. But if we have a lot of zone 7a plants, they are probably going to die or seriously injured.

Now let's complicate life even more. We have plants that are zone 4 and 5 rated. We expect them to survive any of our winters. We have a mild fall and all of a sudden on Halloween we hit zero degrees. Or all of a sudden on Valentine's Day it's 70 degrees, and it stays warm like that for ten days. But then by late February we are back down into single digit temperatures. Even those zone 4 and 5 plants are going to be damaged by temperatures that never got below zero. They got damaged because sudden changes didn't allow them to go into dormancy in a normal fashion, or they were coaxed out of dormancy too early by a late winter warm spell.

We have now had a week of temperatures that ranged from just below zero to not quite up to freezing. Those temperatures, in and of themselves, will not damage most of our hardy landscape plants. But couple that with dry soil or some of the ice, or any of the other unusual weather we've seen the last 36 months and we might expect some problems. Peach trees may not bloom next spring. They are still alive and growing well, but no blossoms, no peaches. Are they winter hardy, or not? It depends on your definition.

Finally, we have to take extra care of evergreen plants. The cold weather seldom bothers them, but dry or frozen soils prevents them from taking up soil moisture. And because they still have green leaf material, they carry on photosynthesis and this requires water. These plants won't winter kill from cold weather, they'll winter kill from dehydration. Winter hardiness isn't just one thing, like temperature. It's a whole complex of things and any one of them can spell trouble for your landscape plants!


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