What Does Cold-Hardy Really Mean?

On the surface we do know that some plants can withstand frosts and long winters. But just what is it that these plants do to become cold-hardy?

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  • Topic: Fall Gardening
  • Author: Cindy Bellinger
  • Keywords: winter, cold-hardy, zone, Other
  • Date: October 2002

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Gardeners toss around the term “cold-hardy” like we know what we’re talking about. On the surface we do know that some plants can withstand frosts and long winters. But just what is it that these plants do to become cold-hardy?

Even the simplest description will take you back to high school chemistry.

First, hardiness has everything to do with acclimation. Plants need to be exposed slowly to changing temperatures to make a safe transition from summer through fall and on to winter. Plants begin adapting during the photoperiod, when the day length shortens. Then when temperatures drop to 40 degrees F for several days, an even more powerful signal is given to plants to prepare for winter.

It’s these environmental cues that stimulate the physiological and biochemical changes that result in a greater tolerance for cold. And when considering what each plant manages to accomplish, you can’t help but respect all plants as tightly composed living systems.

The early metabolic changes that occur include producing higher concentrations of dissolved sugars, amino acids and other soluble organic molecules. Then within the cells a higher concentration of chemically-bound water begins. This helps with the elasticity of the protoplasm, which is what makes a plant remain resilient during freezing.

Outwardly, it’s during this stage that the fall colors appear. When a plant’s metabolism is altered, it breaks down the chlorophyll, which in turn makes the aspens turn yellow, the maples turn red, and the oaks change from green to golden orange. Autumns with more sunny days produce more colored leaves because the breakdown is slower.

What gives a plant the ability to withstand cold temperatures is the absorption of essential elements from the soil such as iron and zinc.

Finally, when plant tissues are first exposed to freezing temperatures, ice crystals physically form outside the cell membranes. The crystals grow larger as they pull water from the protoplasm. When temperatures warm back up, these crystals melt and release water back into the cells. This action re-hydrates the plant so it can resume metabolism. In frost-sensitive plants, the water does not enter back into the system, and the plant eventually dies.

Hardiness is a complex relationship of factors, something us normal gardeners really don’t need to understand. We can leave the details to the researchers. It just helps knowing some of the finer points that take place inside plants as they move into winter. It lets us appreciate them even more.