CORVALLIS, Ore. – The formation of ice within a plant cell is lethal and is the most common way plants are injured in the winter.
But many plants in cold climate zones have evolved strategies for surviving sub-freezing temperatures. They cope by either tolerating or avoiding freezing, according to retired Oregon State University plant physiologist Les Fuchigami.
Some plants can tolerate freezing by forming ice outside the plant or outside the plant cell walls, in a process called extracellular freezing, said Fuchigami, who has been studying dormancy and cold hardiness of nursery and fruit crops for more than 30 years in OSU’s Department of Horticulture.
“When ice forms outside a plant’s cell walls, it is generally not lethal,” said Fuchigami.
Freeze-tolerant cells have ways of preventing ice from forming in the cell. Instead, water outside the cell freezes, thereby causing water from inside the cell to move outside the cell to form more ice, he explained.
The water moving out of the cell reduces the amount of freezable water within the cell. Inside plant cells, sugars and other things then become more concentrated, forming natural antifreeze.
Another way plants avoid freezing is by deep super cooling, staying in a liquid form below the melting point of water, zero degrees C.
Many species of plants fluctuate seasonally in their cold hardiness.
“Changes in hardiness occur in response to the changing day-length and temperature,” Fuchigami said. “From spring to summer, many plants are in a non-hardy state, where they will succumb soon after the first freezing event.
“The hardiness of plants increases in the fall and early winter in response to shortening day-length and/or low or freezing temperatures,” he added. “The development of hardiness in the fall and early winter is a slow, gradual process. The loss of hardiness in late winter and early spring is dependent on temperature. Plants lose hardiness faster with warmer temperatures. Once growth begins in the spring, plants lose their ability to acclimate and become susceptible to freezing temperatures.”
Roots are less hardy than stem or above ground tissues because they are exposed to less cold under the soil, Fuchigami said.
And in general, tissues or organs that are actively growing are the least hardy. So avoid fertilizing until risk of freeze is over.