Cold temperatures may have damaged plants

Marianne Ophardt, horticulturist for WSU Benton County ExtensionDecember 12, 2013 

It has been so cold, gardeners are wondering if the frigid weather damaged their landscape and garden plants?

Here are some factors involved in cold-weather damage to plants:

-- Hardiness and acclimation: In autumn, as the days shorten and the temperatures cool, a plant's ability to withstand the temperatures increases. Plant tissues become hardy, and plants reach their maximum potential hardiness in mid-winter. This process is called acclimation and involves complex physiological changes. When there are frigid temperatures, the time of winter and the temperatures in the weeks preceding the cold weather, as well as the severity and duration, are factors in whether a plant will experience damage.

-- Maximum hardiness and zones: Plants gain their maximum genetically determined potential winter hardiness in mid-winter. This is genetically determined, but is also is influenced by weather conditions, plant exposure and plant health. To see if a particular plant is hardy enough to withstand the extreme temperatures, consult the USDA Plant Hardiness Zone Map.

Using weather data from a span of 30 years (1976-2005), the map is based on the average extreme annual minimum temperatures across the country. Each of the 13 zones on the USDA Hardiness Zone map represents a difference of 10 degrees, and the divisions of "a" and "b" in a zone indicate a 5-degree difference. The Tri-Cities is rated as "Zone 7a" with the average extreme minimum temperature being 0 to 5 degrees. The coldest parts of Benton and Franklin counties are in "Zone 6b," with the average extreme minimum temperature being 5 to minus 5 degrees.

Trees, shrubs and perennials are rated as hardy in a particular zone based on the coldest temperatures they can survive. For example, a Zone 7 plant should survive the average coldest weather in the Tri-Cities -- if it has achieved its maximum hardiness.

-- Is wind chill a factor? Humans and animals are subject to wind chill because it is an index created to reflect the heat loss that occurs to warm blooded animals from wind when it is cold outdoors.

The wind chill index is not a concern when it comes to plants, but winds can have a drying affect, especially on evergreen plants like pines and rhododendrons.

That is why it is important to not let plants go into winter drought-stressed, and to water trees and shrubs during mild fall and winter when the soil isn't frozen.

-- Will plants have escaped damage from our recent weather? Winter is far from over, so it is hard to predict, but I suspect we may see some damage. The plants most likely to have been damaged are Zone 7 (or above), those planted in late fall and those that were unhealthy or drought stressed before the frigid weather arrived.

One factor that probably limited the damage and aided in the acclimation process was the fairly cold weather we were experiencing before the low temperatures.

-- Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.

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