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Wintry weather worries Mid-Columbia gardeners

KENNEWICK -- Here it is the last week of November and snow is falling outside and there's at least 6 inches of the white stuff blanketing the ground.

It's hard for me to reconcile that last week we were raking up leaves and this week we're shoveling snow.

Apparently, La Nia is strengthening over the Pacific Ocean and meteorologists are prognosticating that we won't have as mild of a winter as we experienced last year.

Some long range forecasters are predicting that the Northwest will be getting the worst of the country's winter cold and snow along with the northern Plains and the western Great Lakes.

However, the Oregon State Department of Agriculture, or OSDA, predicts that there will be above normal mountain snow with wet and stormy conditions statewide through December. OSDA also predicts cooler than normal west-side temperatures and milder conditions on the east side through the winter.

The good news is that they are predicting above normal snowpack in the mountains.

I'm talking about the weather because the abnormally early frigid air this week could damage some of our garden and landscape plants because its coming so quickly after a period of fairly mild weather. Plants go through a physiological process in the fall that prepares their tissues for winter's cold temperatures. Their tissues gradually "harden" or acclimatize themselves to cooler temperatures, attaining their maximum hardiness in mid-winter when the coldest temperatures normally occur.

Keep in mind this region's average low temperature for November is 36 degrees while our average low for December and January is about 28 degrees. November temperatures in the 20s certainly is not normal, but the blanket of snow during this cold weather should provide some insulation to plants that aren't any more ready for winter than I am!

Weather is a common topic of conversation for many, but especially for gardeners who worry about their plants. Here are two frequently asked winter "weather" questions asked by gardeners:

Q: Is wind chill a concern when it comes to trees, shrubs and perennials?

A: The wind chill index was developed in terms of warm-blooded animals, including humans, to indicate heat loss based on the factors of air temperature and wind. Since plants aren't warm blooded and can't feel, the wind chill factor does not affect plants. If the air temperature is 5 degrees and the wind chill factor is minus 15 degrees, it's only the 5 degree temperature that affects a plant.

Q: When gardeners talk about "hardiness zones," what do they mean?

A: USDA developed a map to help gardeners know which landscape plants could be planted in different parts of the country and survive the average coldest winter temperatures experienced in those regions. The USDA map divides North America into 11 zones. Based on climate data for the average annual minimum temperatures, each zone is 10 degrees warmer or cooler than the adjacent zone.

The USDA Hardiness Zone Map uses temperature data from 1974-86 for the United States.

On the USDA map, the Tri-Cities is in Zone 6, with minimum temperatures from 0 to minus 10 degrees.

The Arbor Day Foundation has used more recent temperature data to reflect climate changes. On its map, we are in Zone 7 with minimum temperatures from 0 to 10 degrees. Trees, shrubs and perennial plants are rated as to their hardiness, such as flowering dogwood, which is rated as hardy in Zones 5 to 8.

Well, our plants can't sit inside with a cup of tea and a blanket, but we can. Stay warm!

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

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