Protect your plants from cold-weather threat

By Marianne C. Ophardt, Special to the Herald

KENNEWICK -- It's been a long time since we experienced such bone chilling weather.

Gardeners can't help being worried about the affects these cold temperatures will have on some of their landscape plants. They should definitely be worried if they were lulled into a false sense of security by the warmer winters of the past several years.

This area is located in Zone 6b of the USDA plant hardiness zone map. Plants rated for "Zone 6" are generally considered safe or "hardy" for our region and should be able to withstand temperatures to minus-5 degrees without damage. However, local gardeners who ignored the hardiness zone ratings of the plants they placed in their landscapes may encounter damage on those plants that aren't fully hardy for this area.

Many gardeners, including myself, have tempted fate by planting some plants that are only hardy to Zone 7 (zero to 10 degrees). Gardeners who like to live dangerously may even have planted Zone 8 plants (10-20 degrees). How have these plants fared? We'll have to wait until spring to find out.

Luckily, the timing of this recent Arctic blast coincides with the time of year that many landscape plants attain their maximum hardiness and ability to withstand the coldest temperatures of winter. The physiological process that enables plants to endure cold temperatures below freezing is called "acclimation."

In fall, acclimation is triggered by shortening daylight and cooler temperatures. Increasingly cooler temperatures advance the acclimation process to the maximum cold temperatures a plant can withstand without damage.

Since this is a physiological process, there are a number of things that can interfere with or delay a plant obtaining its maximum hardiness during the winter. One thing is the weather. If temperatures are very mild during the fall, dormancy and the onset of a plant's hardiness will be delayed.

Once a plant finally attains its maximum winter hardiness, a period of extremely mild temperatures causes the plant to become less hardy or "de-acclimate." The warmer the weather and the longer its duration, the greater the loss of hardiness. Sudden big drops in temperature right after a period of warmer weather can lead to injury even if it's mid-winter when the plant should be at its hardiest.

Will the mild temperatures of the past month have affected the hardiness of our plants? Maybe.

We can't do anything about the weather except talk about it. However, there are some cultural practices that promote late season plant growth at a time when growth should be stopping. This hinders the acclimation process. Cultural practices that encourage late-season growth should be avoided in the fall. These practices include:

1. Fertilizer applications in the fall. It stimulates new growth and should be avoided. This is difficult if you have trees or shrubs located in or near to the lawn where two fall applications of fertilizer are advised. Try to avoid fertilizing near landscape plants, especially those that aren't completely hardy for this area.

2. Watering heavily during the fall until irrigation water is finally shut off. This also keeps plants in a vigorous and growing state. It's best to gradually reduce the frequency of watering with the onset of cooler weather in the fall. However, it's important not to drought stress landscape plants during the fall, in hopes of rushing them into a dormant state. Research has shown that drought stress in the fall makes plant more vulnerable to winter injury from cold temperatures.

3. Almost any time of year, except late summer and fall, is suitable for pruning woody plants. Pruning late in the growing season can stimulate new growth, also impeding the natural acclimation process. Late winter, when plants are fully dormant and buds are starting to swell is one of the best times to prune trees and shrubs. Deadwood can be removed safely at any time of year and insect or disease infected wood should be removed when noticed regardless of the time of year.

Problems that compromise a plant's health, such as disease, insects and root damage, also will affect a plant's potential hardiness. Young plants also are more vulnerable to winter cold damage, as well as plants that were transplanted in the fall.

All we can do for now is talk about the weather and hope our plants are hardy enough to endure the cold weather along with us.

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

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Saturday, Dec. 20, 2008

Protect your plants from cold-weather threat

By Marianne C. Ophardt, Special to the Herald