Washington State University Extension

Garden Tips


GARDEN TIPS – written by
Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA
published-DECEMBER 12, 2014

Weather-wise, this has been a strange fall. We had a wonderfully balmy October. Frost and cool temperatures were delayed well past October 15th, the average date of the first fall frost. Unfortunately, the extended abnormally warm weather was followed by an abrupt spell of bone-chilling low temperatures.

Many plants were just not ready for the severe cold and may have experienced damage from low temperatures that they could normally withstand in mid-winter. Further complicating the issue was the warmer-than-normal weather that followed this frigid spell. Then, the warm weather was immediately followed by another period of severe cold.

Gardeners knew these departures from the norm could mean trouble for the trees, shrubs, and perennial plants in our yards and gardens. The frost-killed leaves still clinging to trees were a hint that something was awry. The abnormal high and low fluctuating temperatures had prompted gardeners to ask me if they should anticipate dead or injured plants next spring. It is one of those questions that I can not answer with a simple “yes or no.”

First, we have to ask if a particular plant was dormant and ready for winter’s cold. Plants hardy in a particular region physiologically prepare or “acclimate” themselves for impending winter cold based on environmental cues. These two main environmental factors are shortening days and gradually cooling weather that work to slow plant growth until a plant becomes dormant. Each species of plant responds to our local cues somewhat differently because they are adapted to the cues from the environment in their native geographic region. The timing of these cues and the acclimation process can differ from species to species.

As fall and winter advance, hardy plants continue to acclimate until they reach “maximum mid-winter hardiness,” assuming the temperatures gradually become cooler. Fluctuations between warm and cold periods interrupt the process. During warm periods, plants de-acclimate or lose some of the hardiness they have achieved and then start to re-acclimate when it turns cooler again.

Environmental cues are not the only factors that figure into plants acclimating to cold weather. Cultural conditions that promote late season growth can delay dormancy and the acclimation process. Pruning and fertilizing in late summer or fall stimulate new growth and delay dormancy. Because of our arid climate we still need to water our plants in the fall, but excessive irrigation also encourages growth and slows the process.

Plants weakened from drought stress, insect or disease damage, environmental extremes (such as our extended record heat last summer), nutrient deficiencies, root problems, or physical injuries may not be able to acquire full potential hardiness. The same goes for plants planted in late summer or early fall that did not have a chance to become established fall.

To protect against cold temperature injury experts recommend:

1. Plant trees, shrubs, and perennials that are designated hardy for the area. (This is USDA Zone 6 for most of our region.)

2. Do not prune or fertilize your plants after the first of August.

3. Cut back on the frequency of watering in the fall, but avoid drought stressing your plants.

4. If planting or transplanting trees and shrubs in the fall, wait until they are dormant.

5. Apply protective mulches on tender plants only after the soil has cooled to 40 to 45 degrees Fahrenheit.

Should we anticipate damage on our trees, shrubs, and perennials? My answer is, “Probably, but only time will tell which plants and how severe the damage will be.”

Published: 12/12/2014 12:33 PM

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