Sometimes our climate does not smoothly transition between seasons. It can experience a heat wave while it is moving toward winter, or a cold snap as it transitions to spring. This can stress and damage plants. For example, it occurred in the Fall of 2010, when the greater Wenatchee area (and North Central Washington generally) experienced a weather event similar to that which occurred in the Fall of 2008 and resulted in damage to, or the death of, many semi-hardy plants, most notably roses.
Temperatures in October 2010 were mild. This continued into November. In the first part of November, 2010 daytime temperatures ran mostly in the range of 50 to 60+ degrees Fahrenheit. Night time temperatures tended to stay above 30 degrees Fahrenheit, except for a few evenings when temperatures fell into the upper 20’s. Many people remarked on the way in which their roses continued to bloom. Then, about 18-19 November, both day and night time temperatures fell sharply with night time temperatures dropping to -1 to -5F in the period of about 22-23 November. Following this event, temperatures came back up somewhat, but remained cold.
Because of the continuing mild temperatures through October and into November, plants failed to “harden off” and go dormant, which would have prepared them for the coming cold. Instead, they stayed active, showing blooms and immature growth. These conditions made the plants extremely vulnerable to damage from the sudden low temperatures when they occurred.
As a result of that weather pattern, damage can be expected in any of a number of semi-hardy plants such as rhododendrons, roses, and laurel. In some cases the damage will be so severe as to cause the death of at least the above ground portions of the plants. Those plants which had been mulched many lose the above ground portion, but retain live root systems.
Among the woody shrubs and trees, the effects of the event may not fully appear until the summer up to two years later.
A caution: although plants may appear to have been damaged/killed we do not recommend immediate removal. Waiting until later in the spring may reveal their ability to recover from the roots.
This type of weather effect can also occur in the reverse calendar season. As plants awaken in rising Spring temperatures, a cold snap can damage emerging growth and flower bud development, severely enough to result in no flowers.
by Orv Vanderlin, Chelan County Master Gardener
Graph of Temperature Highs and Lows
This graph illustrates the high and low temperatures recorded at the WSU Tree Fruit Research Center in Wentachee, WA during the period October 15 to December 14, 2010.