The winter of 2008/09 was rough on broadleaf evergreens in Missouri landscapes. Boxwoods, hollies and southern magnolias have more brown tips on their leaves this spring than we have seen for a number of years. I have noticed that bayberry shrubs, which normally have nice green foliage in spring, are currently leafless in Central MO. Although we are still investigating the causes of damage in some cases, we can blame a lot of it on the weather.
Broadleaf evergreens are subject to browning from desiccation of their leaves by cold, dry winds in winter. If the cold does not last too long, broadleaf plants can take up water from the soil to replenish that which has been lost to the atmosphere. However, if the temperature stays below freezing for many days, water can not move through the frozen stems. If the soil is frozen, this problem is compounded. A review of weather condition over the past winter reveals that we had two periods which could easily explain the damage we are currently seeing.
Some of the damage may have been done just before Christmas. Weather data from a station at South Farm in Columbia show that the minimum air temperatures on December 21 and 22 were 2 and 0.5 degrees F, respectively. Maximum wind speed on December 21 was 36 mph. Then, on January 15 and 16, minimum air temperatures were -4 and -3 degrees F, respectively. On January 17, the soil froze to a depth of 8 inches and the average wind speed was 12 mph. Soil freezing to this depth is fairly unusual and may have been due to a combination of unusually cold temperatures and lack of snow cover. The soil actually remained frozen at 8 inches until February 8, with the average wind speed ranging from 6-12 mph over that 23 day period. A summary of weather data for this period can be viewed at http://agebb.missouri.edu/weather/stations/boone/framesfm.htm.
Another factor which may have contributed to evergreen damage is low soil moisture. From October 1 through February 7, we recorded a total of 5.4 inches of precipitation at South Farm. If evergreens were not irrigated in fall, they could have gone into these high stress periods under drought stress. Given the unusual combination of cold temperatures, frozen soil, strong winds and low precipitation, it is not surprising that we are seeing some ugly broadleaf evergreens this spring. The take home message from last winter is that we should plant broadleaf evergreens where they are protected from the wind, water them in the fall and mulch them well to prevent soil freezing. Also, keep in mind that the average minimum winter temperature in most of Missouri is supposed to be -10 to -15 degrees F. The winter if 08/09 may cause some of us to reconsider planting more of the marginally hardy, broadleaf evergreens that have survived several of our recent, mild winters.
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REVISED: September 30, 2015