News release: Cold Damage

Tim Baker, MU Extension Horticulture Specialist
102 N. Main, Suite 1, Gallatin, MO 64640

Release Date: April 14, 2016

Headline: Cold Damage

Our recent cold snap on the morning of April 9 has been of concern to orchard owners, especially in the northern part of our area.  Apples were at the “pink” stage, and some blossoms had opened up into a full bloom.  The low temperatures encountered that morning were cold enough to give us some concern. Before I discuss what our situation might be, I thought I would give you a little background information.

First, we can divide cold damage into two types: cold damage that occurs during the dormant season, and cold damage that occurs during the spring.

Dormant season cold damage will occur if you are growing a plant too far north, and out of its native range.  The USDA cold hardiness zone map addresses this problem.  Do you want to grow Southern Magnolias in northern Minnesota?  Sorry, I’m afraid you’ll get winter injury.  They just can’t take that kind of winter.

You can get other types of dormant season injury, especially if the plant has not hardened off properly.  If you have a warm fall, and then a sudden cold snap occurs, even plants that normally would not get injury will have problems.

But that is not what happened this April.  In this case, the plants were coming out of dormancy.  Above normal temperatures in February and March promoted good spring growth. Then the mercury fell on April 9 to temperatures that may have caused significant damage in our fruit crop.

The key to spring damage is the stage of the flower bud.  I will use apple trees as an example. If apple trees are fully dormant, they can take very cold temperatures, even below zero, with no problem.  But when the buds start to swell, they cannot take those same cold temperatures without damage.  When an apple bud is at the “silver tip” stage, for example, a temperature of 15 degrees will take out approximately 10 percent of the blooms.  If it gets down to 2 degrees, you might see as much as a 90 percent loss.

As the buds progress, those critical temperatures rise.  At “half-inch green”, the temperatures for damage are 23 degrees and 15 degrees for a 10 percent and 90 percent bud kill.

At full bloom, it’s worse.  A temperature of 28 degrees will see a 10 percent kill, and if it gets down to 25 degrees, you’ll lose around 90 percent of your crop.

In Gallatin, I measured a low temperature that morning of 24 degrees.  That’s not good, needless to say.

There are other factors that go into whether damage is caused or not, such as the number of hours below a given temperature. Dr. Patrick Guinan, Missouri State Climatologist, sent me a map which shows the low and the number of hours below 25 degrees encountered that morning.

Albany had the longest time below 25 degrees, at 4 hours, and reached a low of 23 degrees.  Kirksville had 3 hours, and reached a low of 21 degrees.  St. Joseph didn’t have any hours below 25 degrees, since their low only reached 27 degrees.  But that still could have caused some damage.

The short message in this is that only time will tell the extent of damage. We should know in a few weeks.