Will Spring Freezes Be a Problem for Fruit Trees This Year?

Chilling Hours Map- Area 120. Midwestern Regional Climate Center.

Factors determining when plants come out of dormancy can be very complicated, and not all are fully understood. One way to look at it is if we look at fruit trees, the more common ones to this area generally need about 800 or more chill hours to break dormancy.  Chill hours are number of hours between 32 and 45 F.  When the temperature is fairly warm, and night temps above 45 F, little to no chill hours are accumulated.  Likewise, when it is cold and temps during the day do not reach 32 F no chill hours are accumulated (think early January this year).  According to the Midwestern Regional Climate Center Chilling Hours Map out of Purdue, the area around Cedar Rapids and to the south is currently at about 700-800 chill hours.  The southeastern part of Iowa along the Mississippi looks to be at slightly higher levels.  The western part of Iowa is closer to 600-700 chill hours.  The chill hours are around 100 hours above normal for this time of year.  Progression of bud swelling generally starts to take place above 50 F degree days.  Degree days are the daytime high added to low and divided by two and subtracted by 50.  Thus, if high is 60 F and low is 40 F there are no degree days.  If high is 70 F and low is 50 F there are 10 degree days.  Apples generally bloom at about 400 degree days.  If the chill requirement has not been met degree days do not count.

The bottom line is we are good for now, but for the rest of this month the less hours between 32 and 45 F the better.  Since we will probably exceed 800 chill hours sometime this month in the eastern part of the state, the hope for March and April is to keep the total accumulated degree days below 400.  The worst-case scenario would be to get several days of highs in the 70s and 80s in March and early April like we did in 2012.  This caused fruit trees to bloom in mid-April instead of the normal early May.  Subsequently there were hard frosts in late April that wiped out most of the tree fruit crop in Iowa that year.


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