Degree Days - Crops in the Cooler Again

August 31, 2009

By Rich Pope, Department of Plant Pathology

After three weeks of near-normal temperatures, Iowa weather wrapped up August with a return to colder-than-normal. For the week of Aug. 24 - 31, we gained on average 50 degree days fewer than normal.  So where does that put us? 

degree day summary for May 1 through August 30

Crop producers who have expressed a small but growing concern about an early frost now have a bit more reason for concern.  Corn silked rather late and many soybean fields had the onset of podset delayed. Even our most advanced corn still needs at least 3 weeks of open fall weather to reach maturity; many acres of soybeans have set fewer than typical numbers of pods.  Both crops will need good September grain filling weather to transport dry matter into the seeds and produce reasonable yields.

Last week Elwynn Taylor and I posted an ICM article that compared degree day deficits from normal throughout several seasons with the 2009 season.  I have updated the graph in that article to show the effects from the past week and posted it below. The last red line segment shows the noticeably cool weather the end of August.

Seasonal graph of degree day progress

The crops need clear, seasonally warm days filled with ample sunshine. And if frost holds off until well into October, we will all be grateful,



Rich Pope is a program specialist with responsibilities with Integrated Pest Management. Pope can be contacted at or by calling (515) 294-5899.

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