Lack of Heat Unit Accumulation Marks 2009's Unusual Growing Season

Lack of Heat Unit Accumulation Marks 2009's Unusual Growing Season

Looking back at 2009 growing season, you can see how lack of growing degree days made a difference in crop performance.

The 2009 growing season will be remembered as producing good yields with very wet corn and soybeans. It reminds us of 1992, when like this year, piles of grain and the whine of grain dryers filled the Iowa countryside.

When considering heat units, maybe lack of heat is a better description to use in this recap of the 2009 growing season. The degree day departure from average graphic shows how the season progressed by crop reporting district in Iowa, says Rich Pope, Iowa State University Extension program specialist in Integrated Crop Management. He compiled and issued the following report last week; looking at the effect of heat units, rainfall and the growing season on corn and soybean  performance and yields in 2009.

This year, the state was remarkably consistent in the amount of heat units that accumulated, with northwest and central Iowa faring just slightly better then the rest of the state. July, late August, and October were all very cold compared with historical records, with July ranking as the coldest on record, and October was in the coldest five.


For most of Iowa, rainfall throughout the season was pretty average, with the slight exception in the east-central and southeast districts, where rainfall was a bit above normal from July on.

There were some very localized exceptions associated with summer thunderstorms that brought locally heavy rain to small areas. Two of those storms were notable because of devastating hail that destroyed crops and caused other damage. The first, on July 24, cut a swath from about Calmar in Winnesheik County to western Dubuque County. The second was a remarkable storm that stripped crops from Ida County in western Iowa to Grundy County in eastern Iowa, and caused particularly intense damage in Hardin County near Eldora and near Callendar in Webster and Calhoun counties. 

This August 10 storm was unusual in that it formed and persisted across Iowa in the mid- to late morning, associated with a slow moving cold front.

The two graphics (below) from NASA show an image of the damage from outerspace that was taken on August 23. Note the defoliation scars that document the extent of these events. ISU researchers have been analyzing damaged ears from these areas for ear rots and potential mycotoxin formation.


July 24, 2009 storm track, northeast Iowa


August 10, 2009 storm track, central Iowa

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