Growing Degree Day Counts Can Deceive

Useful tool for figuring when insects may appear. Tom J. Bechman

Published on: Jun 7, 2006

Parts of April were cool, parts weren't. May was clicking along until a cool spell in the middle of the month wrecked averages. Based on that recent memory, you might suspect Growing Degree Days through late May were running behind normal in Indiana.

According to National Weather Service information, reported in FMC's May 19 Hatch Trak, total accumulated growing degree days for the spring were running from around 100 to 200 ahead of normal in the northern half of the state, and up to 400 ahead of normal in southern Indiana, particularly southwest Indiana.

Growing Degree Days are simply a measure of heat units. Insects and plants tend to function and perform maturity-wise based on total heat unit accumulation, not whether there's a stretch of 10 days or more that are cool. That stretch, like the one in mid-May, definitely slows development, but the pest or crop may already have been running ahead of schedule. So even with a cool spell, crops and/or crop pests can mature quicker than normal.

My strawberry patch near Franklin is a prime indicator. My earliest variety has produced first berries anywhere form May 12 during a very warm spring a few seasons back to June 1, during a cool spring. This year the first picking of that variety came off around May 20-23, average to slightly ahead of schedule.

For the record this point in the spring is one of the fastest periods of GDD accumulation. It's why crops grow quickly, and why insects that depend upon GDD accumulation to mature, such as corn rootworms, reach active stage very quickly.

For example, based on 30-year data from the Indiana Ag Statistics Service, total accumulation of GDDS from May 23 to June 20 jumps from a 30-year average of 458 on the May date to 962 on the June date, nearly double.

The negative side, of course, is that if crops aren’t yet in the ground do to weather delays, they've missed a sizable portion of potential GDDs for the season. It's a hidden factor that you may not think about consciously when making late-planting decisions, but which lurks in the background. Late-planted crops depend upon continued warm weather to help make up for a late start.