Growing Degree Days

As stated by David Rhodes of Purdue University, the growth and develop - ment of both plants and insects is strongly temperature dependent. Their temperature-dependent growth and development is often described as requiring a certain number of "heat units". Daily heat units are calculated as the difference between the average of the daily maximum and minimum temperatures and a certain critical basal or developmental "threshold" temperature. The threshold temperature varies between species. For corn the threshold temperature is 50 degrees.

Farmers and gardeners can use Growing Degree Days based on “heat units” to forecast and avoid problems with pests and predict harvest. Growing Degree Days takes into account insects as well as many plants depend on environmental temperature for development (although some plants depend on day length more than temperature). The formula for calculating growing degree days is “maximum temperature + minimum temperature divided by 2, minus base temperature (for corn and most other warm season plants 50 degrees F) = GDD.”

This information is of great interest to me as this is the first year I have been involved in growing vegetables for market and not just my family. In the past I’d never considered the effect weather had on my crop beyond the health of the plant. When the corn was ready we picked and enjoyed.

In February we made our final plant selections based on taste and reported plant performance, choosing varieties we felt confident would meet the needs of the program and satisfy our customers. We also planned for farmers markets and calculated beginning harvest dates for local grocers. Our predicted date for beginning markets was mid July.

At my families farm I chose ‘Sugar Baby’ and ‘Ambrosia’ for sweet corn varieties. We planted on May 4th and I was excited to consider our having sweet corn, starting with Sugar Baby, around July 7th. We harvested our first sweet corn this past week, on July 22nd, over two weeks past what I had calculated.

We chose ‘Trinity’ and ‘Ambrosia’ for our organic sweet corn crop at Black Hawk Organics. Rain and wet fields kept us from planting on schedule and on May 23rd, almost two weeks later than planned, we finally got our corn in the ground, making our anticipated harvest to begin around July 21st. We are now predicting August 4th (our first u-pick field day) as when the corn will be ready. We’ve learned our first big lesson in market farming. NEVER count on the weather!

For more information on Growing Degree Days visit: http://www.isws.illinois.edu/warm/cropdata/ .