By Jeff Burbrink
Ag & Natural Resources Extension Educator, Purdue Extension LaGrange County

LAGRANGE — Many people are talking about how slow things were growing during our cooler than normal spring. Gardeners told me their tomatoes were just sitting there. Farmers were concerned that their corn and beans were not making any progress. It felt like the warm season crops were just eeking along, staying short, yellow and seemingly stunted.

Jeff Burbrink

When the first 80-degree temps finally arrived, you could almost see the plant growth take off. Meanwhile, cool season plants like cabbage, broccoli, cauliflower and the bluegrass in your lawn were doing quite well, thanks to the cooler air.

Agronomists have developed a way to measure temperature and use it to predict growth. The system is called Growing Degree Days or sometimes heat units. Most plants have a preferred temperature range in which they thrive. In the case of corn, temperatures between 50 and 86 are the ideal. In a year like this, GDD is much more useful than a calendar at predicting plant growth.

Each plant has its own preferences. Alfalfa is adapted to relatively cool weather and has a base temperature of 41 degrees Fahrenheit. In contrast, field corn, sweet corn, sorghum, and soybeans have a base temperature of 50 degrees. Tomatoes, a very tropical plant, thrive between 50 and 92 degrees, as long as they have plenty of water. There is little growth occurring below or above a crop’s base temperature preferences.

Here is an example how the calculation is made: Imagine a day where the recorded low temperature is 60 degrees, while the high is 80 degrees. The base temperature for corn is 50 degrees. The average temperature for the day is 70 degrees [(60+80)/2 = 70]. The GDD accumulated that particular day is 20 GDD (70-50).

What happen when the temperature falls below the base? Let’s say we had 45 degrees as the low, and 75 degrees as the high. Since there is essentially no growth below 50 degrees, we adjust the calculation. Our average temperature is 62.5 degrees [(75+50)/2]. And our GDD accumulation is 12.5 GDD for that day (62.5-50).

If the temperature exceeds the upper temperature range (86 degrees for corn), the GDD is similarly adjusted to reflect the crops preference. For instance, if our low for the day is 74 and the high is 93, the average temperature is 80 degree [(86+74)/2] and the GDD accumulation for the day is 30 GDD (80-50) for a corn crop.

How is GDD used? GDD have become an important way to predict growth. The seed corn industry knows very precisely how many GDD it takes to produce a tassel and ears on all of their seed corn parent lines, helping them predict when to plant the “male” rows days after the “female” plants are in the ground. That is important, because you want the pollen to be shedding when the ear-bearing “female” plants are producing the silk.

GDD are also used to track insect growth. Alfalfa weevil, a common pest locally, is very predictable. Using 48 degrees as the base, we are able to predict when the weevil eggs hatch (300 GDD), and when the damage is likely to begin (375 GDD). We also know that after we pass 800 GDD, we are likely not to find the larva feeding on alfalfa anymore.

Many ag and horticulture industries use GDD to predict and schedule planting and treatments during the year. The turf industry uses GDD to predict the optimum timing for crabgrass preventer, Japanese Beetle emergence, dollar spot treatments, and other forms of weed, disease and insect control.

Michigan State coordinates a website that tracks GDD as several temperature baselines. It uses data from the National Weather Service and your zipcode to report the accumulated GDDs and also predict a week in advance in your precise location. While the site is geared towards the turf industry, I find it useful as a place to look up our GDDs for all crops and insects. And they do all the math for me!