Simply Science: Growing Degree Days

5:36 PM, Jun 6, 2013   |    comments
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MINNETONKA, Minn. - In this week's Simply Science we take a step into the worlds of agriculture and math for a lesson in growing degree days.

"Growing degree days are a way of measuring where we are in the life-cycle of a plant," starts Chris Cook, Head of Agronomy at Syngenta.

Instead of regular days, folks in agriculture follow growing degree days, or growing degree units. That is a measure of heat accumulation that determines plant growth. 

Cook goes on to explain, "We get some heat and it starts to measure that heat and says, okay after we've gotten a certain amount of heat I'm going to take the next stage in my life."

That next stage might be sprouting from the ground, developing tassels, or ripening to full maturity.

"So from the day you plant it, everyday you start adding on growing degree units. For a 105 day hybrid corn plant, which is planted here in central Minnesota and near the Twin Cities, that would take approximately 2700 growing degree units to go from seed to harvest, adds Cook."

We collect 20 to 25 growing degree units per day during the growing season.

"The only way it counts is if its 50 degrees or more. Otherwise the plant's not growing.  At under 50 degrees you're just not getting any interactions within the plant, so we just don't count it.  Above 86 isn't counted either, because that's just too much heat," says Cook.

Figuring it out takes some math... finding the day's average temperature, minus the base temp specific to each plant.

Simply put by the Head of Syngenta Agronomy, "Below 50 degree you don't count it.  Above 86 you don't count it. You add your low plus your high for the day, you add them together. Divide them by two. And subtract the base, which for corn is 50."

Insects also follow growing degree days, rather than the calendar.  Keeping track of growing degree units helps farmers time pesticide use for those insects that arrive within a window of GDD's each year.

Home gardeners can work in growing degree units as well.  You probably don't want to spend your gardening days computing mathematical equations, so here is an apps that will keep track of your growing degree days for you:

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