Q: I have not been able to absorb the science of growing degree days. I get the idea of it — in my terms, it seems to be a measure of the increasing warmth of nature as it shakes off winter. But what is a person supposed to do with this information?
— Lorry Kormanik, Copley Township
A: You’re correct. Basically, growing degree days are a cumulative total of the warmth we’ve experienced in a given year. They’re determined by a mathematical formula, but suffice it to say we don’t start accumulating growing degree days until the days get warm enough for most plant growth to kick into gear.
Probably the most practical use for growing degree days is their ability to help us practice integrated pest management. It allows us to time pest treatments most effectively with the least harm to the environment.
I’ll use my own experience as an example: A few years ago my euonymus plants became infested with tiny insects called euonymus scale. I know that my best chance of controlling the scale insects without blasting them with chemicals is to spray the plants with earth-friendly horticultural oil right when the scale eggs hatch and the baby bugs are most vulnerable. I also know from Ohio’s growing-degree-day calendar that those eggs hatch when we reach 406 GDDs. So every year, I keep checking the calendar, and I spray any existing eggs as soon as we hit that number.
Since various things in nature always happen at the same time, it’s possible to skip checking the calendar and just use natural phenomena as visual cues. In my case, I could keep an eye on the spirea plants in my neighborhood. Spirea first blooms at 406 GDDs, the same time the euonymus scale eggs hatch. So I could just watch for the first spirea blooms and then treat my euonymus scale as soon as I see them.
Ohio’s GDD calendar is at www.oardc.ohio-state.edu/gdd. It allows you to plug in your ZIP code, so you can get information specific to your area.
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