Top 10 things to know about growing degree days

The ups and downs in temperatures over the last several weeks made us miss the snow and at the same time, made some great walking days in T-shirts. These temperature swings are not completely unusual, but they do impact bloom times and insect emergence. While we were ahead just over a week ago, we’re getting back to a steadier temperature. These steadier temperatures slow growing degree days.

Here are 10 things to know:

1. Growing Degree Days (GDD) are a measure of plant and insect development during a growing season. It is based on accumulating temperatures and is the best way to predict development. It is much more accurate than using notes from previous years or a conventional calendar.

2. The development of plants and insects are dependent on several factors: availability of water; access to sunlight; and nutrients / food. But acuminating temperatures is the main factor. GDDs let you know how much temperature accumulation there has been for a given date.

3. GDD are created when the average daily temperature exceeds a minimum threshold, usually 50 degrees. If the day’s temperature is above the threshold, one or more GDD are accumulated (added). Warmer days add more GDDs than cooler days. If the average temperature for a day is below a minimum, no GDDs are added. GDDs accumulate over time.

4. Each plant and insect has its own development timeline, determined by research and experimentation. Some plants and insects need more warmth for a long period than others to complete development. So as GDD continue to build up, more biological events happen.

5. Biological development of plants and insects always follows the same sequence. No exceptions. If you know the GDD, you can predict a plant or insect development for a given date using the GDD website.

6. Development never goes backwards. If the temperature stays below the minimum threshold, then development slows down or stops, but does not go backward. If temperatures get too cold for too long, then the plant or insect could be adversely affected.

7. To know the GDDs for a date and location in Ohio, use Ohio State University’s GDD calendar at http://go.osu.edu/growingdays. The site uses temperature information from OSU weather stations to calculate an estimated GDD by ZIP code.

8. The calendar will tell you the GDD is for today’s date (or dates in prior years), what development events have already happened (if any) and what development events are next. You can use this calendar to plan controls for insect pests and weeds.

9. For a specific spot like your home, keep in mind the GDD calendar is for your entire ZIP code. Your home and yard might experience slightly more or less warming than the calendar shows. One reason for this is microclimates such as a sunny, south-facing location that warms faster than a shady, north-facing location.

10. Phenology and GDD are complementary. Phenology is the study of the timing of the life-cycle events in plants and animals: flowering, leafing, hibernation, reproduction and migration. Phenology can also be used to predict events by watching changes in nature. For example, when the Eastern redbud first blooms, spotted lantern fly eggs are almost ready to hatch. Phenological data is used by scientists studying climate change.

Sprague is an Ohio State University Extension Master Gardener Volunteer in Mahoning County.


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