Growing degree days: nature’s timetable


OSU Ext. master gardener volunteer

As we all know, Ohio’s weather is unpredictable. Some years, spring comes early and gardeners can get a head start and have a successful year. Other years, winter drags on, making our springs late. Either way, late frosts can set gardeners back. With this uncertainty, gardeners have learned they can’t rely on the calendar alone when it comes to Mother Nature.

But what else can be used to time horticultural and pest-management activities? Luckily, the relative progress of the growing season can be estimated using a measure of accumulated heat called growing degree days (GDD).

GDD information helps gardeners anticipate when buds and blooms on landscape plants will appear and the ripening of vegetables in the garden. Farmers use GDDs to know when the bad pests will impact their growing crops. GDDs are used to time herbicide application on lawns to control broadleaf weeds. GDDs are also used to time the development stages of pest insects for the most-effective use of pest controls as part of an Integrated Pest Management plan.

The development of plants and insects are both temperature-dependent. Both respond to accumulating heat. GDD is used to capture heating data and use this data to predict the development of plants and insects. But how do you measure accumulating heat?

A growing degree day is the amount of heat that accumulates above a base temperature during a 24-hour period. For the Northeastern United States, including Ohio, a base temperature of 50 degrees is used when calculating GDD. GDDs accumulate anytime the average temperature for a day is more than 50 degrees. Gardening GDD calculations are made each year beginning the first day of March.

Fortunately, no one in Ohio has to calculate and track GDDs on their own. The Ohio Agricultural Research and Development Center (OARDC) does it for any ZIP code in Ohio. Visit For example, if Japanese beetles plague your grapes and roses every year, you simply need to check the website starting in early June to see when the first ones will appear. Knowing this will help you control this insect before it does its damage.

For a better understanding of growing degree days, visit

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