Have you ever heard, if you don’t like the weather in northwest Ohio or southeast Michigan, just wait — it will change?
From a recent tease from Mother Nature of warm weather, to rain and wind, to snow last week for some of our northern readers, and then back to spring — things are always changing.
It is those changes that make it nearly impossible to predict which day your dogwood tree is going to bloom, or which day the monarchs will return. Those events are not like your birthday — something that happens every year on the same day, but rather, it can be close but different each year, usually within a week to three weeks, depending. But there is something called growing degree days (GDD) and plant phenology, a biological calendar that happens in a sequence, but not tied to a specific date or time.
Over the years, I have tried to intertwine the topic of GDD and plant phenology into many programs and even into the garden column. If you know me, you know that I like insects — even the unlikable ones — but today’s topic really connects the timing of plants and pests. Can it get any better? I hope that you find this tool as useful as I do.
While you can dig deep into this topic, I liken it to a biological calendar that incorporates the beauty of tree and shrub blooms and insect activity into one comprehensive list. While this calendar isn’t tied to a specific date from year to year, the sequence or order remains the same each and every year. And that should make sense, as both plants and insects depend on the weather to make progress in their development.
And while I could spend the entire column expanding on the topic’s background, I will share a couple of my favorite links, with brief explanations, and then jump in with two feet as we look at how this year compares to year’s past, the numbers, and what that means.
On the Ohio State phenology calendar website, weather.cfaes.osu.edu/gdd/, click on the link and type in your Ohio zip code and the website will do the work. Not from Ohio, but rather a neighboring state? You can use the closest Ohio zip code to your location, or see if your state has a similar resource.
I have used 43615 for the work that I have done, and it is my office’s zip code at the Toledo Botanical Garden. Once you see the list, you can compare what you are seeing in your own landscape, neighborhood or community and where you fall on the list.
You can click on the link to see the latest summary, and feel free to search additional summaries by searching GDD on the website. You can manipulate the Ohio zip code and the date if you want to see what was happening on a specific date in history, or compare years of data.
The warmer the temperatures, the more GDD units we accumulate and rapidly move through the biological calendar. This model uses a base temperature of 50 degrees. This means that when temperatures stay below the base temperature, there is not progression through the plant and pest species listed in the calendar.
Let’s break it down and look back in time. As of Friday, we have accumulated 137 GDD units. The GDD on April 11 in previous years looked like this: 2024: 157, 2023: 107, 2022: 98, 2021: 192, 2020: 83, 2019: 72, 2018: 57, 2017: 174, 2016: 122, 2015: 53, 2014: 42, 2013: 68, 2012: 266, 2011: 59, 2010: 191, 2009: 103, 2008: 75, 2007: 122, 2006: 82, 2005: 93.
A 20 year GDD average for Toledo on April 11 is 111 GDD units, with a range from 42 to 266. This year we are just a little ahead of the 20-year average.
Next week, I will start the weekly column to see how the temperatures have moved us along in this biological calendar and will try to keep you posted as we move throughout the season.
But what does that look like in our landscapes and neighborhoods? What is blooming, or becoming active in the insect world, and what is on the horizon? Here is a quick list of horticultural happenings: saucer magnolias that did not get hit by the recent cold snap should be blooming (133 GDD), the black stem borer is emerging as an adult (136 GDD), common quince is blooming (137 GDD), the dreaded Callery pear is beginning to bloom (142 GDD), and European pine sawfly is hatching from their overwintering eggs (144 GDD).
Check out this tool that gardeners should be using, and don’t hesitate to reach out if you have any questions about GDD, plant phenology, or anything horticulturally related. Extension is here for you!
Amy Stone is an extension educator with the Ohio State Extension – Lucas County, Agriculture and Natural Resources. Contact her at: stone.91@osu.edu.
First Published April 15, 2025, 11:00 a.m.
