I frequently receive questions from gardeners about the best timing for various gardening practices. Questions such as when is the best time to plant tomatoes or when is the proper time to treat the lawn for crabgrass are common.
Also common are questions about the timing related to the expected appearance of specific garden insects such as flea beetles, tomato hornworms or Japanese beetles.
While knowing the historical (and changing) dates of the first and last frost can be helpful to gardeners when trying to determine when to plant specific crops, and historical weather data is useful when determining when to expect certain insects to emerge in the garden or landscape, gardening solely by the calendar can be challenging, particularly with a changing climate.
A more useful approach for gardeners may be the concept of phenology and a weather-measurement system called growing degree days.
Phenology is the study of recurring biological phenomena and the relationship to weather. Bird migration, hunting and gathering seasons, blooming of wildflowers, budding of trees and the seasonal appearance of insects are examples of phenological events that have been recorded for centuries.
The word "phenology" comes from the Greek word "phaino," which means to show, to bring to light or make appear.
Because the development of both plants and insects is temperature-dependent, plants can accurately track the environmental factors that determine when insects are active.
For this reason, plant development can be used to predict insect emergence in the garden and landscape. The use of plant phenology to predict insect activity is an old practice, with recorded observations dating back at least 300 years.
Research at the Ohio State University has shown that plants bloom and insects emerge in virtually the same sequence every year, no matter what kind of weather occurred that particular winter or spring.
For this reason, the flowering sequence of plants can be used as a biological calendar to predict insect activity, and to time other gardening practices that are dependent on a particular stage of plant development, such as propagation or weed control.
Growing degree days
Growing degree days (GDDs) are a measurement tool for tracking the accumulation of heat units, which are necessary for the growth and development of plants and insects.
The number of GDDs increase as the temperature increases throughout the winter, spring and summer seasons. This calculation allows gardeners and farmers to track the growth stages of various plants and insects based on accumulated heat units over time.
Growing degree days are a measure of temperature accumulation over the course of the growing season, spring through autumn. The value resets to zero and starts on Jan. 1 each year and the number of growing degree days only starts increasing once the average daily temperature exceeds a certain threshold, also referred to as the base temperature, which is 50 degrees Fahrenheit.
The warmer the average temperature each day, the faster the increase in GDD values. The GDD number is cumulative; it increases with each passing day.
As of Jan. 20 this year, no growing degree days have accumulated in the OSU campus zip code of 43210. In 2023, 23 growing degree days accumulated in the same zip code as of Jan. 20.
The 20-year average (2006 to 2025) of growing degree days accumulated as of Jan. 20 in the 43210 zip code is eight GDDs.
This variation by year demonstrates how the development of plants as well as insect and wildlife activity can vary from year to year based on weather, not simply the calendar date.
No need to do the math yourself
You don't have to track weather data and calculate heat units yourself in order to use growing degree day data in Greater Columbus, as the College of Food, Agricultural, and Environmental Sciences at OSU maintains a phenology calendar website, which includes up-to-date weather and growing-degree-day accumulation data by Ohio zip code.
The site is located at weather.cfaes.osu.edu/gdd/.
When you enter your zip code and the current date into the website, you will see how many growing degree days have accumulated for your particular location. Specific microclimate conditions in your garden or landscape, such as full sun or shady locations, may mean that actual growing degree days may be slightly different than the number accumulated in your zip code, but this difference is typically small.
The phenology calendar website also includes historical weather and growing degree day data and a calendar with color photographs showing expected upcoming plant and insect phenological events for your zip code based on the number of accumulated growing degree days.
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For example, if you enter your Greater Columbus zip code and today's date, you will see that one of the first plant and insect phenological events which will occur in our area is the first bloom of silver maple (Acer sacharinum) buds when 34 GDDs are accumulated.
Full bloom of silver maple will occur when 42 GDDs have accumulated. The phenology calendar shows that one of the first insect-related phenological events in Greater Columbus landscapes is emergence of white pine weevil (Pissodes strobi) adults at 84 GDDs, followed by the egg hatch of Eastern tent caterpillar (Malcosoma americanum) at 92 GDDs.
Using phenology and growing degree days to track expected plant development and insect emergence in the garden and landscape can be an interesting tool to understand how changing weather patterns affect living organisms in the local environment.