Hurray! The official start of spring has arrived. With the sunny and warmer days, do you know when your favorite tree or shrub is going to flower? Can you predict when insects are going to arrive and possibly start causing damage to your landscape?

What if I told you that you can? You would think I had a crystal ball! Not quite, however, we do have a tool called Phenology.

So, what is Phenology? Phenology in horticulture is based on growing degree days (GDD). Growing degree days is the measurement of the growth and development of plants and insects during the growing season. Development does not occur at this time unless the temperature is above a minimum threshold value, or what is also referred to as the base temperature. This base temperature can vary for different organisms and is determined through research and experimentation.

The actual temperature experienced by an organism is influenced by several factors, and these factors will ultimately affect that organism’s growth and development. We can probably all agree that depending on the weather, an organism’s temperature may be a few degrees more or less than that recorded. For example, an organism in direct sunlight will likely experience higher temperatures than those in full shade, and of course, somewhere in the middle if the organism is in partial shade, or both sun and shade at some point throughout the day. What it comes down to is the actual location can result in those temperature differences. Fertility and nutrient levels in the soil can also affect the growth rate of insects and plants. The presence of weeds and precipitation may indirectly influence development as well. Due to these factors and some other scientific considerations, a base temperature of 50 degrees Fahrenheit is considered acceptable for all plants and insects.

The phenology tool for plants and insects was developed by researchers with Ohio State University Agriculture Research and Development Centers (OARDC). This tool is available online: https://weather.cfaes.osu.edu/gdd/.

The website uses growing degrees days that ultimately provides a biological calendar that “marries” a list of plants at their first and full bloom and insect activity. This calendar is a sequence of events that includes both plants and insects and ties to each organism to the GDD.

It is important to note that while the actual number associated with growing degree days is based on weather stations across Ohio, there can be some differences based on microclimates or the actual weather in your backyard. However, the sequence of activity is always in the same order. As you use GDD, it is always recommended to get outdoors and compare what the website is telling you, and what should be happening, based on what you are seeing. For example, the first plant on the Ohio list is first bloom of the silver maple at 34 GDD. Check out the website, type in your Ohio zip code, see what the website says your GDD is, and then head out to the field and make the seasonal observations that is included in the list. Are you seeing silver maples blooming in your area?

For comparison, these are growing degree days in Bowling Green late March and early April last year. Thursday, March 25, 2021, the GDD was 75, and Thursday, April 1, 2021, the GDD was 94. Last year, Red Maples were in full bloom by March 25, and by April 1, the Forsythias were in full bloom.

Insect phenology also comes in handy when we are trying to predict egg hatch for controlling Bagworms (Thyridopteryx ephemeraeformis). Based on phenology Bagworms hatch from their eggs at 630 GDD. This is the same time Washington Hawthorns (Crataegus phaenopyrum) are starting to flower!

By using the horticultural Phenology tool, you too can predict the flowering and insect activity for your favorite landscape plants!