For the most part winter in Northern Ohio has been relatively mild. January, we did have a week or so of below freezing and even below zero temperatures; however temperatures rebounded to somewhat mild conditions.

With the mild temperatures will this harm our outdoor landscape plants?

The answer to this question is 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. Development occurs at a time when 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.

Various factors are used to figure out the base temperature. One of the factors is direct sunlight. Direct sunlight is stronger in March as opposed to January. It may be the same temperature outdoors however the southern exposer is warmer than the northern exposure mainly due to shady condition depending on the month of the year. Soil temperatures also come into play most outdoor organisms do not develop until the soil temperatures have warmed. Though day temperatures may be well above freezing the night temperatures have been below freezing. Moisture 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.

Botanists and other researchers including those in the technology department at The Ohio State University have created a user-friendly phenology tool. This tool is available online: https://weather.cfaes.osu.edu/gdd/default.asp

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 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 the first bloom of 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 your own seasonal observations.

Even though we have had a mild winter, phenology gives us a tool to show us our outdoor landscape plants at this point in winter are doing just fine.

By using the horticultural phenology tool, you too can predict the flowering and insect activity of your favorite landscape plants.