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Phenology: Here comes spring...sort of

If you're tiring of the long Cincinnati winter and chomping at the bit to get those gardening gloves dirty, take heart.

Meteorological spring starts March 1. How is this defined? The average temperature in March is warmer than December. Therefore, while the calendar seasons are defined by the position of the sun, the meteorological seasons are defined by average weather, temperature, and seasonal changes. For practical purposes, this makes sense! Therefore spring begins Monday!

Whether we're tracking seasonal changes by astronomical changes or meteorological yardsticks, nature can have a mind of its own. Gardeners and horticulturists have known for some time that gardening by calendar dates can be dicey.

Weather and temperatures can vary greatly from year to year, with trees leafing out at widely varying dates and also altering insect emergences and plant disease manifestations. In 1992, temperatures in the upper 70's in early March induced plants to leaf out by the end of the first week of the month, followed by the obligatory freeze and resultant damage. Last year, late winter was cool and trees did not leaf out until April.

The intuitive realization of this variation has led to research regarding the correlation between temperature, plant bloom times, and pest emergences. What has been found is that there is a close correlation. This area of research is called phenology.

Phenological research measures growing degree day accumulation (GDD) and correlates that to plant bloom times and pest emergences. With this knowledge, we can track degree day accumulation and watch what plants are blooming, then predict with great accuracy what pests should be emerging and becoming active.

Researchers at The Ohio State University have taken this phenological research and produced the Growing Degree Days and Phenology for Ohio website which tracks phenological progress throughout the state of Ohio. Phenological data was obtained from research by Dan Herms & John Cardina, and web site development was managed by David Lohnes.

The website software allows the user to enter any zip code within the state of Ohio, then correlates this location with weather data from one of several automated weather stations in Ohio to display the accumulated degree days. The software then correlates the accumulated GDD's to research data to predict what plants should be in bloom at the current time and what pests should be expected to be emerging or have already emerged.

One limitation of the website is that the limited number of weather stations may mean that the station used may not be representative of your zip code. This error is usually not off by more that a week or two, and can be checked by the bloom time information which is also on the site. For example, if the site says red maple is at first bloom (predicted at 44 GDD), but the red maples in your neighborhood are in full bloom (predicted at 75 GDD), you can predict that your location is approximately 30 GDD ahead of the website and, as a result, white pine weevil (84 GDD) should be about ready to emerge at your location.

The bad news is that, due to the cold winter, we stand at zero accumulated GDD over most of the state as of this writing. Therefore, whether you celebrate the first day of spring based on astronomical spring or meteorological spring, phenological tracking would suggest we are still in winter. However, with average temperatures going up and the sun angle getting higher, it is only a matter of time before growing degree day accumulation begins and the world begins to come alive!



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