Phenology: The Garden Helper You Might Not Know About

Phenology: The Garden Helper You Might Not Know About


I have a forsythia bush in my yard.  It is always the first plant in my yard to bloom in the spring, followed by the Bradford Pear and then the Red Bud tree. The date of blooming can vary wildly, anywhere from late February to early April.  I first found out about phenology from an article in Urban Farm magazine which is now, sadly, out of print.  According to that article, “Phenology is the study of recurring natural events and their relationship to weather, and it has been used as a predictive tool for more than 300 years.”  Those recurring events include both plant (like blooming and sprouting of perennials) and insect events (like the hatching or adult emergence).

Ohio State University has a wealth of information on phenology on their website and it states that “research at The Ohio State University has shown that plants bloom and insects emerge in virtually the same order every year, no matter what kind of weather occurred that 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.”

The sequence of events can happen faster or slower but the sequence remains the same.  Another tool of phenology is the measure of heat accumulation called Growing Degree Days.  Growing Degree Days are a running total of average daily temperature in excess of a base temperature (usually 50 degrees).  So if the average daily temperature (the high temp plus the low temp divided by 2) is 60 degrees, the Growing Degree Days would be 10. The growing degree days are cumulative for the month or year. There are some other more nerdy ways to figure growing degree days but fortunately there’s an app for that!

Last year I made a record of the growing degree days when certain events happened, like the Bradford Pear blooming and the ants appearing in my kitchen.  Two years in a row the ants have invaded during the same week in April.  So  this year I’ll put out some ant control just before we reach the same number of growing degree days as last year when the ants appeared.  You could also use growing degree days to help determine when to release the mason bees you bought (they need to be able to find blooming plants within 300 feet) or when to apply pest control measures.

Growing degree days can help you decide when to plant annuals if you refer to your past year’s successes or failures.  The measure of accumulated heat is more precise than simply using the calendar or the predicted last frost date.  For those who are not garden nerds (like me) you could simply record the sequence of events in your garden like when things come up and when they bloom.  For instance, if you know the blueberries bloom right after the raspberries, you will be ready to fertilize the blueberries during early bloom (so get that special blueberry fertilizer ordered).

Peonies coming up
Peonies coming up

Last year we had a higher number of Growing Degree days in January than February.  This year the February number alone is 95, more than the cumulative total for both January and February last year.  I think it’s going to be an early Spring!

plum buds


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