Do you remember last spring…it came all at once in a massive explosion of color, leaves and pollen. Snowdrops, Squill and Daffodils all came at the same time, while Maples flowered and dispersed their pollen. During the last week of March in 2012, Forsythia, PJM Rhododendrons and Star Magnolias were already in full bloom and my tree lilacs, an early tree to leaf out, were already in leaf. However, this year you have to search hard to find any buds swelling and only this week did Squill start to appear and Daffodils start showing flower buds.
Why the dramatic difference from one year to the next? The answer comes in three words– Growing Degree Days (GDD). By tracking this measurement you can tell when plants will flower and when the bugs will hatch. While the calendar is a good guide for Baseball’s Opening Day, there can be quite a variation when it comes to plants and insects, since they go on their own calendar that is based upon weather and temperature.
One GDD is equal to the number of degrees the average daily temperature is above 50 degrees. You take the average of the high and low for the day and subtract 50.
(High Temperature + Low Temperature) / 2 – 50 = GDD
You can do it yourself or rely upon your local State School Extension. I use UMass — click on the Landscape Message tab for timely updates.
This season we haven’t had many warm days and the evenings have remained very cold, so as of today, April 5th we have had a paltry 1.5 GDD for the season. Last year we had 103 GDD by the end of March. These numbers seem small since last year we accumulated nearly 3,300 GDD, but a lot of activity happens in these first couple hundred GDD. Late in the summer with 90+ degree days and warm nights we can add 300 GDD in a week, while in some years we may only accumulate a total of 300 GDD by the end of May.
So, is spring late this year or were we early last year? I pulled some annual information from UMass and looked at GDD and soil temperature for the last 10 years.
Year GDD Soil Temp
The data reveals that we might be a little late this year when compared to the past decade, but last year was most definitely early. With all the late snow we had this year maybe we are just hoping for spring to come. By this time last year we had already had three good weeks of spring. I remember that I had mostly cleaned up and prepped my gardens by the end of March. That is certainly not the case this year as I still have a chunk of snow in my back yard.
One of the nice things about spring coming slowly is that the flowers are nicely spaced out and last longer in the cool weather. The later that Magnolias flower gives them a better chance of lasting, as they get easily burned with frost.
The dreaded Winter Moth that is devastating many of our shade and ornamental trees emerges within the first 50 GDD, and last year the arborists had already sprayed once for the moth and this year they are just waiting for the plant and insect world to waken from its slumber.
During the first 50 GDD you can expect to see PJM Rhododendrons, Forsythia, Star Magnolia and Pieris starting to flower. It is also the time that the Witch Hazels finish and drop their flowers. My Witch Hazel is still in bloom, lasting more than two months and without the heat the flowers have stayed fresh, and will be around at least for another week or two.
At 100 GDD you start to see the native Dogwood (Cornus florida) and Crabapple flowers. Check out this Cornell IPM Fact Sheet on flowing times for plants.
No matter how much you analyze, hope or pray, spring will come when it comes and there is nothing I can do about it.