In the May/June 2015 issue of Urban Farm (UF) magazine, there is an article about phenology. When I saw the cover and the title of the article “Use Phenology When Planting Your Peppers” ( written by Les Wilke), I was confused. Because, well, I really was confused. I was confusing phenology, the study of naturally occurring events as a predictive tool for other naturally occurring events, with pheromones, which are often used to attract or bait bug traps.
But it turns out, phenology is something entirely different! And after reading the article, I was intrigued. I checked out a few of the websites they mentioned, as well as a Penn State page that had information about phenology in Western Pa. Phenology uses a measure, called GDD to determine when things are going to happen. It’s all very scientific, but crazy simple at the same time. It sounds a lot like old wives’ tales being scientifically proven! Which is pretty freakin’ cool.
GDD stands for Growing (or Grower) Degree Days. And the entire phenology system is based on GDDs. Basicly, GDDs measure the heat accumulation in your area and then can be used to predict natural phenomena. It can be complicated, but the basic formula, according to PSU is [(temp min. + temp max)/2]-base temp. (Base temp is often 50F because below 50F, most pests don’t develop.)
Because, as you may know, many things in nature are heat, or cold, dependent. Many things emerge, bloom, grow, or sprout, at very specific temperature ranges. Using GDD we can predict naturally occurring events from year to year, by using the current year’s temperatures to calculate the current GDD and comparing it to previous years GDD at the time of the event in question.
Think of it this way, The date of Easter changes each year, based on an elaborate lunar church calendar. Every Easter my family had a family portrait taken under the same Magnolia tree. It didn’t matter if we were out there shivering in the snow, or if Easter was later in the calendar year, that tree was almost always in bloom around Easter.
Another example, and one that was given in the UF article, was the old adage about planting your peas on St. Patrick’s Day. Well, I have not yet had any luck with peas – going simply by the March 17th date. Some years it’s already so warm the peas aren’t happy, and some years its too cold, and again, the peas aren’t happy. According to the article, in 2012 the GDD on March 17th (in OH) was 95, but in 2014 it was only 12! That’s quite a big difference! And that means that it was really warm in 2012, but really cold, in 2014 on the same date. Using GDD, you can plant peas at the most ideal time for peas – sometimes that may be before the 17th, and sometimes it may be after. But it’s too late for me to test this theory this year, as I already planted my peas – much later than the 17th of March.
I used one of the GDD calculators mentioned in the article in UF, to see what our current GDD is, and here’s what I found for the past 2 months in our area:
Growing Degree Days (GDD) Forecast
Mar. 1 – Apr. 29
2015 = 95.0 GDD
Average** = 7.0 GDD
**Based on 30-year climatological temperature average for the selected ranges
For more information:
USA National Phenology Network
For a good explanation of GDD, check out the Penn State Extension Office site.
Unfortunately, Urban Farm magazine is no longer in print. But they do still have a website for “farmers” like me. While you can’t find the article from the magazine on the website, there is other useful information on the site.