When? Growing Degree Days

As I have written about before, the most often questions asked of this farm are: How is the crop going to be this year?, and When will be able to pick?  Natural questions deserve natural answers, but we can’t characterize nature well enough to answer these questions precisely.  Crop abundance and quality is mostly affected by pollination and precipitation, and pollination is affected by temperatures during blossoming.  “When” is primarily affected by temperature.  After blueberries blossom and are pollinated, the berries and mature (to size), then ripen.  How long the growing, maturation, and ripening takes is not well quantified or understood with respect to environmental variables, hence why forecast tables use large windows of likelihood: “high-bush blueberries usually ripen from late July to mid August.” Ripening varies for different varieties and we see ripening that extends into September most years for some rows of our bushes. 

Although we do not understand the rate of maturation and ripening fully, we can track some variables, like temperature, in a way that helps us guestimate when the bushes might be ready for picking. One method commonly used by farmers and horticulturists is called “growing degree days” (GDD), which is calculated as the sum of the difference between mean daily temperature (i.e., average of the maximum and minimum temperature for the day) and a “base” temperature, which corresponds to a temperature below which there is not much growing/ripening happening (typically 50 F (10 C). See http://en.wikipedia.org/wiki/Growing_degree-day, and references cited therein for more information about GDD and crops. I am not aware of anything specific for blueberries.  If you come across something, I would appreciate your sharing it.

Below is a table that lists our opening dates for the span since we have owned this farm and the GDDs as of the opener using temperatures recorded and GDDs calculated for the Houghton County Airport. A couple things to note: GDDs is not a precise predictor, it is an indication; and the data is from Houghton County Airport and farm climate can be quite different than the airport. The GBbF weather station has not collected data for a full growing season yet. Another thing to note is that we open in mid August over half the time. End-of-July/early-August openers are less often and typically for only very warm years. 

GBbF Opening Days and Growing Degree Days Calculated based on Temperatures Recorded at the Houghton County Airport. (Data and calculations from wunderground.com)

Year	Opening Date	Opener GDDs at HCA (wunderground.com)
2014	TBD	787 (as of August 1)
2013	August 17	954
2012	July 28	1136
2011	August 13	1104
2010	July 31	1105
2009	August 15	828
2008	August 9	829
2007	August 1	1091
2006	July 24	880
2005	July 29	942

We are HOPEFUL that we can open August 16 this year!