growing degree days (GDDs)

 
Keith Stewart., Storey's guide to growing organic vegetable & herbs for market, [2013]

pp.78─79
   I learned the importances of day length the hard way with my broccoli crop.  In my early days of farming, it took me a few years to figure out why I couldn't get decent heads of broccoli in fall.  Invariably, they would be only 2 or 3 inches in diameter by November, when damaging frosts are likely to occur.  I gave the plants more fertilizer and more generous spacing, but these measures didn't help much.
   My mistake, in turned out, was to assume that the “days to harvest” numbers given in the seed catalogs were reliable throughout the growing season.  Indeed, they were fairly accurate if we seeded broccoli in spring and allowed it to enjoy the longer growing days.  But eventually, it became clear to me that to get decent heads of broccoli in fall, it was necesssary to give the plants at least another month of growing time. 
─“”

p.79
Growing degree days

Aside from a simple visual inspectin, one way to gauge when a specific crop should be ready for harvest is to keep track of growing degree days (GDDs), which are a culmulative measure of warmth over time.  In most parts of North America, GDDs (they are really units rather than actual days) occur when the average temperature for the day is above 50 degree F (10 degree C).  For example, if the high temperature on May 15 was 78 degree F and the low 42 degree F, the average for the day would be 60 degree F.

   (78 + 42) / 2 = 60
   Since 60 degree F is 10 degrees above the baseline of 50 degree F, we have an accumulation of 10 GDDs.  Average daily temperatures below 50 degree F are ignored, rather than subtracted from the running total.  (Depending on their locatin and subject of study, scientists might use a GDD baseline that is above or below 50 degree F.)
   Because plant growth and development are strongly influenced by the amount of warmth that is accumulated over time, the total GDDs accrued during a given period can be used to determine when a specific crop should be mature and ready for harvest.  GDDs are an aspect of the very interesting science of phenology ── the study of how climatic and cyclic phenomena influence plant and animal growth and behavior. 
   A farmer might use GDDs to predict other phenological cycles, such as when certain insect pests will emerge from dormancy and become active.  This method of keeping track of accumulated warmth is a more reliable indicator of plant growth and other cyclic events in nature than the calendar method, which provides dates based only on the historical record for a given area.  Various websites keep track of GDDs for different North American regions and locales.  Just type  “growing degree days”  in your web browser to learn more. 
─“”
   ( Storey's guide to growing organic vegetable & herbs for market / by Keith Stewart.
Guide to growing organic vegetables & herbs for market
Growing organic vegetables and herbs for market
includes index.
1. organic gardening.
2. farmers' markets.
3. vegetables──organic farming.
4. herbs──organic farming.
SB453.5.S84  2013
631.584──dc23
             )
·‘’•─“”
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