"Plants do NOT, however, primarily respond to photoperiod, though there is a correlation.  The stronger correlation to bloom date is growing-degree-days"

Partly true.  Some respond almost entirely to degree days.  Particularly true of early spring bloomers.  Maples are a good example.  Three or four days in the 50s F and the soft maples bloom.  Can be as early as late Feb or as late and late March locally.  Others respond almost entirely to photoperiod and those tend to be early summer bloomers or later.  Nectar bearing golden rod species are a nice example of the later case.  The first day I can smell GT odor on my hives has not changed in 40 years regardless of cool versus hot summer or lots of rain versus a dry summer.  I will smell it the first good fly day after August 26 to 28.  Cold wet and rain during that time period can postpone it as much as a week.  That first days date has not changed for 40 some years.  Some of my local GR species start to bloom as early as July 1 but those do not bear nectar.

Dick
 

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