Jim, would it solve your horticultural problem to rephrase the line in question as

          It fell about the Martinmas time
          When green leaves all had fallen. ?

(Not being too precious about all these great posts as I'm coming to the song from the point of view of finding a version that works for a singer -- from a psychological point of view, I suppose.
I feel very privileged to have a good library of ballad collections and am not averse to the Pick'n'Mix approach to constructing a text, but I always admit to my concoctions.
So, apologies to Richie et al for playing fast and loose with original sources and adding to later confusions.)