"Flower Carol" is in The Oxford Book of Carols (1928, no. 99), with this interesting comment attached:
This is a free translation, with a doxology, of the words proper to the melody of No. 136, 'Tempus adest floridum', the Spring carol which Neale unfortunately turned into a Christmas carol by writing his rendering of the legend of 'Good King Wenceslas'. We have therefore reprinted the proper tune here, with the suggestion that it should be sung as a Spring carol, and that 'Good King Wenceslas' might be gradually dropped. (p. 207)
Interestingly again, the underlined words was deleted from the 1964 reset edition (p. 215).
~Masato