Dull roots with spring rain.
The first several lines are meant to echo another celebrated, and very different, description of April in English poetry: the prologue of Geoffrey Chaucer’s Canterbury Tales (see note on line 1 above).
Spring stereotypically rekindles a lover’s flame, so you can read “stirring dull roots” as having sexual undertones, whether phallic (“stimulating limp penises”) or gender-neutral (“making people horny”).
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