There is a -na adjective『うららかな』, which means "warm, bright, and cheerful" So, 『春うらら』means "warm, bright, and cheerful spring" >why is みなぎる花粉 a description of spring? In Japan, a lot of people suffer from pollen allergies caused by a particular type of trees called "sugi" or in English "Cryptomeria japonica" and they release their pollen massively in Spring. So a lot of Japanese people who have pollen allergies associate spring with pollen. 『みなぎる花粉』is from the second phrase of a Japanese poem "Haiku," right? In a Haiku, traditionally, you have to include a word/phrase that symbolizes or implies the season of the Haiku. And that word or phrase does not have to be words like "spring" or "summer"; it can be a figurative word/phrase or a metaphor. And in Haiku, people often use metaphors.