Kupu is to sprout, grow, germinate. It can mean offspring as well. Lau means leaf. Spring, ke kau kupulau is when "leaves sprout"
that is so cool !
kupulau does not require "kau" before it. " ʻOluʻolu ʻo Maui i ke kupulau" is perfectly acceptable. The only season that requires the kau prefix is summer kau wela.