는 puts an emphasis on 봄에 and marks it as the topic of the sentence. It also has the connotation of "only in spring", so this phrase means that flowers bloom in spring, but not necessarily in other seasons.
Would it be wrong to write it as "봄에서 꽃은 펴"? Since this sort of sounds more like a fact would it be more appropriate to have 은 rather than 이 on 꽃? Or does it sound better if you have the topic marker on "spring" instead?