"in Just-" serves double-duty: it's both adopted title and first line. Cummings makes up a hyphenated word (Just-spring) and then splits it up, putting half into the title and half into the first line.
Wait…how can we tell that Just-spring is one word? Well, we can’t for sure. We do know that Cummings makes up hyphenated words in other parts of the poem, like "puddle-wonderful" and "mud-luscious." Using our incredible Shmoop deductive reasoning skills, we’re concluding that "Just-spring" follows in this pattern. As in, it hasn’t been spring for very long.
Back to our title, though: our speaker seems to be throwing us headlong into his world. We think we’re just reading a title…and then, WHAM! All of a sudden we’re off and running, smack in the middle of the poem itself. In a weird way, the immediate change from title to poem mimics the suddenness of spring’s arrival. We’re in winter, we’re in winter…and then one morning, you wake up and the sun’s beaming. It’s spring!
Come to think of it, the title/first line mimics also mimics the rest of the poem formally. We never really get to a stopping point in this poem. The last word of a line isn’t ever just the end of phrase. It usually links directly into the next line. Hmm...isn’t that sort of like the constant growing and blossoming of things in spring? Well, yes. Yes, it is.