It seems weird at first that Cummings just drops in lines like this one at different parts of his poem. But what he's doing here is reminding us that, no matter what's going on in our lives, the seasons keep passing in their endless cycle. He does this to remind us that all our personal dramas always unfold in nature.
they sowed their isn't reaped their same (7)
Cumming compares having kids to planting seeds and harvesting them. He takes a bit of a shot at American families, though, when he suggests that people tend to raise a bunch of kids who are all the same and kind of boring. Parents would probably be offended if you ever told them their kids were just like everyone else's kids.
when by now and tree by leaf (13)
Cummings thinks of all the moments in our life as a bunch of "nows." He also thinks these moments connect to our lives the same way individual leaves are connected to a tree.
bird by snow and stir by still (15)
Again, Cummings uses images of nature to remind us that all the little parts of our lives are connected to a larger whole. And that whole is nature. Individual human lives are pretty tiny things compared to all of nature, but this is exactly why they're valuable—because they're part of something bigger.
stars rain sun moon (21)
Cummings returns to these types of images as though they were a chorus in a song. He wants us to think about individual human lives, but he never wants us to forget that the cycles of nature ultimately are in control of what's going on.
noone and anyone earth by april (31)
When anyone and noone are finally dead, Cummings reminds us that the two of them return into the earth. It's almost like a sort of homecoming, since all humans are born into nature and they eventually die back into it, too.