I immediately thought of a snake shedding its skin: a combination of “fine green” and “paper thin” and “iridescent” with being “split like a sheath” right away makes you feel sort of uncomfortable, which is the point since her son is uncomfortable in his own skin.
“Glossy”, “bulbs”, “Gabriel”, “toes”, and “crocus” are guttural words, like groaning, croaking (frog) helps us hear the sound of his growing pains.
She sets up the image of a plant that comes at the end with “Spring”, but also uses it to further show us that he’s growing, that he’s full of life that can’t help itself, that must grow. And this leads us to the best image in the poem of comparing him to a sprout growing towards a light. I kept thinking of those old PBS Nature videos of time-lapse shots of flower sprouts growing out of the black earth, twisting and writing with unnatural speed.
She ends the poem strangely with a desperation, a drowning, as if his very life depends on growing or else he will die suddenly. This is something only a parent would think about – a kid is too busy trying to older, to grow, whereas she knows that after growth comes decline, that death is inevitable and life is a struggle – it’s Darwinian here, too.