I remember the neckcurls, limp and damp as tendrils; (1)
From the very first line, Roethke draws a direct connection between Jane and the natural world. We can really see him blending her characteristics with natural imagery. The line starts off with "neckcurls," an immediately recognizable physical description of Jane's hair. By the end of the line, Roethke transforms her hair, through the magic of figurative language (a simile simile), into the "limp and damp" tendrils of a plant. Neat trick, Ted.
The leaves, their whispers turned to kissing; (8)
Leave it to Roethke to take a fairly common personification of leaves (that they whisper in the wind) and turn it up a notch. Our perception of leaves in the wind, how we experience them, is changed by Roethke's unexpected personification. He is able to marry his perception of the natural world to his internal, psychological world. For the speaker, remembering Jane's happiness changes how he experiences the sound of the leaves.
Waiting like a fern, making a spiny shadow. (15)
Roethke doesn't just compare Jane to any old plant or bush. He's specific. Roethke knows his foliage and he picks a fern because he knows that connecting Jane to this plant will help us connect with her. Fern's have long, lush, complex leaves. They cast intricate shadows. But their beauty and complexity are easy to miss. They're kind of like the wall-flower of the plant-world. They blend in. They don't have bright, colorful flowers or a sweet scent. With his extensive knowledge of flora and fauna, Roethke is able to find a plant that gives us a sense of Jane physically and emotionally. If he had decided to compare Jane to an apple blossom we would have gotten a very different sense of her. Connecting Jane to the natural world makes her easier for us to understand and see.