The tulips are too excitable, it is winter here. (1)
This isn't really a "nature poem" in the typical sense, but based on the first line, you might think it was. In this opening moment, there's no way to tell that we aren't outside, except for this: tulips don't grow in wintertime. So something's off here.
They pass the way gulls pass inland in their white caps (12)
Check out how the speaker sees something natural (seagulls) in a very unnatural, human space (the hospital). Why do you think she chooses this particular image to describe the nurses?
My body is a pebble to them, they tend it as water (15)
On the one hand, this is just a metaphorical way of expressing the way the nurses treat her. But on the other hand, it's yet another way that the natural world creeps into almost every line of this poem. Once you start looking, you start to see the natural world just about everywhere.
Between the eye of the sun and the eyes of the tulips, (47)
Nature gives us plenty of pretty images in this poem, but it's also a bit of a threat. Its powerful forces fill the poem with life, but they also make our fragile speaker feel vulnerable and maybe even a little imprisoned.
Now the air snags and eddies round them the way a river Snags and eddies round a sunken rust-red engine. (53-54)
Here the tulips are turned into something man-made, a rusted engine stuck in a river. That's the way it goes with these tulips. They are natural, sure, but there's also something not quite right about them, like an engine that's been plunked in the middle of a beautiful river.