13 years old

[Wjames]

Mom was at the kitchen sink, sweating
and snipping beans. Her lips pursed,
then she asked you where you'd been.  

You'd wandered off again, down the lane  This is a good intro imo, it places you in the mind of the narrator.
the tractors made as they rolled through the fields. you could cut 'made as they'
You'd followed their tracks to the hill
where the ferns and the trillium grew, you could cut the two 'thes' in this line - with them there, there is a lot of them in this section.
checked the ground for poison ivy, 
then sat down and wondered why.

Why your palms were still moving.
Why your head swayed without wind.
Why you had goosebumps in the middle of summer.
Why shade hurt your eyes. I think this line is implying light is painful on their eyes, but even the shade hurts as well - I think if you add 'even' before shade it might make it clearer.

The trillium bloomed every year in May,
but white to purple took only a week—    A nice idea.
a floral Brigadoon. 

The tall corn covered you from the sun
on the way back, like a boyfriend drapes
his arms around a girl.  This is nice.
The gentle scrapes of their rough leaves
felt real as beds and dinner plates. This is an interesting simile - beds and dinner plates are no more real than anything else to me. They do imply traditional womans role in a house, though, making beds and preparing dinner? 

The door to the kitchen opened itself;
another person's arm pulled the handle.

How to tell her that she didn't have a child? I agree this could be a good ending.
She'd steamed the life out of those beans. 

She didn't care where you'd been
or whether your hands were real,
only that they could shell a bushel of peas. The last four lines are the first thing about the mother-daughter relationship in the poem (or about the mother) - because we know so little about the mother and the daughters relationship, it lessens the impact. I think you could potentially add more details earlier in the poem. 

Hey Lizzie, I enjoyed reading this. It is not entirely clear, which makes it kind of nice to think about.