I thought it was a wonder the treelets had persisted through a North Dakota winter. They'd had water perhaps, but only feeble light and a few crumbs of earth.
When I read this, I immediately thought of the mother who was attacked. In the first 83 pages, Joe describes how lifeless his mother is compared to how she used to be. She is isolating herself, saying very few words, and not eating. I feel like the treelets represent the mom because both the mom and the treelets are getting very little nutrients and sunlight. They're not receiving a lot of the basic things they need for survival. I feel like the "North Dakota winter" is the aftermath of the mom's attack. It's dark, cold, and dreary and Joe doesn't think she's going to get through it. He is hopeful, but he's not sure. This could also be a little bit of foreshadowing because the treelets persisted through the winter, so maybe the mom will make it out too.