The speaker gives his most specific setting description in the first stanza—and it ain't pretty. He starts by giving us the grim details of the room his grandmother's been bed-ridden in: "There were some dirty plates/ and a glass of milk/ beside her on a small table/ near the rank, disheveled bed—" (1-4). With just a few words, he makes us feel just how depressing this room is. It's dirty, it smells, and it seems like no one has changed the sheets of the grandmother's bed. It makes us wonder where this sickroom is. Is it a low-rent nursing home that doesn't take care of its patients? Is it the grandmother's own house? Wherever it is, there seem to be other people around, because the grandmother says, "They're starving me" (10). Wherever this dirty sickroom is it seems to be a place of neglect.
Eventually, the speaker convinces his grandmother to go to the hospital, and the next setting we hear about is an ambulance. The speaker doesn't give us any imagery at all to help us get a sense of what it's like in the ambulance. Did he figure we'd seen enough insides of ambulances on TV? Was he just slacking off? Nah, he probably didn't describe it because it's just not where he wants our focus to be. The grandmother's attention is on the row of elm trees that the ambulance is passing. The speaker doesn't bother describing the ambulance because he wants us to see what his grandmother is seeing. She watches the outside world flash by through the windows of the ambulance, and at the tail end of the poem she rejects it all. She's tired of this world, its sick rooms, its ambulances, and its trees. Come to think of it, she turns away from the whole setting.