See in text (The Garden)
The modal verb “can” indicates an ability to perform an action. When it is in a past form, could, this states that the ability to perform the action is remote, or impossible. Notice here that the speaker repeats “can not,” which connects to the earlier repetition of “could.” While in the previous stanza the speaker states that were it not for the heat, she could perform those actions, here the heat is so intense that the fruit actually cannot perform even the simple act of falling. In either case, the tone conveyed is one of listlessness, in which the heat has paralyzed everyone and everything in the garden.