In the opening lines of the Mrs. Smith’s garden chapter, the sun and the moon are used in a traditional manner, to represent natural harmony, a natural passage of time. Sounds simple enough, but Updike takes it further. The night before Becky’s funeral, Rabbit dreams he’s in an empty track field (like Mrs. Smith’s garden, but without the flowers), and the phrase "the cowslip swallows the elder" is broadcast from a disembodied voice, kind of like a sports announcer. In the dream he understands that the cowslip is the moon (death) and the elder it’s swallowing, or eclipsing, is the sun (life). He dreams that life and death are all part of a beautiful cycle, and that he must found a new religion to spread this word.
The elder and the cowslip are both flowers, so we can connect this back to Mrs. Smith’s garden, too. Since it’s suggested that the field has something to do with sports, we could connect this to Rabbit’s high school basketball career that he’s so nostalgic about. Since the field is empty and possibly represents two times in his life when he was happy with what he is doing, and since he’s told to leave the field to start a new religion, the dream could mean that he’s ready to let go of his attachment to things that only partially satisfy him, and to reach for a higher goal. When he wakes from the dream, it no longer makes sense. He lacks a way to apply it to his life. We don’t necessarily see fulfillment of the dream prophecy by the end of the novel, unless you consider his running off again "founding a new religion." But hey, maybe we can. Maybe he’s starting a religion of running. Or maybe that’s a bit of a stretch.