These sweet potato vines were rooted at my After-School gardening classes last Winter, and transplanted this summer at a restaurant sidewalk planter. Today I noticed that they're flowering.
I'm not sure if sweet potatoes bloom based on variety or conditions. I don't recall any of mine blooming before, and most that we see do not have flowers. Sweet potatoes mostly reproduce asexually (as we created these very clones from a tuber), so the flowers are less necessary. They're close relations to morning glories, which do indeed need to flower for their species survival.
The flowers resemble tobacco and petunias, but those are in the nightshade family, while sweet potatoes are in the convolvulaceae family.