I spotted this guy out on a creek bank while fly fishing recently. Very pretty though it looks out of place. I should say that the location is south central Missouri along the bank of Huzzah Creek
but they are sometimes hard to tell apart. One difference is that the flowers of Convolvulus arvensis are typically under an inch in diameter while those of Calystegia-sepium are usually much larger, 2 - 3 inches in diameter.