Case it point--these black-eyed Susans. My husband, the resident gardener around here, isn’t certain where they came from, but his best guess is that they are “volunteers” delivered by the wind.
Black-eyed Susans are biennial, which means they live for two years. In the first year, the plant grows a rosette, which is a group of leaves growing from the center, low to the ground. In the second year, the plant sends up flower stalks. At the end of the second year, the plant dies. http://www.fcps.edu/islandcreekes/ecology/black-eyed_susan.htm
He has decided not to mow that area of the lawn so that these visitors can multiply freely where they will, for alas this is their last year. Ah, I trust that the wind sends new seeds.