For those of you that think they don't need to bag their flower heads to prevent cross pollination. Read this little blurb from "The yearbook of Agriculture", 1961
The tobacco flower tends to be self-pollinated, but self-pollination is not sufficiently assured to make protection unnecessary when pure seeds are desired. The actual amount of natural crossing has been found to vary from season to season and from location to location and with the degree of isolation. Three years of systematic tests in 1956, 1957, and 1958, in which two marker-carrying gene varieties planted adjacently and at distances of one-twentieth, one-tenth, one-fourth, and one-half mile, showed most crossing in adjacent plantings, and the greatest distance did not always prevent crossing.
The tobacco flower tends to be self-pollinated, but self-pollination is not sufficiently assured to make protection unnecessary when pure seeds are desired. The actual amount of natural crossing has been found to vary from season to season and from location to location and with the degree of isolation. Three years of systematic tests in 1956, 1957, and 1958, in which two marker-carrying gene varieties planted adjacently and at distances of one-twentieth, one-tenth, one-fourth, and one-half mile, showed most crossing in adjacent plantings, and the greatest distance did not always prevent crossing.