Flowers have one mission in life: to make seeds. Some flowers are much better at this than others. These are varieties of flowers to plant in your garden if you desire them to reseed themselves year after year. On the other hand, these are varieties of flowers to avoid if you do not desire your flowers to reseed themselves year after year in your garden.
A cool-weather-loving flower, johnny jump-ups, also called violas (Viola cornuta) are one of the first bedding transplants set out in springtime. They look like tiny pansies and are in fact one of the parents used in the development of pansies. Johnny jump-ups bloom prolifically from early spring until the heat of summer and each spent flower develops many seeds. When the seeds ripen the seed pod explodes, sending its seed in all directions. Thereafter, johnny jump-ups will come up in the garden bed in which they were planted, as well as every other garden bed on your property and quite possibly the lawn as well. Violas are prolific reseeders. Grow violas in partial shade to full sun, although they prefer partial shade as the weather warms. Keep them well watered, especially when hotter weather arrives. When they begin producing fewer blossoms, cut them back by about half of their height and they may bloom again when cool weather arrives in autumn.
A prairie flower, bachelor buttons (Centaurea cyanus) are also called "cornflowers." They produce whorls of tiny flowers that create the look of a larger flower, with each tiny flower producing a seed. Bachelor buttons are most often a bluish-purple, but can also be pink, white or wine-colored. They grow about 12 inches high and will continue to produce flowers until killed by frost. Bachelor buttons will come up as volunteers in your garden for many years after you initially grow them. They can be pulled, left in situ, or easily transplanted to a more desired location in your garden.
With finely divided, lacy foliage and daisy-like flowers, cosmos (Cosmos bipinnatus) are a highly desired annual flower. Their large size---over 4 feet high under ideal growing conditions---makes them an ideal addition to the back of the annual flower border. They have a branching habit and produce more flowers from the crotch between the branches and the main stem. Cosmos produces long skinny black seeds that spread themselves all over the flower bed. Volunteer cosmos seedlings emerge in mid spring and bloom by midsummer. They are easily transplanted to a more desired location and will grow on without missing a beat after transplanting.