All annuals have a one year life cycle, they start as a seed, grow, flower, produce a seed and die at the end of the season. Many annuals, such as marigolds and petunias produce seed but the seed does not survive cold winters, so the plants do not come back next year. Some annuals though are very hardy and their seed not only survives over the winter, but it benefits from a cold spell. These charming annuals frequently self seed each fall and return to populate our garden the following spring as reliably as the perennials.
Many classic cottage garden flowers such as cleome and larkspur are hardy annuals that produce swaths of color for your garden. The seeds of hardy annuals can be spread out in fall or very early spring, so that the seed can germinate in cool soil. A few frosts and snow cover benefits the seeds and ensures that they germinate at just the right time. Once germinated, the seedlings need to be thinned so that they are not crowded and they grow quickly. Typically the annuals bloom in spring or early summer and may take a break when very hot weather hits but they often return to flower again at the end of summer. The seeds produced that first year fall around the plant where they wait until next spring before germinating again and creating a casual group of colorful plants.
As the years go by your self-seeding patch of colorful annuals will slowly grow in size but can be restrained by pulling extras seedlings out of the ground as soon as they have germinated.
The best choices for hardy annuals are the heirloom plants which are stable in both color and size. Hybrids have specific traits and although the seed will germinate the plants are unlikely to be the same as your originals.