Growing annual flowers offers two advantages over growing perennial flowers. First, annuals bloom profusely throughout the growing season. Secondly, many favorite annual flowers will self-sow and weave their way through your garden year after year. Only open pollinated and heirloom varieties of any plant will grow true from seed, meaning the seedlings will be exactly like the parent plants. But there are many open pollinated, self-seeding annuals to choose from and even if you do get some unexpected seedlings, who's to say you won't like them?

That's where new plants come from.

When growing self-sowing annual flowers, you will need to allow the late season blooms enough time to go to seed. If you've been deadheading all summer to keep the blooms coming, stop deadheading by the middle of August. The seeds need to ripen and that usually means the flowers must dry completely. Hopefully you'll have plenty of other fall bloomers to distract from the browning annuals. That's about all it takes. They don't call them self-seeders for nothing. If nature cooperates, you'll be seeing volunteers in and about your gardens next year and for years to come.

Sowing Seeds Yourself

Annuals that have self-seeded give your garden a natural look. However, sometimes they seed to enthusiastically or plant themselves where you wish they hadn't. Luckily self-sown annuals are easily transplanted to other spots in your garden or potted up for friends. Or you could take matters into your own hands and simply save the seeds and sow them yourself. You can do this by scattering the seeds directly into the flower bed or by starting the seeds indoors next spring.
If you choose to direct seed your own annual flowers, be sure you know and provide the conditions the seeds require to germinate, including:

• Light Some seeds need light to geminate and should not be covered with soil. Scatter these seeds and lightly press them into the soil with the back of a hoe or a board. Other annuals require darkness, which can be easily achieved with a top layer of soil.
   
  
• Scarification There are several annual flowers that protect their seeds with hard coverings. Morning Glories are a good example. To improve the odds of these seeds germinating, scarify or nick the outer covering by rubbing with sand paper or chipping the coating with a sharp knife. I don't recommend the knife method. These seeds are hard and tiny and it's so easy to miss... I prefer to soften the seed by soaking it over night.
   
  
• Cold Besides moisture, there are annual flower seeds, like poppies, that require a period of cold before they are triggered to begin germination. Nature takes care of this for us, when the seeds are left on the ground during winter. If you are starting seeds indoors, place the potted seed in the refrigerator for the recommended amount of time.

When to Sow Seeds Outdoors When to sow annual flower seeds outdoors depends on the type of seed and your climate. The best indicator is nature. If certain annuals reliably self-sow in your garden, you can bet they prefer being sown in the fall. Annuals that disappear after a season could simply be sterile hybrids or they may prefer warmer germination conditions. You can save seed from many marigolds, but it is rare for them to self-sow in cold climates.
USDA Hardiness Zones 5 and Lower
Short season gardens stand the best chance of getting a lot of self-seeded volunteers during winters with good snow cover, for insulation from cold, drying winds. Obviously only seeds that can handle a period of cold are going to self-seed in your gardens and you will need to ensure that you have allowed the seed heads to mature before your first frost. The option of saving seed and direct sowing it in the garden in the early spring, just before you expect the frosts to subside, might work best for gardeners in colder zones.

USDA Hardiness Zones 6 - 8
You moderates have the best of both worlds. You have a longer period in fall to allow seeds to ripen and drop. You also probably get enough of a cold spell for seeds that need a chill to germinate.

USDA Hardiness Zones 9-11
Gardeners in zones 9 and higher can also allow seeds to self-sow in the fall. But your annuals will likely grow and flower in winter, rather than waiting for spring. Unless your weather is very dry, you will probably get several seasons of volunteers.

Here's a List of Easy to Grow Self-Sowing Annual Flowers.