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.
These self-sowers let their seeds go naturally, but if you want to keep them in the flowerbeds you will need to collect the seeds and direct sow them yourself. You can even get REALLY specific with your seed placement by trying out his cool idea from HGTV called, Seed Tape.
It's best to let seeds ripen on the plants until they are almost ready to disperse on their own. It's a common mistake to separate seeds from the plant before they've finished developing; any collected too early will not be viable. Once seeds have finished enlarging, they will generally change color (from whitish or green to tan, brown, or black) and begin to dry out. Pods will start splitting open, berries or fruits will shrink and wrinkle, or flower heads will begin to fall apart, dropping the seeds within.
On many plants the seeds don't ripen all at once (check daily), and it can be useful to tie a small paper bag or the toe cut from panty hose over the seed heads to prevent dispersal before you can harvest. If this method isn't practical, collect the seed heads or pods before they are completely dried out (i.e. papery dry), or the seed may either have dispersed or will fall out when you shake the plant stems while breaking off the pods or heads. Cutting the stems will help to prevent this sort of seed loss. As long as you wait until the flower seed heads have turned dark and fairly dry, or until the bracts ("leafy" shape that surrounds the petals of a flower where it attaches to its stem) or the pods themselves have quit enlarging and begun to dry out, you can cut and take these indoors to finish ripening in a cool, shaded place inside a closed paper bag (this will prevent you from losing seeds which disperse explosively, such as Impatiens and Phlox).
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 germinate 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.