Answer: Quite a number of flowers will readily self-sow, including annuals such as sweet alyssum (Lobularia maritima) and pot marigolds (Calendula officinalis), and perennials such as purple coneflowers (Echinacea purpurea), Virginia spiderworts (Tradescantia virginiana), gloriosa daisies (Rudbeckia hirta), and garden phlox (Phlox paniculata).
Note, however, that subsequent generations of flowers may not be identical to the parents, especially if the parents are hybrids.
If you want to increase your population of any particular flower, let the seedheads form on the plant and then pluck them off, open the seedpod, and sprinkle the seeds in the garden. If you want to renovate the garden, or till the soil, then collect the dry seedheads and store them in an airtight container in a cool, dark location over the winter months. In the spring, after you've prepared the soil, sprinkle the seeds where you want the plants to grow.
To naturalize an area, you just need to be able to identify the baby plants so you don't mistake them for weeds and accidentally pull them out. Try to keep most of the spent flowers picked off during the early part of the summer or the plants will put their energy into producing seeds instead of producing more blooms. Let the very last flowers in the late summer become your source for seeds for the new plants. These end-of-the season seedheads are the ones you should harvest for sprinkling in the garden or saving for next spring.
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