The unpredictable nature of self-seeding flowers allows for informal garden effects, such as the cheerful little violas known as Johnny-jump-ups along the fringes of a gravel path; spires of volunteer larkspurs among the roses; or a cottagey mix of nicotiana, cosmos, and feverfew.
In any area, the list of annuals that self-seed depends on climate; more volunteer seedlings tend to appear where winters are milder. Self-seeders include California poppy, cornflower, and calendula, plus nostalgic favorites such as four-o'clock, tassel flower (Emilia coccinea), corn poppy, annual baby's breath, and gloriosa daisy.
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Most of these prefer bare soil (or gravel) for germination, but a few willingly pop up through mulch. Some self-seeders, such as cleome and love-in-a-mist (Nigella damascena), verge on weediness, germinating in crowded thickets if not reined in. Use a hoe to dispatch the unwanted extras in spring. Thin the remaining seedlings so each has enough room to develop.
Of course, learning to differentiate among flower and weed seedlings is essential. Because seedlings are bound to show up in unexpected places, gardeners who are willing to relinquish some control over garden design—and who are not fastidious about deadheading spent flowers—are most likely to enjoy this laissez-faire approach to gardening. When tidying flowerbeds in fall, shake the remaining seed heads where you want new seedlings to appear. Nature will do the rest.
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Planting Self-Seeding Annuals
To start a new colony of self-seeders, wait until after a killing frost in fall to make the initial sowing. Cultivate the ground shallowly to loosen the top inch of soil and to remove weeds. In a jar, shake the seeds with some dry sand; the sand facilitates distribution of the seeds and makes it easier to see where the seeds have landed. The exact proportion of seeds to sand doesn't matter, and you can mix more than one type of seed in the jar. Then sprinkle the sand over the ground. Except for seeds such as poppies that prefer to remain on the surface, rake gently to cover, then water.