Answer: Sweet Williams (Dianthus barbatus) bloom for a few weeks in late spring or early summer and dead heading makes them look better and preserves the plants' energy but doesn't seem to increase the blooming time. These plants are considered to be short lived perennials or in some cases biennials, so you may wish to let at least some of them set seed to perpetuate the patch. They can also be divided or propagated by tip cuttings if you have favorites. If you save seed, be sure it is fully ripe before you collect it (but catch it before it shatters); it should germinate without any special treatment.
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