From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-1906-779974
Question: I planted some Sweet William last year, and they bloomed
beautifully this year. The flowers are now starting to die(after
several weeks in full bloom). What should I do now? Should I shear
them back? Is there any way to get the plants to flower again?
Answer: Dianthus barbatus (Sweet William) is a vigorous biennial
providing dense clusters of flowers on tall stems. Sown from seed in
late spring, they will bloom the following year. Once they've
flowered, they won't flower again. You can shear back the spent blooms
and stems to keep the plants looking attractive, or you can leave them
alone, allowing the flowers to produce seed and deposit them in the
bed. This self-sowing habit will perpetuate your Sweet Williams, so
you'll have plants of flowering age each year.