The Columbia Encyclopedia, Sixth Edition. Columbia Encyclopedia World Factbook American Heritage Coll.
Annuals propagate themselves by seed only, unlike many biennials and perennials.
They are thus especially suited to environments that have a short growing season. Tender and half-hardy annuals do not mature and blossom in one ordinary temperate growing season unless they are started early under glass and are set outdoors as young plants. Hardy annuals are usually sown where they are expected to bloom. Quite often they reseed themselves year after year. Blooming is prolonged by cutting the flowers before the seeds can form.