perennial, any plant that under natural conditions lives for several to many growing seasons, as contrasted to an annual or a biennial. Botanically, the term perennial applies to both woody and herbaceous plants (see stem) and thus includes numerous members of the kingdom. In horticulture, however, the term is usually restricted to hardy herbaceous perennials, particularly border plants such as alyssum, chrysanthemum, iris, peony, phlox, pink, and sedum, all of which characteristically die down to the ground each year and survive the winter on food stored in specialized underground stems (corms, rhizomes, and tubers in horticulture; bulbous plants are not considered perennials). Perennials form seeds each year after reaching maturity, but since plants grown from seeds do not normally bloom until the second season (unless forced), most garden perennials are propagated by dividing the rootstocks (see propagation of plants). In fact, division every few years—as well as judicious pruning—is usually necessary to prevent the plant's becoming straggly and weak. Perennials, including the woody perennials, may have a rest period of some duration during their life cycle. In the plant different parts rest at different times and resume growth independently, e.g., the buds of deciduous plants, which form in late summer and remain dormant until spring. Even in tropical areas where plants appear to retain their leaves the year round, some plants lose all their leaves for a brief period and others grow new and drop old leaves on a continuing basis, as do most conifers.
See R. W. Cumming and R. E. Lee, Contemporary Perennials (1960); J. U. Crockett, Perennials (1972); A. M. Armitage, Herbaceous Perennial Plants (1989); R. R. Clausen and N. H. Ekstrom, Perennials for American Gardens (1989); P. J. Harper, Designing with Perennials (991).