With a name derived from the Latin word "perennis," meaning everlasting or uninterrupted, perennials are plants that live for more than two years. They are divided into two distinct groups -- woody (e.g., trees and shrubs) and herbaceous (e.g., primulas and pulmonarias). In horticulture, perennial refers to the herbaceous or non-woody group. The longevity of perennials differs from that of ephemerals, which live for up to eight weeks; annuals, for one year; and biennials. for two years.
Perennials germinate from a seed as long as the conditions are warm, damp and dark. A seed coat is designed to withstand adverse conditions while it is dry or dormant, and it protects the embryonic plant during germination. Seed plants are self-sufficient because the food necessary for embryonic growth is stored and absorbed by the embryonic leaves called the cotyledon.
Perennials spend between two and eight years producing non-flowering growth that starts with roots, then continues with leaves and stems, which are eventually discarded for the winter. Annuals such as Helianthus annus complete this stage within months, and biennials, for instance Digitalis purpurea, spend the first year producing vegetative growth. By the time these groups complete this phase of the life cycle, generations of ephemerals like Cardamine hirsute have lived and died.
All plants that reach maturity and flower, producing seeds, are called angiosperms. The perennial flowering season starts in spring and finishes in fall. Most herbaceous perennials will flower only once a year, but lupins and delphiniums can be manipulated into a second flush by cutting them back after their initial bloom.
Just before winter, perennials discard their leaves and adapt their roots and stems to store food and water. (These are released in spring to promote new leaf growth.) Evergreen perennials, especially those in tropical regions, don't shed their leaves, because they are in their native habitat and can grow all year. In immoderate climates, perennials such as Gunnera manicata discard their leaves and require covering with fleece, such as straw, to protect them from winter temperatures.
The final stage of a perennial's life is death, though the seeds produced in earlier years usually will preserve the species. Perennials such as alliums, gypsophilia and geraniums can live for a long time, which is desirable for gardeners. With unwanted perennials weeds, such as Urtica dioica, a premature death is desired. Exhaustion of roots, through the removal of vegetative growth, is a suitable option for perennial weed eradication.