Some plants manage to maintain life in a dormant state through the winter and begin growth on the same roots and stem year after year. These plants are referred to as perennials and differ from annuals, which complete their life cycle in a single year and must start from seed produced from earlier generations each spring. More species of flowers and herbs fall into the perennial category than do vegetables.
The coneflower, daisy and coreopsis are just a few examples of flowers that grow year after year from the same roots and stems. Expect a perennial to spend the first growing season establishing itself as a plant. Flowers will occur during the second season and, if conditions are right, every year thereafter. In some cases, annual flowers will appear to be perennial, as the seeds repopulate the plant in the same spot as the previous year's growth. However, this is still a new plant, rather than a continuation of growth from the same root after a dormant period during the winter.
Sunchokes are among the few perennial vegetable plants. This plant is related to the sunflower and yields small tubers that are harvested for the table. Plant sunchoke tubers in the spring, and harvest them in the autumn or leave them in the ground to grow again the next year. Welsh, or winter, onions are also a perennial that will return year after year if not harvested.
Biennials require two growing seasons to mature and produce seeds. The plant grows from seeds during the first season and becomes dormant through the winter. In the second growing season, it blooms and produces seeds before dying. Carrots are an example of a biennial, although they are commonly harvested during or at the end of the first growing season.
Some plants behave as perennials in mild climates and as annuals in colder climates. If the growing season is long enough for the plant to go through its complete life cycle in a single summer, it behaves as an annual. If the season is not long enough, the plant comes back the next spring. The black-eyed Susan is an example of this type of plant.