What are Annual, Perennial and Biennial plants?
The difference between annuals, biennials and perennials relates to the life cycles of the plants.
Annuals grow from seed to full maturity within one growing season. Marigolds, ageratums, and zinnias are typical annuals. They flower, set seed and die, all within a single year.
Biennials such as Canterbury bells and some foxgloves, complete their life cycles over two years, sometimes producing a few flowers in the first season, but most often just making foliage growth and establishing their root systems.
Perennials live longer than two seasons and may outlive many shrubs. In fact, strictly speaking, shrubs and trees are perennials, but when gardeners talk of perennials they generally mean plants that don’t develop permanent woody stems.
Perennials occur in several types. Some, such as Acanthus, are evergreen and don’t have a period of total dormancy, though few flower continually except in very mild climates.
Herbaceous perennials – those most common in temperate climate gardens – usually have a period of dormancy when they die back to a permanent rootstock. Most commonly this is during winter, but plants from hot dry areas many be dormant in summer or during periods of very low rainfall.
Some herbaceous perennials have developed the ability to use their roots or stems as food storage organs to enable them to survive extended periods of dormancy. Known as rhizomes and tubers, these storage roots can often be separated from the parent plant and grown on as new plants, in much the same way as bulbs and corms. Dahlias and alstroemerias are well-known tuberous plants, while bearded irises are probably the most widely grown rhizomatous plants. Some plants have specialized rhizomes known as stolons, which spread across the surface of the ground, or just below the surface, taking root as they spread.
Some perennials are treated as annuals, either because they cease to be attractive as they age, or because they are incapable of surviving cold winters. Petunias and impatiens, for example, may live for several seasons if protected from frost, but they become leggy and untidy, so they are usually replaced annually.
It is not uncommon for a genus to contain both annual and perennial species, such as the annual and perennial cosmos, or species with differing growth habits, like the fibrous and tuberous rooted irises and begonias.
In cultivation, the differences between annuals, biennials and the various types of perennials tend to become blurred. The important thing is how you use the plants, and with annuals and perennials you can give free rein to your imagination.