A biennial plant is a flowering plant that takes two years to complete its lifecycle. In the first year the plant grows leaves, stems, and roots (vegetative structures), then it enters a period of dormancy over the colder months. Usually the stem remains very short and the leaves are low to the ground, forming a rosette. Many biennials require a cold treatment, or vernalization, before they will flower. The next spring or summer the stem of the biennial plant elongates greatly, or "bolts". The plant then flowers, producing fruits and seeds before it finally dies. There are far fewer biennials than either perennials or annuals.
Under extreme climatic conditions, a biennial plant may complete its life cycle in a very short period of time (e.g. three or four months instead of two years). This is quite common in vegetable or flower seedlings that were exposed to cold conditions, or vernalized, before they were planted in the ground. This behavior leads to many normally biennial plants being treated as annuals in some areas. Flowering can be induced in some biennials without vernalization by application of the plant hormone gibberellin, but this is rarely done commercially.
From a gardener's perspective, a plant's status as annual, biennial, or perennial often varies based on location or purpose. Biennials grown for flowers, fruits, or seeds need to be grown for two years. Biennials that are grown for edible leaves or roots are grown as annuals, e.g. beets, Brussels sprouts, cabbage, carrots, celery, lettuce, parsley, and Swiss chard. If a normally biennial plant is grown in extremely harsh conditions, it is likely to be treated as an annual because it will not survive the winter cold. Conversely, an annual grown under extremely favorable conditions may have highly successful seed propagation, giving it the appearance of being biennial or perennial. Some short-lived perennials may appear to be biennial rather than perennial. True biennials flower only once, while many perennials will flower every year once mature.
Examples of biennial plants are parsley, Lunaria, silverbeet, Sweet William, colic weed, and carrot. Plant breeders have produced annual cultivars of several biennials that will flower the first year from seed, e.g. foxglove, stock, and hollyhock.
- Detailed Information on Sweet William Dianthus barbatus. Dave's Garden.
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