A biennial plant is a flowering plant that takes two years to complete its biological 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. Flowering biennials include hollyhocks, foxglove, dianthus and Canterbury bells. Most biennials are actually vegetables — beets, Brussels sprouts, cabbage, carrots, cauliflower, celery, chard, collards, endive, kale, kohlrabi, leeks, onions, parsley, parsnip and rutabaga — that produce food the first year but don’t complete their growing cycle and drop seed till the second.
Under extreme climatic conditions, a biennial plant may complete its life cycle rapidly (e.g., in three months instead of two years). This is quite common in vegetable or flower seedlings that were vernalized before they were planted in the ground. This behavior leads to many normally biennial plants being treated as annuals in some areas.
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 for just one year (and not grown on a second year to run to seed).
Generally speaking, biennial seeds are sown in summer and the plants develop though the rest of the season; so that by the time winter sets in they will have made substantial plants. Then, in early spring, they’re off to a flying start and provide us with a colourful display.
You can end up with blooms from biennials every year if you stagger your own new plantings with the existing plants’ self-sowing. For instance if you plant seeds the first year, they will grow and then bloom and sow their seeds the second year. In the third year as those seeds are sprouting, plant second-year transplants, which will bloom that year and set seed, which will sprout in year four when year one’s self-sown seedlings are blooming. From there forward you should have some second-year plants in bloom every year.