Biennials, as the name implies, are plants with a two-year life cycle. A majority of biennials grow only foliage in the first year of the cycle and flower in the second, after which the plants die. Most biennial plants can be treated as perennials in their placement, planting, care and culture.
Though their growth habit defines biennial plants as short-lived, their freely self-seeding habit usually ensures that biennials return year after year. Because the plant develops its root structure during the first year in preparation for the reproduction cycle in the second year, most biennials do not flower in the first year after planting. Once they flower and set seed, plants usually die, but seeds that fall from the parent plant generally go on to start a new biennial cycle.
Several old-fashioned garden favorites are biennial plants, including hollyhock, dusty miller, foxglove and sweet William. Not only do these plants put on long-lasting flower displays from spring through summer, they are among the hardiest garden flowers available in the nursery trade. Other well-known biennial garden flowers include Canterbury bells, maiden pinks and black-eyed Susans. Biennials are also present in many vegetable gardens, and include edibles such as parsley, chives, onions, carrots and raspberries.
Care and Culture
Like many other types of flowering perennials, most biennials perform best in moist but well-drained soils. Optimum flowering is achieved by placing plants in locations where they will receive full sun, or at least six hours of direct sunlight each day. Plants that are watered generously after planting establish more quickly and will be better able to withstand summer heat and stretches of drought.
Biennials reproduce by flowering and forming seed, which then drops to the ground and sprouts even as the parent plant's life cycle comes to an end. This is necessary in cold climates where the plant must enter dormancy during the winter months in order to survive into its second year. In warmer climates where temperatures rarely dip below freezing, biennials can behave more like short-lived perennials. Individual plants may live for several years rather than just two, and a plant will flower each year during its life.
Flowers that take two years to flower can be discouraging to gardeners who are immediately eager for the showy flowers of plants like the hollyhock or foxglove. By sowing seed in mid-summer rather than spring, some of these plants will grow to flowering size before winter dormancy and then flower in the following year. Staggering plantings every other year is another method of ensuring that something will always be flowering from year to year.