Biennials are distinguished from annuals in that they take two years to complete their life cycle, rather than just one. In the first year, these plants produce a small rosette of foliage only. In their second year, they bloom, set seed and die. If planted in a location to their liking, many biennials can be counted on to reseed every year, making them almost as reliable as perennials.
Canterbury bells, Campanula medium, take their name from the bell-like, 3-inch deep flowers which bloom from May to July. Some varieties even have a flat petal under the cup, and are called cup-and-saucer plants. The flowers are available in vivid blue, rose, white and pastels. At 2 to 3 feet high, they are best planted in the middle of a sunny border.
The forget-me-not, or Myosotis, is distinguished by its delicate sky-blue flowers in early spring. This plant makes a good partner for spring bulbs such as tulips, daffodils and hyacinths, or early blooming perennials like lady's mantle and coral bells, said Dayton Nurseries in its online plant encyclopedia. It prefers a cool, shaded area with moist but well-drained soil. Besides the well-known blue forget-me-not, flowers also come in pink and white.
Foxglove, or Digitalis, is easily grown from seed and can be planted in sunny to partly shady spots. In its first year you will see a good-sized clump of foliage, and the second year you will be rewarded in June and July with tall (some varieties can reach 5 or more feet) flower stalks which hold graceful thimble-shaped blossoms. The flowers, which are attractive to butterflies, bees and hummingbirds, are delicately freckled and come in shades of white, yellow, pink, rose and lavender.
The hollyhock, Alcea rosea, is an old-fashioned cottage garden favorite. The plants' dramatic height of 6 feet or more make them useful as a screen, or as a planting along a fence or wall. The soft, showy flowers can be single or double, and are available in red, pink, white, yellow and purple. Given full sun and well-drained soil, hollyhocks will bloom from July through September. Gardens Ablaze recommends that hollyhocks be given plenty of room, since they are susceptible to rust, a fungus which is encouraged by poor air circulation and wet conditions. Their foliage can also be damaged by pests, such as Japanese beetles.
Sweet William, or Dianthus barbatus, is a member of the carnation family and has sweetly-scented flowers from May to July. The foliage is low, mounding and grass-like, making it an excellent choice for rock gardens, edging, or the front of the border. The flowers, which are flat and grow in large clusters, come in shades of white, rose, red, purple or bi-color. Plant them in full sun and keep the blossoms deadheaded to encourage new flowers to form. Sweet William is a good choice for the butterfly or scent garden.