How biennials grow
Biennials are an interesting group of plants. They spend the first year producing a sturdy root system, with only a simple rosette of leaves above ground. The leaves die back to the ground in winter and the root system goes quietly to sleep. The following spring, the plant awakens to send up a flower stalk, which produces blossoms and seeds.
At the end of this second season, the plant usually dies, having completed the task of spreading its seeds. Some biennials can be pushed to come back another season if you pinch out the flower stalk before it sets seed. But why would we do that when seeds are what bring back the plants for years to come?
Because biennials put all their energy in the second year into producing only flowers, seeds ripen early enough to germinate and begin growing in late summer and fall, completing the first phase of their life cycle at this time. In spring, the new plants are ready to bloom.
Once you start the cycle, you’ll have plants blooming every year. There are some tender perennials that live for only two years and are often referred to as biennials, but true biennials do not flower in their first year.
If purple is your passion, try the Korean angelica (Angelica gigas, Zones 4 to 9). A huge plant that grows up to 6 feet tall, this biennial boasts purple-veined leaves and masses of purple flowers that bees and wasps love (photo 1).
Add a splash of color to the back of the perennial border with 6-foot-tall delphiniums (Delphinium Pacific hybrids, Zones 3 to 7). Their lavender, pink, or blue blossoms will repeat during the season if deadheaded.
Another back-of-the-border star is hollyhock (Alcea rosea, Zones 3 to 9). The plant’s large tropical leaves add substance, and spires of crepe-paper blossoms in all hues of rose, red, pink, salmon, and even darkest purple-black (my favorite) add unrivaled drama and make great cut flowers. Give these sun-loving plants plenty of room, as they grow 1 to 3 feet wide and up to 8 feet tall.
Foxgloves (Digitalis spp., Zones 3 to 8) are tall, old-fashioned favorites with a cottage-garden look. The drama of the biennial foxglove with flower spikes of all shades of pink, purple, and fuchsia is well worth the wait. It’s a narrow plant with 3- to 5-foot-tall spires, and it performs well in full sun or partial shade.
British gardeners love tall, spiky mullein (Verbascum spp., Zones 5 to 8), topped with yellow, peach, apricot, or white flowers, because of the dramatic statement it makes in the perennial garden or meadow garden, as well as in cut-flower arrangements. Mullein needs full sun and average soil, and it grows 4 to 6 feet tall.
For a lovely, prominent plant in the perennial garden, try Canterbury bells (Campanula medium, Zones 5 to 8). This clumping plant grows about 3 feet tall. It blossoms best in full sun, with flowers of white, pink, and blue.
Other colorful full-sun favorites are the 2-foot-tall English wallflower (Erysimum cheiri, Zones 3 to 7), which blooms in shades of yellow, mahogany, red, and orange, and clary sage (Salvia sclarea, Zones 5 to 9), a 3- to 4-foot-tall culinary plant with soft lavender-pink blossoms. Clary sage has been used for years to flavor wine, and it makes a tasty sage fritter. Both grow in moderate to poor soil.
If you like cutting flowers from your garden for indoor arrangements, try money plant (Lunaria annua, Zones 3 to 9), an upright plant that grows 2 to 3 feet tall with rich sage-green leaves and clusters of pink, white, and purple blossoms. Its papery seedpods make an attractive addition to floral arrangements. Money plant will tolerate partial shade.
Bee blossom or gaura (Gaura biennis, Zones 5 to 9) is a wildflower with white to light pink, butterfly-shaped flowers. It can grow up to 10 feet tall in the wild but usually reaches only 6 feet tall in the garden. It does best in full sun to partial shade.
Caring for biennials
Almost all biennials will thrive if you grow them in well-drained average soil and add organic matter regularly. Unlike annuals, they do not require frequent fertilization; a yearly dose of general garden fertilizer or compost will keep the seedlings coming up year after year. Some require full sun, but others will thrive in partial shade.
Most biennials will continue to produce blossoms through the season if you make a practice of deadheading the spent flowers. Leave a few, however, to reseed themselves.
Because they reseed so readily, some biennials are considered invasive in certain areas.
Check with your local cooperative extension service to make sure your favorite biennials aren’t invasive in your climate.
Kate Jerome is a garden writer and author of several books on houseplants.