Tall perennials make fine additions to perennial borders and work along foundations, as well. Other uses for tall perennials include specimen plants, container plants for a patio or deck and as hedges when the species grows with a robust nature. If blue is a colour you like for your flowers, a number of tall blue perennials exist for your landscape.
Two types of indigo fit the bill of being tall blue perennial species -- one is false blue indigo (Baptisia australsi) and the other is a cultivar of it known as Purple Smoke. Both types grow to 4 feet tall. False blue indigo develops best in full sun; in too much shade, it requires staking. False blue indigo flowers emerge on 12-inch long spikes, with the indigo-blue flowers coming out in the middle of spring. Purple Smoke has tolerance to drought, in part due to its long taproot. Its flowers are a violet-blue shade and bloom in April and May. These tall blue perennials are options for poor-quality locations where other flowers cannot grow.
Among the types of asters growing to tall heights are the Tartarian aster (Aster tataricus), glossy-leaved aster (Symphyotrichum puniceum) and the somewhat smaller Stokes aster (Stokesia laevis). Tartarian aster is a Siberian plant growing to 6 feet and blooming late in the season, with its violet-blue flowers opening in September. Glossy-leaved aster is a 6- to 8-foot high species that is ideal for the background of a perennial border. The flowers turn out in August and are a shade of blue mixed with violet. Stokes aster has a hybrid called Omega Skyrocket that possesses pale blue flowers. The plant grows from 3 to 4 feet tall and blooms all summer long.
Blue sage (Salvia azurea) is a perennial of the Deep South, growing to between 3 and 5 feet tall. A hybrid called Grandifolia grows to the same height. Blue sage is suitable for cottage gardens and for naturalised areas. Both blue sage and Grandifolia flower beginning in July and stay in bloom through October. Wood sage has a cultivar called Lye End (Salvia x sylvestris), featuring lavender-blue flowers in May and June. This sage species thrives in sandy soil that drains well. Cut it back once it ceases flowering. It will spring back to be between 3 and 5 feet tall the following year.
The spiny leaves of thistles are something to avoid contact with, but these perennials grow tall and produce some blue flowers. One species, the globe thistle, (Echinops bannaticus) hails from Europe and grows to 6 feet in height. Its blue flowers are out in July and August at the top of the plant. The steel blue flowers of the small globe thistle, named because it tops out at "only" 4 feet, are a good fit for cut flower gardens, notes the Missouri Botanical Garden. A hybrid known as Veitch's Blue has flowers a darker shade of blue than those of the parent species.