Perennials are not the only flowering plants holding a patent on blue flowers, although their numbers are greater than are other types. For example, perennials such as blue stars, bellflowers, blue bells, bluets, nepatas and irises feature blue flowers, as do varieties of asters, lungwort, thistles and geraniums. Other sorts of plants bloom blue during their flowering season, making the choices for a landscaping project plentiful when it comes to this color flower.
Vines
Among the vines, the clematis has a reputation for blooming for a long time; an array of species and cultivars has blue flowers, including Ramona and Blue Light. Ramona is a vine for small garden trellises, lampposts and fences, generating light blue flowers as wide as 7 inches from May into September. Other vines known for their blue flowers include nightshade, blue passionflower, blue trumpet vine and blue jasmine. Wisteria vines originating in China and Japan have blue flowers. Types such as the Chinese wisteria cultivar Prolific require a stout support, since their weight becomes an issue on structures that lack sturdiness. Prolific has lilac blue flowers during May and June.
Broadleaf Evergreens
Grow pigeon berry as an annual for its blue flowers outside of U.S. Department of Agriculture plant hardiness zones 10 and 11. It is a broadleaf evergreen shrub in these hot climates, but suitable as a container plant in other regions as long as you protect it from frost and cold. Cape leadwort, from South Africa, lives in a zone as chilly as zone 8, but is another broadleaf evergreen requiring protection from any hint of real cold. It has pale blue flowers all through the growing season. Other broadleaf evergreens with blue flowers include a plant called butterfly bush (Clerodendrum myricoides), but not because it attracts butterflies; its blue flowers bear resemblance to flying butterflies.
Annuals
Blue-eyed Mary at a quick glance looks like forget-me-nots, but this blue, native wildflower of the eastern states is an annual from a different species. It grows in large masses in the wild, making it a good fit for woodland and cottage gardens, but it does die by the middle of summer. Borage, cornflower and the floss flower are other annuals with blue blooms. Cool summer climates are best for a California annual called baby blue eyes, which grows to 6 inches and features blue cup-shaped flowers.
Deciduous Shrubs
Deciduous shrubs such as bluebeard cultivars like Longwood Blue generate blue flowers, with a long summer blooming period that enhances their value in the landscape. Shrub borders and group plantings are appropriate venues for bluebeard shrubs. Some have a cold hardiness extending into parts of USDA zone 5. Another deciduous shrub for blue flowers is the bigleaf hydrangea cultivar Nikko Blue, growing to 6 feet in zones 6 through 9. Lilac forms such as Little Boy Blue have much better cold hardiness, growing to zone 3 in shrub form.
Source: http://www.ehow.com/info_8065925_flowers-blue.html
