Annual Blue Flowers

Lobelia

• Just 3- to 9-inches high, lobelia (Lobelia erinus) is a spreading annual happiest in spring and autumn's cool temperatures. At its peak of bloom, lobelia has masses of small, delicate deep- or sky-blue flowers. Airy, medium green or bronze foliage adds to its appeal. Plants in hot summer climates benefit from a partly shady location, according to the Cornell University Home Gardening website. Elsewhere, it likes full sun. Shearing back the plants in hot summer lets them rest before their autumn bloom. In the right conditions, lobelia's blue graces gardens from spring through fall. Good drainage is essential for this plant.

Torenia

• Torenia (Torenia fournieri) is a multiple-branched, 12-inch high annual with profuse, bi-colored flowers from summer to fall. The tubular blossoms pair deep blue -- bordering on purple -- lower lobes with lavender-blue upper ones. Yellow-blotched interiors complete the color palette, notes the Missouri Botanical Garden Website. Torenia's light green stems and foliage further brighten the shady spots it loves. This plant works well as a border edging or window box plant. It thrives in organically rich, well-drained moist soil.

Miniature Lupine

• Miniature lupine (Lupinus bicolor) has smaller versions of its perennial relatives' dense flower spikes and compound, lobed foliage. This pea family annual's blue-and-white blossoms appear on upright stems between March and June. Only 16 inches high, miniature lupine has clumps of greenish-gray foliage. They provide a cooling garden touch when the plant isn't in bloom. Pair miniature lupine with orange or yellow poppies for contrast. A sunny spot with sandy, well-drained soil will keep the plants looking their best, notes the Lady Bird Johnson Wildflower Center.

Ageratum

• Ageratum (Ageratum houstonianum), sometimes called floss flower, is an aster family annual. Mounding ageratum cultivars stand from 6 - to 16-inches high, while taller varieties can reach 3 feet, according to the University of Illinois Extension's HortAnswers website. All have flat, fuzzy flower clusters. "Blue Hawaii" and "Blue Horizon" are two of several blue choices. The spring-blooming plants perform best in full sun and moist, well-drained locations. Fertile soil keeps them flowering until frost. Mounding plants are charming in rock gardens and along walkways. Tall ones provide unusual cut flowers