Though among the more unusual flower colours, a number of plants will provide beautiful blue blooms. Perennials, annuals, shrubs and vines all have varieties that produce shades from electric blue to pale blue. Consider the hardiness zones, growing preferences, sizes and growth habits of the plants before deciding which one to place in a particular location.
Balloon flower, Platycodon grandiflorus 'Astra Semi-Double Blue,' is a perennial that is hardy in USDA Planting Zones 3 through 8. This is a short, clump-forming plant that is suitable for borders, reaching a height of 1 foot and a width of 8 to 9 inches. The 3-inch-diameter flowers look like little balloons just before they open. The blooming period for this plant is from June through August. The foliage is blue-green in colour with oval to oblong toothy leaves. Balloon flower plants grow best in a light, organically rich, loamy soil with full sun or partial shade. They will not thrive in wet soil that does not drain well.
Bluebeard, Caryopteris x clandonensis 'Heavenly Blue,' is a deciduous shrub that is hardy in USDA Planting Zones 5 through 9. Also known as blue spiraea or blue mist, this compact shrub grows to a height and width of only 2 to 3 feet. It is a later-blooming plant that produces clusters of periwinkle blue flowers beginning in July and continuing into September. The silvery-grey foliage is oval or elongated and emits a fragrance when brushed or rubbed by hand. Bluebeard is easy to grow in loose, loamy soil that gets full sun exposure and is slightly drought-tolerant. It will not thrive in wet locations that do not drain well.
Long Leaf Speedwell
Long leaf speedwell, Veronica longifolia 'Blue John,' is a perennial that is hardy in USDA Planting Zones 4 through 8. This plant grows to a height of 1 to 1 1/2 feet and a width of 6 to 9 inches. It produces 20-inch-long spikes of pale blue flowers from June through September that attract butterflies and hummingbirds. The blooms are ideal for cut-flower arrangements. Blue John foliage is dark green with elongated leaves. The plant grows well in average soil that drains well with full sun exposure, although it prefers nutrient-rich, moist soil.
Blue bugloss, Anchusa azurea 'Loddon Royalist,' is a clump-forming perennial that is hardy in USDA Planting Zones 3 through 8. It grows to a height between 1 and 3 feet and a width of 1 to 1 1/2 feet and produces deep blue, 3/4-inch-diameter flowers in May and June. The foliage and stems are deep green and hairy with 4- to 8-inch-long lance-shaped leaves. This plant will grow in most soil types as long as the soil drains well, but it prefers nutrient-rich soil. It is highly susceptible to crown rot when planted in wet, poor-draining soil. In warmer climates, zones 7 and above, place the plant in a spot where it will receive shade from the hot midday sun. In zones 6 and below, it does best with full sun exposure.
Bigleaf hydrangea, Hydrangea macrophylla 'Nikko Blue,' is another option in blue-flowering deciduous shrubs. It is hardy in USDA Planting Zones 6 through 9 and grows to a height and width of 4 to 6 feet. Big, blue, 4- to 5-inch-diameter orbs of flowers bloom from June through August. The foliage is medium green with large, oval-shaped, toothy-edged leaves. The hydrangeas prefer a moist, well-drained location that gets some shade from the hot midday sun. The soil must be highly acidic with a pH of 5.5 or lower in order for this shrub to produce blue flowers. Soil with a pH between 5.5 and 6.5 will cause the blooms to be purple or a mixture of pink and blue, and a neutral to alkaline soil pH of 6.5 or higher will produce pink blooms.