True blue and deep purple are rare and highly sought-after colors in the natural world of flowers. Unlike the reds, yellows, and whites that dominate most landscapes, cool-toned blooms possess a unique aesthetic quality that is both calming and visually arresting. These colors can transform a garden space, providing an unmatched sense of depth and tranquility. This exploration offers guidance on identifying, cultivating, and utilizing the best blue and purple varieties in landscape design.
The Chemistry Behind Blue and Purple Tones
The vibrant colors in blue and purple flowers are primarily due to a group of pigments called anthocyanins, which are water-soluble compounds stored within the plant’s cell vacuoles. These pigments are part of the flavonoid family and are responsible for a wide spectrum of colors, including red, purple, and blue. The precise color expressed is largely dictated by the cellular \(\text{pH}\) level inside the vacuole, functioning much like a natural litmus test. In highly acidic conditions (low \(\text{pH}\)), the pigment tends to appear red or pink. As the environment becomes more neutral or slightly alkaline (higher \(\text{pH}\)), the pigment shifts toward violet and deep purple.
Achieving a true blue color is a significant biological challenge, requiring a relatively alkaline \(\text{pH}\) (often 7 to 8) in the vacuole, which is difficult for most plant cells to maintain. Plants that manage this often use complex mechanisms, like co-pigmentation or metal ion binding, to stabilize the blue form. The most common base pigment that leads to blue and purple is delphinidin, which is chemically predisposed to these cooler tones.
Key Blue and Purple Flower Varieties
Annuals and Tender Perennials
For immediate and season-long color, annuals offer a reliable source of vibrant blues and purples. The Morning Glory, specifically the ‘Heavenly Blue’ variety, produces large, trumpet-shaped, sky-blue flowers that climb vigorously, often reaching heights of 6 to 10 feet. This vine prefers full sun exposure to maximize its bloom production. Trailing Lobelia is a low-growing plant that produces masses of tiny, bright blue flowers, perfect for spilling over the edges of containers. It thrives best in part shade in hotter climates.
Perennials and Bulbs
Perennials provide structure and color that returns reliably each season. Delphiniums are renowned for their towering, stately spires of true blue and purple flowers, often growing 3 to 6 feet tall and requiring full sun to partial shade. Lavender, a classic Mediterranean shrub, features aromatic, spikey purple blooms and silvery-green foliage, requiring intense sunlight and well-drained soil. Irises, like the deep purple or blue-veined German Iris, offer sophisticated, architectural blooms in late spring and early summer.
Shrubs
The Hydrangea is the most famous shrub for color manipulation, exhibiting blue, purple, or pink flowers depending on soil chemistry. Large-leaf varieties like Hydrangea macrophylla can be coaxed into blue-purple shades. Rose of Sharon, such as the ‘Blue Chiffon’ cultivar, is a tall-growing shrub that offers soft blue or violet blooms from mid-summer into fall.
Optimizing Color Intensity in the Garden
Gardeners can actively influence the color intensity of certain flowers, particularly those whose pigments are highly reactive to soil conditions. The most dramatic example is the Hydrangea, where bloom color is directly tied to the availability of aluminum ions in the soil. To achieve blue or deep purple Hydrangea blooms, the soil \(\text{pH}\) must be acidic, ideally between 5.0 and 5.5. This acidity increases the solubility and uptake of aluminum by the plant. Achieving this low \(\text{pH}\) requires applying soil amendments like aluminum sulfate or elemental sulfur, which should be done carefully and incrementally after a soil test. For many blue and purple flowers, intense afternoon sun exposure can cause the delicate pigments to fade. Positioning these plants to receive morning sun and afternoon shade helps maintain the deepest color saturation.
Design and Pairing Principles
The cool tones of blue and purple flowers offer specific advantages in landscape design. Using these colors in the background of a garden bed creates a sense of greater distance, making the garden appear larger and adding visual depth. This effect occurs because cool colors tend to recede visually. For a high-energy, contrasting display, blue and purple blooms pair powerfully with their complementary colors: yellow and orange. Planting deep purple Salvia next to bright yellow Coreopsis creates a vibrant contrast that draws the eye immediately. Alternatively, a monochromatic scheme using various shades of blue and purple fosters a serene, calming, and cohesive atmosphere. Darker shades often take on a luminescent quality under the low light of dusk, making them excellent choices for planting near evening seating areas.