Cosmos flowers, members of the genus Cosmos, are a popular choice for gardeners due to their delicate, daisy-like appearance and ease of cultivation. The two species most commonly grown as annuals are Cosmos bipinnatus and Cosmos sulphureus, both native to Mexico. These plants provide a spectacular range of color across the warm and cool ends of the spectrum. The simple structure of the ray florets surrounding a central disc makes the flower an ideal canvas for color, ranging from the palest white to deep crimson and fiery orange.
The Spectrum of Common Colors
The color palette of cosmos is broadly divided between the two main species. Cosmos bipinnatus, often called the garden cosmos, is known for its cool and pastel tones. This species features shades including pure white, soft pink, magenta, and rose. Cultivars within this group can also display deep crimson and subdued lavender hues. The most common varieties of C. bipinnatus are characterized by their single row of petals and feathery, fine foliage.
Cosmos sulphureus, conversely, is recognized for its vibrant, warm colors. This species is commonly referred to as the sulfur cosmos because of its original bright yellow coloration. The color range extends through various shades of gold, brilliant orange, and fiery red. Newer varieties even include tones that approach mahogany, offering a deep contrast to the yellows and oranges.
How Cosmos Flowers Produce Their Hues
Flower color in cosmos is determined by two primary classes of pigment molecules. Anthocyanins are water-soluble pigments responsible for the red, pink, and purplish colors found mainly in C. bipinnatus. These molecules are stored within the cell vacuoles of the petals. Their exact hue is influenced by the concentration and the acidity (pH) of the cell sap. A more acidic environment tends to produce redder colors, while a less acidic one might shift the color toward blue or purple.
Carotenoids are the second major pigment group, responsible for the yellows and oranges seen in C. sulphureus. Unlike anthocyanins, carotenoids are fat-soluble and are housed in specialized organelles called chromoplasts within the petal cells. Specific carotenoids, including beta-carotene and beta-cryptoxanthin, are responsible for the vibrant yellow-orange shades. White petals are produced when the genes responsible for synthesizing both pigment groups are inactive, resulting in a complete absence of color molecules.
Specialty Colors and Cultivated Novelties
Modern horticulture has expanded the cosmos palette far beyond common solid colors through selective breeding. Novelty varieties often feature bi-colors, where a petal displays two distinct shades, or picotee patterns, which show a contrasting color along the petal’s edge. Cultivars like ‘Apricot Lemonade’ showcase a blend of soft apricot and lilac blush that fades to a pale yellow, illustrating complex color gradients. Other unique forms include striped or splashed patterns and double-flowered types, where the genetic expression of color is uneven across the petal surface.
The most dramatic color novelty is the deep, almost black, maroon of the Chocolate Cosmos, Cosmos atrosanguineus. This rich, velvety color is due to a high concentration of anthocyanins and a related flavonoid compound called chalcone. The color is so intense it appears dark chocolate brown, fitting for a flower that also releases a distinct scent of vanilla and hot chocolate on warm days. However, a true blue cosmos remains a challenge, as the specific genes required for blue-shifting anthocyanins are absent in the Cosmos genus.