Centaurea cyanus aka Cornflower

Centaurea cyanus is an annual in the aster family. Gardeners call it cornflower or bachelor's button. Kew places its native range in the central and eastern Mediterranean. NC State gives a broader origin of Europe and western Asia. It now grows across the United States and southern Canada. It occurs in fields and beside roads and railways. Once widespread in crop fields, it has declined under modern farming.

Garden plants reach about 1–3 feet [0.3–0.9 m] tall. RHS gives an upper height of 3.3 feet [1 m]. Their spread reaches about 1–2 feet [0.3–0.6 m]. The stems start upright, then branch above the base. Woolly hairs give the green stems a gray cast. Tall plants may lean without stakes or nearby plants. The leaves grow one per node along the stems. They have pointed tips and a narrow, lance-shaped outline. Some leaves have lobes, and long hairs cover their surface.

Blue flower heads appear from late spring through summer. Missouri Botanical Garden lists the main season as May to July. Each head spans about 0.1 feet [0.03–0.04 m]. Large outer florets spread around a core of smaller florets. The inner florets have a violet-blue shade. Plants may also bear pink or white blooms. The fruit is a small achene that ripens in fall. Fine hairs cover it, with short brown bristles at its tip. Birds and butterflies feed on the flowers and fruit.

Cornflower grows best in full sun and well-drained soil. It accepts poor soil and needs no added fertilizer. It tolerates drought but prefers a medium level of moisture. Light shade suits it, though deeper shade makes stems flop. Sources list USDA zones 2–11 and UK hardiness H6. Sow seeds where plants will grow in spring or autumn. Removing spent blooms extends flowering and limits seed spread. Plants reseed freely and can become weeds. They suit borders, pots, cottage gardens, and meadows. Their blooms also last well as cut flowers.

Serious pest and disease problems are uncommon. Aphids, mealybugs, wilt, rot, rust, and powdery mildew can occur. Young shoots and flowers are edible, either raw or cooked. Fresh florets add color to salads and serve as garnishes. Flowers also supply blue coloring for sweets, ink, and dyes. Dried petals add color to potpourri. Herbal use has a long history, but eye-treatment benefits remain disputed. Pet sources class cornflower as non-toxic. Eating large amounts may still cause nausea or vomiting.