Black cosmos, with their striking appearance and unique fragrance, offer a distinctive garden addition. Their captivating flowers range from deep, velvety maroon to an almost black hue, creating a dramatic visual contrast. Their delightful chocolate or vanilla-chocolate scent adds intrigue for gardeners seeking unusual ornamental plants.
Characteristics of Black Cosmos
Cosmos atrosanguineus stands out due to its unique attributes. Its flowers range from deep crimson to rich brownish-red, often appearing nearly black, with a velvety texture. Blooms are typically 1.5 to 2 inches across, adding a delicate yet intense focal point.
Its distinct chocolate or vanilla-chocolate fragrance becomes more pronounced on warm, sunny days. Black cosmos is a tender perennial from Mexico, growing from fleshy tuberous roots. It can be grown as a perennial in USDA hardiness zones 7-11, or as an annual in colder regions. The plant forms an upright, bushy clump, reaching 18 to 30 inches tall and spreading up to 2 feet.
Planting Black Cosmos
Successful establishment of black cosmos requires careful site selection and adequate sunlight. They thrive in full sun, needing at least 6 to 8 hours daily for robust blooming. Protection from strong winds can prevent their slender stems from breaking.
Well-draining soil is crucial for black cosmos, as they are susceptible to root rot in waterlogged conditions. Sandy loam is ideal, with a preferred pH range of 6.0 to 8.0. For heavy clay soils, incorporating organic matter like compost improves drainage and aeration.
Planting should occur after all danger of frost has passed. Seeds can be sown directly outdoors in late spring once soil temperatures reach 65-75°F. When transplanting nursery starts, place them in mid to late spring, spacing them 12 to 18 inches apart for proper air circulation and growth.
Ongoing Care for Black Cosmos
Consistent moisture benefits black cosmos, especially during dry periods. Water deeply and regularly, about once a week, allowing the soil surface to dry slightly between waterings to prevent overwatering and root issues. Established plants show some drought tolerance, but consistent hydration ensures continuous vigor and blooming.
Fertilizing black cosmos should be done sparingly, as overly rich soil or excessive nitrogen can promote lush foliage at the expense of flower production. A balanced or low-nitrogen fertilizer can be applied monthly for container-grown plants, following product instructions to avoid over-fertilization. For garden beds, adding compost to the soil before planting is often sufficient, with additional feeding generally unnecessary unless the plant shows signs of nutrient deficiency.
Regular deadheading (removing spent blooms) encourages continuous flowering and maintains plant health. Light pruning can shape the plant or encourage bushiness. While generally resilient, black cosmos can be affected by common garden pests like aphids and slugs, or diseases like powdery mildew, especially in humid conditions. Good air circulation helps prevent fungal diseases.
In colder climates (below USDA Zone 7), where black cosmos may not survive winter outdoors, lift and store the tuberous roots. Similar to dahlias, dig up tubers after foliage dies back in fall and store them in a cool, dry, frost-free location until spring. In milder zones, a protective layer of mulch in autumn can help tubers overwinter in the ground.