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How to Grow Cosmos

Cosmos bipinnatus
Cosmos bipinnatus

Cosmos is an old-fashioned favorite that bears clouds of daisylike flowers on long stems above feathery foliage.

Cosmos can be started from seed indoors and transplanted into the garden two or three weeks after the last frost in spring. It will bloom from late spring into autumn.

Mass cosmos in large bands or groups in flower beds and borders. Grow cosmos in containers. It also be used in meadows and wildflower gardens.

Cosmos wants full sun and well-drained soil.

Cosmos sulphureus
Chocolate cosmos, Cosmos atrosanguineus

Get to Know Cosmos

  • Plant type: Tender annual
  • Growing zones and range: Summer annual in all zones
  • Hardiness: Tender; killed by frost but tolerant of high heat; Cosmos atrosanguineus is hardy to 15°F (-9°C).
  • Height and width: 1 to 6 feet (.3-1.8m) tall, 10 to 36 inches (25-91cm) wide depending on the variety
  • Foliage: Lacy, threadlike to feathery leaves are mid green
  • Flowers: Single or double disc and ray flowers; serrated flower petals may be daisylike, or may be crested, double or quilled around yellow center.
  • Flower colors: Yellow, orange, scarlet, pink, deep rose, white, and bicolors.
  • Bloom time: Summer
  • Uses: Mass in beds and borders; very good cutting flower.
  • Common name: Cosmos
  • Botanical name: Cosmos spp.
  • Family: Asteraceae
  • Origin: Southern United States and Central America

Where to Plant Cosmos

  • Plant Cosmos in full sun or partial shade.
  • Cosmos will grow in poor to average, well-drained soil.
  • Cosmos prefers a soil pH of 6.5 to 7.
  • Good garden companions for Cosmos include Achillea, Belamcanda, Coreopsis, Diascia, Gaura lindheimerri, Linaria.

Cosmos Uses

  • Mass plant Cosmos in beds and borders.
  • Cosmos is a good background for intermediate and dwarf annuals and perennials.
  • Cosmos make excellent cut flowers.
Cosmos in the garden
Cosmos bipinnatus in the garden

When to Plant Cosmos

  • Set Cosmos seedlings in the garden after all danger of frost has passed in spring.
  • Sow seeds indoors 3 to 6 weeks before the last frost; seeds will germinate in about 5 days.
  • Sow seed directly in the garden after the last frost.
  • Set out plants after the last frost when the weather is settled and warm.
  • Make several successive sowing to assure a good supply of flowers which will bloom until frost.

Planting and Spacing Cosmos

  • Sow seeds indoors in six-pack or flats or one seed per 4-inch plant; cover seeds with 1/4 inch of soil. Keep seeds warm at 65°F to 80°F (18°-26.7°C); seeds will germinate in about 5 days.
  • Sow seeds outdoors in smoothly prepared; space seeds 3 inches (7.6cm) apart 1/4 inch deep.
  • Set container grown plants in the garden when the weather is settled and warm after hardening them off for several days.
  • Space Cosmos 10 inches (25cm) apart.
Cosmos sulphureus
Yellow cosmos, Cosmos sulphureus

How to Water and Feed Cosmos

  • Cosmos requires moderate water; water as needed. Cosmos can tolerate drought.
  • Cosmos does not need feeding; prepare the planting bed in advance by adding plenty of aged compost or commercial organic planting mix.
  • Mulch around plants with aged compost to keep the soil evenly moist.

Cosmos Care

  • Stake Cosmos as needed especially plants growing in shade or windy areas.
  • Anchor tall plants will additional soil heaped atop the roots.
  • Trim spent blooms to promote more blooms.

Cosmos Pests and Diseases

  • Cosmos is commonly pest and disease-free.

Cosmos Propagation

  • Germination takes 5 to 7 days at 60°F (16°C).
  • Basal cuttings can be rooted with bottom heat in spring.
 Cosmos bipinnatus
Cosmos bipinnatus

Cosmos Varieties to Grow

  • Cosmos atrosanguineus, chocolate cosmos. Tuberous-rooted perennial; grows 24 to 30 inches (61-76cm) tall with coarsely cut foliage; deep brownish-red flowers to nearly 2 inches (5cm) wide; has strong chocolate or vanilla fragrance.
  • C. bipinnatus. Flowers are pink, rose, lavender, or white with golden discs. ‘Sensation’ is a classic strain with white and pink flowers that can grow to 10 feet (3m) tall. ‘Sonata’ is compact 2-foot tall flowers in pink, deep rose, white.
  • C. sulphureus. Flowers are 1 to 2 inches (2.5-5cm) wide; they may be yellow, orange, or rusty red; leaves are broader and plants are smaller than C. bipinnatus; ‘Bright Lights’ is a cultivar that grows to 3 feet (.9m) tall and has yellow, orange, and scarlet flowers.

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