Cosmos Sulphureus Care: Sulfur Cosmos Growing Tips

Members of the Asteraceae family have always been popular with gardeners, and cosmos have become a hallmark flower in recent years.

One of the most beloved members of this new trend is Cosmos sulphureus (KAHS-mos sul-FER-ee-us), whose often firelike appearance has earned it the common name of sulfur cosmos.

Orange yellow Cosmos sulphureus flower
Semi Double Orange Cosmos sulphureus bloom | sansa55-DepositPhotos

Other common names include:

  • Klondike cosmos
  • Orange cosmos
  • Yellow cosmos

The attractive Cosmos sulphureus annual hails from Mexico and other parts of Central America, but its broad temperature tolerance allows it to be planted throughout most of the United States.

Popular cultivars of sulfur cosmos include:

  • Brightness Red
  • Diablo
  • Klondyke Mix
  • Ladybird Dwarf Gold
  • Ladybird Dwarf Lemon
  • Ladybird Dwar Orange
  • Ladybird Dwarf Red
  • Polidor
  • Sunny Gold
  • Sunny Red
  • Sunset
  • Tango

Sulfur Cosmos Care

Size & Growth

These popular easy to grow flowers usually reach a height of 2 to 3’ feet with a 1 to 3’ foot diameter, but heights up to 6’ feet have been recorded.

The green lanceolate leaves grow alternately and can grow up to 12” inches long.

Each leaf has 2 to 3 deeply cleft leaflets.

The stems are hairy and blooms arrive on peduncles measuring 4 to 8” inches long.

Flowering and Fragrance

The main attraction for Cosmos sulphureus begins in summer and continues until the first frost, with daisy-like flowers measuring up to 2.5” inches across.

These ray flowers range in color from yellow, orange-yellow, red, or deep orange; with cultivars producing blooms with a wider range including scarlet and variegated petals.

Light & Temperature

Cosmos love full sun. You’ll want to plant them somewhere that receives full sun at least 6 hours per day.

You can plant these semi-hardy plants outdoors in USDA hardiness zones 2A to 11b, as well as keeping them as indoor container plants.

Watering and Feeding

Cosmos sulphureus is drought-tolerant once the roots are established, and only require minor to moderate amounts of water.

Avoid using fertilizers as cosmos is adapted for poor soils and will overgrow if overfed.

Soil & Transplanting

The cosmos is adaptable to many soil types, with a well-drained clay, sandy, or loamy soil being preferable.

Avoid rich soils, as the plant will grow too tall and become unstable.

Grooming And Maintenance

You may choose to cut back taller plants as needed.

Cutting back to a height of 12″ to 18” inches as flowers dry out can extend the plant’s bloom time.

Deadheading spent flowers will also encourage additional blooms.

Tall plants will require support, preferably being tied with a loose string to a bamboo pole embedded 3 to 4” inches away from the stem.

How To Propagate Orange Cosmos

  • Seeds are the primary method of propagation for sulphur Cosmos.
  • Harvest seeds in the fall for planting the following spring.
  • Either sow the seeds indoors 4 to 6 weeks before the estimated final frost or outdoors once the final frost has passed.
  • A soil temperature of 65° degrees or higher is ideal.
  • Gently rake the seeds into the loose soil, being careful not to plant too deep.
  • Ensure the soil remains moist for 5 to 10 days after sowing.
  • Germination generally takes 7 to 21 days.
  • Cosmos is also able to self-seed when the seeds are allowed to fall onto bare ground.
  • Self-seeding allows you to increase the density of your blooms but may become aggressive in warmer climates.

Cosmos Sulphureus Pests or Diseases

This semi-hardy plant has few health risks, with the main pests being aphids and Japanese beetles.

Other pest include: root-knot nematodes

It is also susceptible to bacterial wilt and canker.

Other diseases include:

The plant is somewhat drought tolerant once established.

Cosmos can spread aggressively if allowed to self-seed unregulated and was declared invasive in the Southeastern US in 1996.

Suggested Sulfur Cosmos Uses 

  • In gardens, cosmos attracts both butterflies and other pollinators.
  • It works great as a border plant, in flower beds, or a container plant in full sun.
  • The cut flowers make excellent bouquets.
  • Due to the low care requirements, this is an excellent flower for children to care for.

The flowers have traditionally been used for dyes in the Americas and currently used to dye wool in South Africa.

The Cosmos sulphureus plant is somewhat edible, with young shoots being consumed raw or cooked in Indonesia, the flowers being used in Thailand for teas and salads, and dry extracts being used in Ukrainian baking.

There is some evidence of medicinal benefits to consuming parts of the plant.

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