Cosmos sulphureus


These popular easy to grow flowers usually reach a height of over a metre and produce a profusion of flowers throughout the summer.

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

The main attraction for Cosmos sulphureus begins in summer and continues until the first frost, with daisy-like flowers measuring up to 5 cm 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.

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.

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.

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.

You may choose to cut back taller plants as needed. Cutting back to a height of 30cm 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.

  • Harvest seeds in the autumn 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.


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