Dictionary of Flowers: Cosmos (Mexican Aster)

Cosmos (Mexican Aster) is pretty in pink. Image used under a Creative Commons licence with the kind permission of Toshihiro Gamo and Flickr

Cosmos (Mexican Aster) is pretty in pink. Image used under a Creative Commons licence with the kind permission of Toshihiro Gamo and Flickr

  • Cosmos
  • Common name: cosmos, Mexican aster
  • Family: Asteraceae
  • Category: annual
  • Height: 10" to 30" 
  • Width: 12" to 18"
  • Sun
  • Blooms: early summer to early fall
  • Attracts: butterflies, birds
  • Growth habit: clumping. Used as a filler in containers
  • Maintenance: easy
  • Soil: average to poor, free draining. Water regularly if grown in containers
  • Garden uses: containers, mixed border, cottage garden, cutting garden
  • Diseases: disease resistant
  • Pests: pest free

A genus of species native to Mexico, cosmos is an easy and fast to grow annual.

The foliage is fern-like and the flowers appear on long stems making them suitable as cutting flowers.

Flower color depends on species: c. bipinnatus has white, pink, and burgundy flowers: c. sulphureus blooms can be yellow, orange and mahogany.

The name derives from the Greek meaning "beautiful".

It's an old fashioned, cottage style plant suitable for both containers and ground planting.Short varieties of cosmos are suitable for container planting.

Taller varieties (up to four feet) are best planted in the middle of a border and might require stalking.

Cosmos grows fast from seed. Plants are also available in nurseries in late spring and early summer. The seeds are fast to germinate.

IWILLWRITECAPTION. Image used under a Creative Commons licence with the kind permission of NAMEOFFLICKRUSER and Flickr

If planted indoors, plant in moist starting mix six weeks before the last frost date. Kept at 75F, they germinate in about five to ten days. Provide the seedlings good light.

Harden off before setting in its permanent location after danger of frost has passed. They can start blooming as fast as eight weeks after planting. Seeds can also be direct sown outside as soon as the last frost is over. Plant seeds about one quarter inch deep.

Whether planting starts or seedlings grown from seed, water well when first setting plants outdoors. Cosmos is somewhat drought tolerant once established.

Cosmos prefers full sun, a regular watering schedule and little fertilizer. Over-fertilized plants tend to bloom less, as do plants grown in part shade. They need to be deadheaded to prolong bloom period. If collecting seeds, stop deadheading in late summer.

The taller varieties suitable for beds and borders benefit from a mid to late summer trim.

If the plants start looking tired, trim to one third of the length. Water well and fertilize. They will start producing new blooms again.

Cosmos prefers average soil, but can tolerate poor soil that drains well. Overwatering cosmos or soils with poor drainage will cause root rot and kill the plant.




IWILLWRITECAPTION. Image used under a Creative Commons licence with the kind permission of NAMEOFFLICKRUSER and Flickr

Though disease resistant, cosmos can develop mildew if grown in too damp weather or if the plants are overcrowded. Improving air circulation should help. If needed, use an antifungal solution.

They are pest resistant. 

Popular varieties:

  • Cosmos bipinnatus 'Sonata' series - two feet tall plants, large frilled flowers in white, pink and dark rose. Heat tolerant
  • Cosmos bipinnatus 'Antiquity' - flowers change color as they mature. They open burgundy and turn warm pink. Dwarf, twenty four inches
  • Cosmos bipinnatus 'Cosimo' - dwarf, eighteen to twenty four inches. Blooms early. Flowers have light centers, darkening towards the edges, from light pink to reddish purple
  • Cosmos sulphureus 'LadyBird' mix - yellow, orange and scarlet flowers, semi double. Dwarf variety
  • Cosmos sulphureus 'Cosmic' series - dwarf (twelve to twenty inches), double and semi double flowers. Yellow, red, orange and yellow
  • Cosmos atrosanguineus (chocolate cosmos) - a species of cosmos with small, dark mahogany red blooms, slightly scented. Perennial in USDA zones 8 and up. Twenty four inches tall.

IWILLWRITECAPTION. Image used under a Creative Commons licence with the kind permission of NAMEOFFLICKRUSER and Flickr

Full List of Dictionary of Flowers Entries

Annuals For Containers

Perennials For Containers

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