The sensation-type cosmos is from tropical America and is grown as an annual primarily for its beautiful flowers (Figure 1). These plants can attain a height of 3 to 6 feet and have an open and sprawling habit. Finely cut and thread-like simple leaves are pinnately cut into deep lobes appearing compound. They are held upright by thin, weak stems. C. sulfureus has foliage that is not as finely divided. The flowers of this series of cosmos occur in the summer for early blooming varieties and in the fall for late blooming varieties. These large, showy flowers occur in a number of pastel colors including red, lavender, pink, white, violet, and rose. They are 2 to 3 inches in width and are daisy-like in appearance. The lovely flowers are borne in capitula with small discs, and the rays are toothed at their apices.
Scientific name: Cosmos bipinnatus 'Sonata White'
Pronunciation: KOZ-mus bye-pin-NAY-tus
Common name(s): 'Sonata White' Mexican aster
Plant type: herbaceous; annual
USDA hardiness zones: all zones (Figure 2)
Planting month for zone 7: Jun
Planting month for zone 8: May
Planting month for zone 9: Mar; Sep; Oct
Planting month for zone 10 and 11: Feb; Nov; Dec
Origin: not native to North America
Uses: border; attracts butterflies
Availability: somewhat available, may have to go out of the region to find the plant
Height: 1 to 4 feet
Spread: 2 to 3 feet
Plant habit: upright
Plant density: open
Growth rate: moderate
Leaf arrangement: opposite/subopposite
Leaf type: simple
Leaf margin: lobed
Leaf shape: variable
Leaf venation: not applicable
Leaf type and persistence: not applicable
Leaf blade length: 4 to 8 inches
Leaf color: green
Fall color: not applicable
Fall characteristic: not applicable
Flower color: white; lavender; pink; rose; red; violet
Flower characteristic: showy
Fruit shape: no fruit
Fruit length: no fruit
Fruit cover: no fruit
Fruit color: not applicable
Fruit characteristic: inconspicuous and not showy
Trunk and Branches
Trunk/bark/branches: not applicable
Current year stem/twig color: green
Current year stem/twig thickness: thin
Light requirement: plant grows in full sun
Soil tolerances: sand; acidic; slightly alkaline; loam; clay
Drought tolerance: moderate
Soil salt tolerances: unknown
Plant spacing: 12 to 18 inches
Roots: not applicable
Winter interest: not applicable
Outstanding plant: not particularly outstanding
Invasive potential: not known to be invasive
Pest resistance: long-term health usually not affected by pests
Use and Management
The sensation-type cosmos are largely used for cutting flowers because they grow too tall and fall over in the landscape. They can be employed as landscape plants if you do not mind them falling over onto other plants nearby. Early pinching causes branching and can increase the density of the plants.
Cosmos needs to be planted in an area of the landscape that receives full sun. It tolerates dry, porous soils and will produce foliage instead of flowers if heavily fertilized. Plants need to be supported or staked to prevent them from falling over. Place these plants 12 to 18 inches apart in the garden.
The propagation of cosmos is primarily accomplished by seed, which can be directly sown in the garden. The seeds of these plants will germinate in approximately one week and bloom in 2 to 3 months.
Cultivars are included in the 'Imperial Pink', 'Sensation', 'Sonata' and 'Vega' series.
Pests and Diseases
Cosmos species may be occasionally bothered by bacterial wilt, canker, powdery mildew, leaf spots, aphids, and Japanese beetles.