This plant is a summer annual about 2-6' tall that is unbranched below,
but branched above, with erect to ascending stems. The stems are light
green to dark red, terete, and either glabrous or sparsely
short-pubescent. Pairs of opposite compound leaves are widely spaced
along the stems. Individual compound leaves are up to 5" long and 3½"
across; they are usually bipinnate with narrowly linear lobes (about 1
mm. across) that are light to medium green and glabrous. The compound
either sessile or they have short petioles up to ¼" long.
stems terminate in individual flowerheads about 2-3" across that are on
long naked peduncles (4-8" long). The peduncles have characteristics
that are similar to the stems. Each flowerhead consists of about 8 ray
florets that surround numerous tiny disk florets. The ray florets are
sterile, while the disk florets are perfect and fertile. The petaloid
extensions of the ray florets are white, pink, or magenta; sometimes
they are bicolored. In addition, these petaloid extensions are obovate
in shape and their outer edges are shallowly
cleft. The circular perimeter of the disk florets spans about ½"
across; it is relatively small in relation to the diameter of the
flowerhead. Individual disk florets are about ¼" long, bright yellow,
and narrowly cylindrical. At the base of the flowerhead, there are
(phyllaries) in 2 series; there are about 8 floral bracts
series. The outer floral bracts are green and lanceolate, while the
inner floral bracts are purple and ovate. Both types of bracts are
about ½" in length. Together, the floral bracts form a shallow cup at
the base of the flowerhead. The blooming period occurs from mid-summer
into the fall, lasting 2-3 months. Afterwards, the disk florets of each
flowerhead are replaced by narrowly cylindrical-angular achenes about
½" long. Each slightly curved achene is slightly swollen toward its
outer tip, where
there is a minute beak. A pair of awns about 1 mm. in length is
sometimes present at this beak. The outer surface of each mature achene
dark-colored. The root system consists of a taproot. This plants
reproduces by reseeding itself.
The preference is
full sun, mesic or dry-mesic conditions, and a mildly acidic to
alkaline soil containing loam, clay loam, gravel, or sand. Once the
seeds germinate, growth and development is rapid. Wild plants tend to
be shorter and bloom less abundantly than garden plants in fertile
soil, otherwise they appear much the same.
The introduced Common Cosmos rarely naturalizes in Illinois, where it
has been reported in only a few counties. Wild
populations of this plant rarely persist. Common Cosmos was introduced
into the United States as an ornamental flowering plant from Mexico or
tropical America, and it is still commonly cultivated. Habitats
include fallow or abandoned fields, roadsides, areas along
railroads, and waste areas. Habitats with a history of disturbance and
relatively open ground are preferred.
Very little is known about the floral-faunal
relationships for this introduced plant in North America. Butterflies
and possibly other insects visit the flowers for nectar and/or pollen.
Insects that feed destructively on Cosmos
(Aster Leafhopper) and the polyphagous
(Gray Looper Moth).
A fallow field near Champaign, Illinois.
Because of its attractive flowers, fine fern-like foliage, and ease of
cultivation, it is easy to understand why Common Cosmos remains a
popular garden plant. There are many cultivars to choose from. While
Cosmos occasionally reseeds itself, it has not been invasive in
Illinois. It is easy to identify in the wild because nothing else
closely resembles it, except other non-native Cosmos spp.
have not escaped from cultivation within the state. Some Coreopsis spp.
have finely divided foliage and similar flowerheads, but they tend to
be shorter plants and their flowerheads are usually yellow.