Cosmos bipinnatus is an annual that is often considered half-hardy, although plants may reappear via self-sowing for several years. The plant height varies from 2–6 ft to (rarely) 9 ft (0.61–1.83–2.74 m). The cultivated varieties appear in shades of pink and purple as well as white. The branched stem is usually densely to occasionally occupied by fine, split up, rough trichomes, some specimens are completely hairless. The petiole itself is inconspicuous, winged, 10 (rarely to 15) mm long, sometimes the leaves are almost sessile.
The partial leaves are linear-filiform to narrow linear with a width of 0.5 to 1 (rarely to 1.7) mm; the tips are pointed, hardened, but not particularly sharp. Its foliage is finely cut into threadlike segments. When flowering, the plant can become top heavy. This problem is alleviated when grown in groups, as the bipinnate leaves interlock, and the colony supports itself.[8]
The achenes become blackish, are smooth or short-bristly. Their shape is spindle-like. They are rounded off into a short, 0.5 to 1.7 mm long, but distinctly pronounced rostrum. The inner achenes are up to 18 mm long, their yellowish beaks are 4 to 5 (rarely to 10) mm long. A pappus is missing or it consists only of two to three awn-like, 1-3 mm large bristles.[9]
The very conspicuous cup-shaped inflorescences have a diameter of usually 5–7 cm (2.0–2.8 in) and contain tongue and tubular flowers, which are surrounded by bracts. There are usually 8 outer bracts, and they are ovate to lanceolate-tail-shaped, 7-15 mm long, 3-5 mm wide. The inner bracts are ovate-lanceolate and 8-12 mm long. They are translucent with many black stripes and a clear edge up to 1 mm wide, sometimes with yellowish or pink pigments, the tip is ciliate. The sprout leaves have gold-yellow, thread-like tips and protrude between the tubular flowers. The broadened base of these spreader leaves is translucent, provided with a yellow line. During flowering, the plant can sag under its weight. This problem can be solved by grouping the feet together so that the leaves hang together.
The mostly eight ray florets are pink to violet or white colored, at the base may show noticeable stains caused by anthocyanin. The tongues are reversely ovate shaped, have a length of usually 20-35 mm and a width of usually 12-20 mm. The tips are almost dull and have three broad, wavy teeth. Below that, they are greatly rejuvenated. In the center of the flower baskets is a large number of tubular flowers (also called disc florets), whose overgrown petals are yellow, turn white in the lower part and reach a length of 5-6 mm. The anthers are brownish-black and about 3 mm long, at the tips are short-triangular, translucent attachments with a length of 0.5-0.8 mm. The branches of the stylus are short and rather dull, with a length of .5 mm.[10]
This plant is native to Mexico, Guatemala and Costa Rica. Since it is used as an ornamental plant in many countries and prone to sedimentation, it is an invasive plant in many areas of the world. It has naturalized in scattered locations across North America, South America, the West Indies, Italy, Australia, and Asia, where it is a garden escape (introduced species) and in some habitats becoming a weed.[8]
Germination takes between 7 and 10 days at the optimal temperature of 75 °F (24 °C); flowering begins between 60 and 90 days after germination
It prefers a soil pH between 6.0 and 8.5, reflecting its native habitat in the alkaline regions of Central America
Flowering is best in full sun, although partial shade is tolerated
Excessive rain can cause cultivation problems, due to the delicate nature of the stems. Heavy rain can cause breakage. Cosmos bipinnatus can tolerate heat as long as adequate moisture is provided, however, it does not handle droughts, strong winds or cold temperatures well. Snails, slugs and aphids have a taste for Cosmos bipinnatus. Successfully cultivated plants can mature 2 to 4 feet (0.61 to 1.22 m) x 12 to 18 inches (300 to 460 mm).[26]
They are not tolerant of frost, but can be grown outdoors in a temperate climate with a warm to hot summer and are therefore called half-hardy in British gardening literature.[27]
^Erich Oberdorfer : Plant sociology excursion flora for Germany and adjacent areas . In collaboration with Angelika Schwabe and Theo Müller. 8th, heavily revised and supplemented edition. Eugen Ulmer, Stuttgart (Hohenheim) 2001, ISBN3-8001-3131-5 , p 929
^Thomas E. Melchert Cosmos bipinnatus. In: Dorothy L. Nash, Louis O. Williams (ed.): Flora of Guatemala. In: Fieldiana: Botany. Volume 24, Part XII, 1976, pp. 230-231.
