This section is from the book "The Standard Cyclopedia Of Horticulture Vol2", by L. H. Bailey. See also: Western Garden Book: More than 8,000 Plants - The Right Plants for Your Climate - Tips from Western Garden Experts.
(from the Greek word with a root idea of orderliness; hence an ornament or beautiful thing). Syn., Cosmea. Composite. Annual or perennial herbs, now popular as flower-garden subjects.
Often tall, usually glabrous: leaves opposite, pinnately cut in the garden kinds flowers typically shades of rose, crimson and purple, with one yellow species, and white horticultural varieties, long peduncled, solitary or in a loose corymbose panicle: achenes glabrous: chaff of the receptacle in C. bipinnatus with a long and slender apex, in other species with a blunt and short apex. - Perhaps 20 species, all tropical American, mostly Mexican. The genus is distinguished from Bidens chiefly by the seeds, which are beaked in Cosmos but not distinctly so in Bidens, and by the color of the rays, which in Cosmos is typically some form of crimson, while in Bidens the rays are yellow or white.
The "black cosmos" (C. diversifolius) is, perhaps, better known to the trade as a Bidens or Dahlia. It has the dwarf habit and dark red early flowers of some dahlias, but the achenes are very puzzling. They resemble those of Bidens in being four-angled, and not distinctly beaked. They are unlike Bidens, and like Cosmos, in being not distinctly compressed on the back. They resemble both genera in having two rigid persistent awns, but, unlike these genera, the awns have no retrorse barbs or prickles. The achenes are linear, as in Cosmos and all our native tropical species of Bidens; but, although narrowed at the apex, they are not distinctly beaked, as in most species of the genus Cosmos. The plant is, perhaps, nearest to Bidens.
Until 1895 there were in the two leading species only three strongly marked colors: white, pink and crimson. These and the less clearly defined intermediate shades have come from C. bipinnatus; and the yellow forms have come from C. svlphureus, which was introduced in 1896. At first cosmos flowers were only an inch or two across. The best varieties now average 3 inches, and sometimes reach 4 or 5 without thinning or disbudding. Pure white flowers of cosmos are rarely if ever found wild, but some of the cultivated varieties are clear white. The group is lacking in bright deep reds. There are no full double forms of cosmos as yet, and, as regards strongly marked types of doubling\, the cosmos may be decades behind the China asters, In the single forms, flat, incurved, or cupping, and reflexed flowers are to be looked for. Stellate forms are now offered; and also dwarfs, and other variants.
It is a mistake to grow cosmos in too rich soil, as one gets too vigorous growth and too few flowers, which are also late. A sandy soil is to be preferred as being earlier, and not too rich. It is well to pinch out the leading shoots of young plants in order to make them bushy and symmetrical, instead of tall and straggling. In the East, for best results it is still necessary to sow seed indoors in April and transplant outdoors as soon as danger of frost is past. Seed sown in the open ground often fails to produce flowers in some northern localities before frost. The early frost kills the typical species, but some of the new strains are said to resist a degree or two of frost.
A. Rays white, pink or crimson: disk yellow. bipinnatus, Cav. Fig. 1077. Glabrous annual, 7-10 ft. high: leaves bipinnately cut, lobes linear, remote, entire: involucral scales ovate-lanceolate, acuminate: fls white, pink or crimson: seeds smooth, with an abrupt beak much shorter than the body. Mex. B.M. 1535. Gn 41:10. R.H. 1892:372. -The older and commoner species. C. hybridus, Hort., is presumably a trade name for mixed varieties of C. bipinnatus, but seeG.F. 1:474 for note.
Fig. 1077. Cosmos bipinnatus.
aa. Rays yellow: disk yellow. sulphureus, Cav. Fig. 1078. Pubescent, 4-7 ft. high, much branched: leaves often 1 ft. or more long, 2- or 3-pinnately cut, lobes lanceolate, mucronate, with rachis and midrib ciliate or hispid; pinnae alternate, entire or 2-3-toothed: peduncles 7-10 in. long, naked: outer involucral bracts 8, linear, acuminate, green, 2 lines long; inner ones 8, oblong, obtuse, scarious, 5 lines long; flowers 2-3 in. across, pale, pure or golden yellow; rays 8, broadly obovate, strongly 3-toothed at the apex, ribbed beneath; anthers of the disk exserted, black, with orange tips: seeds linear, 1 in. long, including the slender beak. Mex. G.F. 8:485 (adapted in Fig. 1078). - introduced 1896; parent of the yellow forms.
Fig. 1078. Cosmos sulphureus. ( X 1/3)
aaa. Rays dark red: disk red.
Otto (Bidens atrosanguinea, Ortg. B. dahlioides, Wats. Dahlia Zimapanii, Roezl). Black Cosmos. Tender annual, 12-16 in. high, with tubers more slender, and requiring more care in winter than those of common dahlias: leaves pinnately parted; leaflets 5-7, entire or slightly serrate, the terminal leaflets largest: peduncles each bearing 1 head 6 in. or more above foliage; rays dark velvety red, sometimes tinged dark purple. Mex. B.M. 5227. Gt. 1861:347. F.C.2:47. J.H. III. 33:403. variety superba, Hort., is sold. - Prop, almost exclusively by seeds. Wilhelm Miller, †