This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
A large genus of annual and perennial herbs of very diverse habit. Involucre globose or oblong; bracts imbricated, scarious, fringed, toothed, or spinous. Receptacle bristly. Florets all tubular, the outer ones often larger and neuter; lobes 5, narrow. Achene flattened; pappus short and bristly, rarely none. There are upwards of 200 species, the greater part of which inhabit the countries bordering the Mediterranean Sea. C. nigra, Knapweed or Buttonweed, is a familiar native example. The name is of classical origin. We must limit ourselves to a small selection.
1. G. Cyanus. Cornflower. - This handsome species is commonly seen in our corn-fields, where its bright blue flower-heads are highly attractive. It grows from 2 to 3 feet high, bearing numerous terminal stalked flower-heads. Leaves linear, entire or lobed, cottony. Under cultivation this species has produced white, pink, and purple varieties.
2. C. Americana (fig. 149). - A very ornamental species, about 3 or 4 feet high. Flower-heads very large, lilac-purple. A native of the southern United States.
Fig. 149. Centaurea Americana. (1/4 nat. size.)
C. depressa is a blue-flowered branching species about 18 inches high; and G. moschata, Sweet Sultan, and G. Amberboi, Yellow Sultan, are Eastern species, the latter with pale yellow and the former with purple or white agreeably scented flower-heads. These two species are also sold under the name Amber-boa.
3. C. Babylonica (fig. 150). - This attains a height of 6 feet or more, and is clothed with a white cottony down: Flower-heads yellow, appearing in July. Levant.
Fig. 150. Centaurea Babylonica. (1/20 nat. size.)
4. C. Ragusina. - This is a frutescent species with silver-white pinnate foliage, now very common in gardens, where it is employed for contrasting with bright-flowered bedding plants. The yellow flower-heads are of medium size and very pretty. But it is for the foliage mainly that this and some allied species are cultivated. Amongst these we may mention, C. dealbata; C. candidissima, syn. C. Cineraria, with leathery pinnate leaves having a large terminal rounded lobe; and C. Clementei. Other perennial species occasionally seen are C. macrocephala with simple stems and immense solitary yellow flower-heads; and C. orientalis, a more branching plant with yellow flower-heads.
A few other species of this group worthy of a place in a large garden are: Carthamus tinctorius, Safflower, a rigid erect annual with leafy involucres and orange-red florets; Carduus Marianus, Blessed Thistle, a more familiar plant, having spiny leaves veined with white and (drooping purple flower-heads; Sonchus macranthus, Onopordon Acanthium, 0. horridum, and 0. Tauricum, and Cdrduus afer, perennials of large size, prickly foliage, and large purple flower-heads.