DAHLIA, a genus of herbaceous plants of the family Corn positae (q.v.). The genus contains ten species indigenous to the high sandy plains of Mexico. The dahlia was first introduced into Britain from Spain in 1789 by the marchioness of Bute. The species was probably D. variabilis, whence by far the majority of the forms now common have originated. The flowers, at the time of the first introduction of the plant, were single, with a yellow disk and dull scarlet rays; under cultivation since the beginning of the I9th century in France and England, flowers of numerous brilliant hues have been produced. The flower has been modified also from a flat to a globular shape, and the arrangement of the florets has been rendered quite distinct in the ranunculus and anemone-like kinds. The ordinary natural height of the dahlia is about 7 or 8ft., but one of the dwarf races grows to only 18in. With changes in the flower, changes in the shape of the seed have been brought about by cultivation; varieties of the plant have been produced which require more moisture than others; and the period of flowering has been made considerably earlier. In 1808 dahlias were described as flowering from September to No vember, but some of the dwarf varieties at present grown are in full blossom in the middle of June.
The large number of varieties may be classed as under the fol lowing heads : (1) Single dahlias. These have been derived from D. coccinea; they have a disk of tubular florets surrounded by the large showy ray florets. (2) Show dahlias, large and double with flowers self-coloured or pale coloured and edged or tipped with a darker colour. (3) Fancy dahlias, resembling the show but having the florets striped or tipped with a second tint. (4) Bouquet or Pompon dahlias, with much smaller double flowers of various colours. (5) Cactus dahlias, derived from D. Juarezi, a form which has given rise to a beautiful race with pointed starry flowers. (6) Paeony-flowered dahlias, a race with large, floppy heads, broad florets and several disk florets in centre.
New varieties are procured from seed, which should be sown in pots or pans towards the end of March, and placed in a hotbed or propagating pit, the young plants being pricked off into pots or boxes, and gradually hardened off for planting out in June; they will flower the same season if the summer is genial. The older varieties are propagated by dividing the large tuberous roots, in doing which care must be taken to leave an eye to each portion of tuber, otherwise it will not grow. Valuable varieties are some times grafted on the roots of others. The best and most general mode of propagation is by cuttings, to obtain which, the old tubers are placed in heat in February, and as the young shoots, which rise freely from them, attain the height of 3in., they are taken off with a heel, and planted singly in small pots filled with fine sandy soil, and plunged in a moderate heat. They root speedily, and are then transferred to larger pots in light rich soil, and their growth encouraged until the planting-out season arrives, about the middle of June north of the Thames. Dahlias succeed best in an open situation, and in rich deep loam, but there is scarcely any garden soil in which they will not thrive, if it is manured. Dahlias flower on till they are interrupted by frost in autumn. The roots are then taken up, dried, and stored in some place where they may be secure from frost and moisture. Ear wigs are very destructive, eating out the young buds and florets. Small flower-pots half filled with dry moss and inverted on stakes placed among the branches, form a useful trap.