After Dr Andreas Dahl (1751–89), a Swedish botanist and a pupil of Linnaeus.
Perennial herbs, shrubs or trees, rarely climbers, usually tuberous. Stems erect, branching. Leaves along stems, opposite or whorled, simple, divided or compound. Capitula radiate, terminal, solitary or clustered, often nodding. Involucral bracts in 2 rows, overlapping, unequal. Receptacle with scales, flat or convex. Ray florets female or sterile, ligulate, variously coloured. Disk florets bisexual, tubular, yellow or purple. Achenes narrow, compressed. Pappus of 2 minute teeth or absent.
Grown as both border and smaller bedding plants as a wide range of garden cultivars which are selected for their vibrantly coloured flowers with strongly symmetrical forms.
Background history: The Dahlia, which grows naturally in C and S America, was encountered by the Spanish Conquistadors in the 16th century, but it had been grown by the Aztecs as a medicinal and crop plant (for animal fodder) as well as an ornamental for many years.
The first dahlias were introduced to Europe around 1790. Early interest was centred in both Europe and the USA where enthusiasm reached a peak in the 1830s to 40s. Breeding and selection of new cultivars is now also active in S Africa, Israel and Australia. The modern garden dahlias (D. ×hortensis Guillaumin) are complex garden hybrids presumed to be derived from D. pinnata with D. mazonii possibly one of the parents, giving rise to magenta-flowered offspring, and D. coccinea to offspring with scarlet or orange flowers. Most colours are now available, including striped and multicoloured blooms, but with the exception of true blue.
The different horticultural groups of dahlias arose as follows:
New cultivars are currently produced in England, although Australia can boast many fine selections. Australian growers have included: Harry Brand from SA, who produced many giant dahlias, often with 'Croydon' in the name e.g. 'Croydon Snowtop'; Mr Baynes, who produced top variety giants; Frank Rossack of Grovedale, Geelong, Victoria, who produced many different kinds; Bert Tilbury, who specialised in small and miniature Cactus, including 'Mrs Rose Fletcher', which was the world's best cut flower for many decades; and Norm Williams,who is known for his Pompon dahlias. Present-day raisers include C. Brooker, J.Daniel, E.Griffen, G. Harding, J. Matulick,N.Nauman, R. Sellick,W.Tapley, F.Turton and R.Yeomans. In Australia in 1919, John Cronin, Director of the Royal Botanic Gardens Victoria, raised a Rosette dahlia that was to be the precursor of the Cactus dahlias - Australia leads the world in the Semi-decorative class of dahlias. Altogether, several thousand cultivars have been available in Australia, the majority of these of Australian origin. Cultivars from overseas are obtained from a few specialist growers.
The International Registration Authority for Dahlia cultivars, the Royal Horticultural Society, UK, recognises 10 horticultural groupings for Dahlia cultivars, subdivisions generally being made on flower size and colour.The Australian Dahlia Council (formed in 1976) has produced a booklet entitled Standards and Judging Rules of the Dahlia (1998) in which 17 different ornamental horticultural show categories are presented.The National Dahlia Society of Victoria (formed 1960) has produced a Dahlia Directory 1996/97 listing most cultivars available and exhibited in Victoria.
Of the few species dahlias grown, D. merckii Lehm. from Mexico is occasionally used as a border plant with attractive simple purple to white flowers and rounded and shortly pointed leaflets less than 5 cm long, while D. pinnata Cav., the Common or Garden Dahlia, has mostly lilac flowers and deeply dissected leaves. However, plants long known as D. pinnata may be D. ×hortensis.
Cultivars by tubers or cuttings, bedding and species by seed.
In C America the tubers were once grown as source of starch-like inulin and used as animal feed.
Large, colourful, nodding capitula.
Country Dahlias at Winchelsea, Vic, has over 20 000 plants on view; other large collections are held by Avonlea Dahlias, Dazzling Dahlias and Jos Van Berkel Nursery.
About 28 species from S North America, C America and NE South America.
Sorensen (1969). Popular: Pescott (195?), Rowlands (1999). Cultivars: International Cultivar Registration Authority is the Royal Horticultural Society, UK (over 20 000 names listed), National Dahlia Society of Victoria Quarterly Magazine, and the South Australian Dahlia Society Year Book.
Source: (2002). Dahlia. In: . Horticultural Flora of South-eastern Australia. Volume 4. Flowering plants. Dicotyledons. Part 3. The identification of garden and cultivated plants. University of New South Wales Press.
