This month’s plant is the dahlia: they are everywhere at the moment, in their incredible variety, and I know almost nothing about them. I’m not even sure I like them all that much – their colours, and especially the complex structures of varieties like the pompom, sometimes look garish and even artificial. I usually have a few in pots, but never succeed in over-wintering the tubers, and I never know what to put them with: it seems to me that you really need a massive herbaceous border, or a layout (like that at Biddulph Grange) dedicated only to dahlias, to use them successfully.

The dahlia borders at Biddulph Grange.

I clearly needed to improve my knowledge and understanding of this enormous plant family: so here goes. Dahlias are herbaceous perennials of the Compositae (or Asteraceae) group; they are bushy and tuberous, and they originated in Mexico. There are 42 species of dahlia, and the reason that there are so many, and such a variety, of hybrids is a genetic one: they have eight sets of homologous chromosomes, while most plants have only two. (They also have other genetic traits which I pass over because I understand them even less.)

Dahlias were originally cultivated as a food crop by the Aztecs; the tubers were cooked and eaten like potatoes, and the long, hollow stalks of the tree dahlia (D. imperialis) could be used for transporting or piping water.

Dahlia imperialis, the tree dahlia.

The first botanical description was furnished by Francisco Hernández (1514–87), personal Physician to Philip II of Spain, who was despatched to the New World to report back on its medicinal plants. He distinguished two related types: the tree dahlia and D. pinnata, and determined their native name to be ‘Acocotle’ or ‘Cocoxochitl’, which he believed to mean ‘water cane’ or ‘water pipe’.

Dahlia pinnata.

Interestingly, it was not until the late eighteenth century that any specimens of the plants were brought back to Europe. An enthusiastic description of the flowers was published posthumously in the account of Nicolas-Joseph Thiéry de Menonville (1739–80), a French botanist who had volunteered to go to Mexico in 1776 to kidnap cochineal insects (and the cactus on which they lived), and establish them in the French colony of Saint-Domingue (now Haiti), thus breaking the Spanish monopoly on this expensive dye. (He also smuggled out vanilla pods, and indigo and cotton seeds – an one-man international act of botanical piracy.)

The cochineal beetle, and the cactus on which it lives/predates.

Menonville’s description of the dahlia flowers caused much interest in Europe, and in 1789, the director of the Mexico City botanical garden sent some tubers to his counterpart in Madrid, Antonio José Cavanilles, who gave the new plants their familiar taxonomic name, apparently in commemoration of the Swedish botanist and pupil of Linnaeus Anders Dahl (1751–89). Cavanilles distinguished three species: D. pinnata, D. rosea, and D. coccinea (the latter two named for their colours).

Dahlia coccinea.

Seeds began to be shared across Europe, to Parma, Berlin, Dresden and Turin. Kew obtained some, though they did not survive. Then, in 1802, Cavanilles sent tubers of his three species to de Candolle in Montpelier, to the Jardin des Plantes in Paris and to William Aiton at Kew. But the wider enthusiasm for these exotica in Britain was due to the plants given by Cavanilles to the society hostess Lady Holland, whose librarian got them to flower at Holland House. Her devoted and somewhat henpecked husband wrote this verse in tribute:

‘The dahlia you brought to our isle / Your praises for ever shall speak; / Mid gardens as sweet as your smile, / And in colour as bright as your cheek.’

Nineteenth-century illustrations of dahlias.

This one is by Redouté, 1833.

Meanwhile, Alexander von Humboldt was also collecting seeds destined for the great European botanical centres, and there were several revisions of nomenclature, caused in part by the exceptional ease through which naturally occurring variants arose.

Dahlia ‘Union Jack’, bred in the 1880s.

To date, systematic breeding has resulted in more than 57,000 named cultivars, classified in fourteen groups, viz. 1. Single-flowered; 2. Anemone-flowered; 3. Collerette; 4. Waterlily; 5. Decorative [?? like the others aren’t??]; 6. Ball; 7. Pompom [smaller than Ball]; 8. Cactus; 9. Semi cactus; 10. Miscellaneous [a useful catch-all]; 11. Fimbriated; 12. Single orchid; 13. Double orchid; and 14. Peony-flowered.

Kaiser Wilhelm, a pompom bred in 1881.

Dahlias, like chrysanthemums, fell out of favour in the late twentieth century (except among individual amateur and professional devotees), possibly because they are so exotic – it’s difficult to make them fit into any scheme of naturalistic planting: like having a bird of paradise arrive on your fatballs – definitely WOW! but really unreal. Also, they are prey to a depressing list of diseases: bacterial wilt, crown gall, leaf spot, powdery mildew, southern blight, smut, stem and tuber rot, cottony stem rot, vascular wilt, mosaic virus and ringspot – to say nothing of the natural disasters that plant is heir to: slugs and snails, and also earwigs, who love burrowing into the hollow stems and collapsing the plants.

A selection of dahlias currently flowering in the Cambridge University Botanic Garden.

Recently, however, there has been a revival of interest, with the Bishop of Llandaff (the variety, not the cleric) leading the charge back into our gardens.

Dahlia ‘Bishop of Llandaff’. Credit: Royal Horticultural Society.

The National Collection of Dahlias is held at Varfell Farm in Cornwall, and frequently wins medals at the Big Four flower shows for its superb displays, while the National Dahlia Society flourishes, and provides novice and nervous growers with all the advice they could possibly need.

Dahlia ‘White Aster’, bred in 1879.

I’m not completely won over (and I simply haven’t got the space: as the Society sagely remarks: ‘it is important to bear in mind that some cultivars can develop into bushes three or more feet across and four to five feet high’), but I’m fascinated by the botanical and social history – and I will doubtless continue to buy some alluring flowering pots every summer, so that the pests can eat the leaves and flowers, and the tubers can turn to mush each autumn.

Caroline