SO why do dahlias offer so much variety in colour and shape? This is simply a happy accident, the result of the fact that the 36 or so wild species of dahlia – mostly native to the sloping uplands of Mexico – have a tendency to be octoploid, rather than diploid or triploid like most plants.
In simple terms, this means they have eight copies of the same paired sets of chromosomes, rather than the two or three copies found in plants such as lilies and roses.
Consequently, there’s plenty of genetic material to play with. So when species dahlias found themselves planted cheek by jowl in botanic gardens, the bees began to hybridise them. Over time, they produced doubles in a variety of shapes, and by the mid 1850s many of the forms we know today – like cactus, decorative, anemone-centred and collerette – were established.
More incredible still, a single dahlia seed head can produce astounding differences in its offspring. In 1958 a seedling from the 1922 dark-leaved, peony-flowered red ‘Bishop of Llandaff’ spawned a khaki-leaved, soft-orange fully decorative dahlia named ‘David Howard’. Both are classic garden varieties, but they look nothing like one another. Dahlias also sometimes produce ‘sports’ – shoots with different flowers and foliage from the main plant.
For breeders, the endless possibilities these versatile plants offer are hugely exciting – although the quest to produce a scented or blue variety continues. New forms arrive each year, and top of my wishlist for 2018 are the anemone-centred ‘Totally Tangerine’ and the blackcurrant and mauve ‘Creme de Cassis’ (both from Rose Cottage Plants).
THE SCIENCE: While most plants are diploid or triploid (we humans are diploid), dahlias are octoploid. That means each cell has eight of each set of the chromosomes of that particular species, rather than just two or three (N = number of chromosomes in a set; see below). With more chromosomes in the mix, there’s more scope for variation – whether this is engineered (by breeders) or accidental.
No other flower offers such a massive range of different colours and flower types – from pompom to cactus
Thanks to genetics, dahlias can produce many permutations, including sports (above), where shoots differ from the main plant