Daz­zling di­ver­sity

Amateur Gardening - - Your Gardening Week -

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.

As­ton­ish­ing off­spring

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

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