Understanding Dahlias


Understanding Dahlias – well, they’re complicated plants, you know.

For starters, Dahlias are tuberous perennials as the flower grows from a swollen root which is called a tuber.

They are also herbaceous perennials as the foliage dies back over winter.

Originating from Mexico, they are tender perennials as the tubers do not often survive in the cold wet winter soils of Great Britain and the northern USA.

And they are both an ornamental flower and an edible crop!


But I do love a dahlia. Consider this whistle-stop tour of one dahlia’s year: –

“There is nothing like the first hot days of spring when the gardener stops wondering if it’s too soon to plant the dahlias and starts wondering if it’s too late”
Henry Mitchell

dahlia tuber shoots, propagation

Which then moves onto the slug patrol – of which more below.

And the glorious display of late summer and autumn blooms in the flower borders and as cut flowers for the house.

Finishing with the frost blackened foliage and a rush to lift the tubers, carefully storing them so they can be brought back to life next year.


Understanding Dahlias – a bit of history

Dahlias originated in central and southern America. Known and cultivated by the Aztecs, who possibly used parts of the plant for culinary or medicinal purposes. The Aztec name for dahlia was ‘acocotl’.

The earliest drawings of dahlias date from the late 16th / early 17th century by a Spanish ‘fact finding team’. They discovered what we know as the Tree Dahlia, Dahlia imperialis, which has hollow stems and grows to 20 feet.

Dahlia imperialis, tree dahlia, author tatiana gerus, photographed at Brisbane Botanical Garden

tree dahlia photographed at Brisbane botanical garden, courtesy of tatiana gerus


  • Originally cultivated for its edible tubers (more on this later) the dahlia had mixed beginnings when it first came over to Europe.
  • The name ‘dahlia’ is not Mexican or Aztec; it is Swedish, named in the 18th century for the botanist Andreas Dahl.
  • But it was during the 19th century that dahlias really got going as an ornamental flower. Hybridisation created many new forms, including a true double flower.

The Great Exhibition at Hyde Park, London in 1851 was paramount in showing off the dahlia to all classes of society. Wealthier devotees of the flower encouraged their head gardeners to breed ever more showy forms. The rise of an exhibition flower had begun and in 1881 the British National Dahlia Society was formed.

Dahlia 'bishop of Llandaff'

By 1936, there were some 14,000 recognised dahlia cultivars. To date, the number has risen to nearer 50,000! And if you really want to be amazed, then get this. All of these dahlia hybrids and cultivars have been developed from just 2 or 3 of the original dahlia species that came from Mexico.

Understanding Dahlias – Wildlife Gardening

Did you know that many dahlias make excellent flowers for pollinating insects?bee on white dahlia in the old English walled garden, Brockwell Park, South London

The single flowered types are generally best, such as the ‘Bishop’ series. All have distinctive purple-bronze tinted foliage to offset their open flowers. Red Bishop of Llandaff; orange Bishop of Oxford; yellow Bishop of York; and Bishop of Dover with a more subtle toned white with a hint of lilac petals.


Understanding Dahlias – Garden Pests

Specifically, rabbits and slugs. If you have a problem with rabbits eating your flowers, then grow dahlias. They may eat the young leaves in a lean spring, but generally prefer other plants. so your dahlias are generally safe from rabbits and deer.

Of course if you have a problem with slugs and snails eating your dahlias that’s another matter. Try the methods suggested in our slugs and snails blog. If growing your dahlias in large containers, then watering in nematodes is another option. This isn’t as efficient when the dahlias are in the border.

snail at Chelsea Flower Show, garden ornament, garden statue

You could try a garlic spray, both as a foliar spray and for watering around the plant. This can be bought ready mixed or prepared at home (very smelly in the making!) Just be aware that some plants are less tolerant of garlic than others; use in a mild concentration to begin with.

Planting them out when the dahlias are larger plants seems to reduce the likelihood of total decimation. But if we have a wet spring I’d suggest trying multiple methods.

Alternatively – and I know you’ll appreciate this is irritating to many a gardener – admit that life is too short and buy your dahlias as plants rather than tubers; treating them as annuals. Or as food crops in an ornamental edible garden.


Understanding Dahlias – Ornamental Edible Gardens

I’m not quite sure why the dahlia didn’t compete well with its ‘cousin’ the potato as a food crop. When brought over to Europe, French horticulturalists in the Jardins des Plantes in Paris tried growing dahlia tubers as a food crop. Much in the way that potatoes had become introduced.

It’s most likely that the reliably sweet, starchy potato tuber won out over the inulin heavy dahlia. For many people, inulin is not as digestible as starch and can cause bloating.

dahlia shadow play, purple bronze foliage, pink white variegated petals, tuberous perennial, tender perennial

However, there may be a turn in the fortunes of dahlias as a food crop. Where we have increasingly smaller gardens, all the plants in them have to work hard to justify their inclusion. A plant which can provide us with lots of colourful drama in the late summer and autumn and then give us a few meals for our family has to be worth considering. It’s also possible to propagate the beginnings of next year’s dahlias from a part of the tuber before eating most of it.  And of course you can compost all the spent flowers and stems to provide soil food for your garden!  NB Do Not eat the dahlia tuber you buy in the spring, it is likely to have been treated with chemicals. Six months in the  soil resolves that – and gives you a fully mature tuber.

lobelia, dahlia, canna, hot flower border, wakehurst place

Now, I haven’t tried this yet. It was on my list for this year but various circumstances, including a total makeover of my own garden got in the way. So the trial will happen next year. What I can tell you is that I have grown – and eaten – Jerusalem artichokes. These are no relation to artichokes, but members of the Asteraceae family like dahlias are. Their bright yellow flowers and edible tubers have long been recognisable on allotments.

More on edible tubers and decorative vegetables in blogs to come and currently in Plews Potting Shed. See some suggestions below.


Understanding Dahlias – and a garden to visit

To finish up, two snippets from the American continent and an English dahlia garden to visit for the August Bank Holiday.

Did you know that in 1926 the dahlia was officially designated as San Francisco’s city flower?

And back to its tuberous roots. Since 1963, the dahlia has been the National Flower of Mexico. It is still an edible crop here as well as an ornamental flower.

That English garden to visit? Funnily enough it’s the dahlia garden where the National Collection of Dahlias is held.
Their special open day is this Bank Holiday weekend, Sunday 27th August 2017. Situated near Penzance, Cornwall (click the link to go to their website).
The garden is open but unattended between mid-July and beginning of October, when the dahlias are in flower.

dahlias, national collection, chelsea flower show 2016, great pavilion

This National Collection has more than 1600 varieties of dahlia! Plenty of beautiful flowers and interesting foliage to make you want to start your own collection. If you’re not able to get to Cornwall, they regularly show at RHS Chelsea Flower Show, RHS Hampton Court Show, RHS Tatton Park Show. And they do put on an excellent display.


Related Gardening Blogs you may enjoy

Autumn Gardening Tasks – Lifting Perennial Tubers
Planting Ideas for Colourful Autumn Borders
Edible Gardens and Ornamental Food
National Gardening Week 2016 – Gardening Questions Answered
Tomatoes and the Aztecs

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