Eight things you need to know about growing dahlias

Lift them at the end of autumn and store the tubers indoors and they’ll be ready for planting again next spring. Get in the habit and you’ll have dahlias for years and years. In mild, sheltered areas, where soil drains very well you can leave them outside over winter and place a thick layer of compost on top. Even easier.

And there are literally thousands of them, in so many variations of flower colour, shape and size that it’s hard to say that you don’t like dahlias! Right now they are pumping out colour in back gardens, balconies and allotments nationwide like a party guest still dancing the night away and wondering why everyone else is yawning and about to go home.

More dahlia advice and inspiration:

• Alan Titchmarsh's favourite dahlias
• Single-flowered dahlias to grow for wildlife
• Dahlia and cosmos pot display
• How to grow dahlias from seed

American origins

Dahlias originate from South America, mainly from the high plains of Mexico at an elevation of 1,500-3,700 metres, with some species endemic to Guatemala, Honduras, Nicaragua, El Salvador and Costa Rica, with 42 species in total. Historians think that dahlias first arrived in the UK at the end of the 18th century and legend has it that the first plants were raised from seed at Royal Botanic Gardens, Kew where the gardeners presumed they were a warm weather plant and killed them with kindness by growing them in tropical conditions, where they all rotted! Oops!

What’s in a name?

In 1791 dahlias were given their European name, named after Swedish botanist Anders Dahl, meaning that in the UK they are perhaps the most mis-pronounced plant of all time (and there’s lots of stiff competition!), with the name common usually pronounced ‘day-lia’ rather than ‘dar-lia’. Dahlias were also given the genus name ‘Georgina’ at the turn of the 19th century and they are still referred to by this name in some parts of eastern Europe.

Seeing dahlias through winter

Dahlias are not completely hardy, meaning that in a mild winter they may come through unscathed if left in the ground, but they need a bit of a help in most gardens. Here’s what to do to keep them happy and raring to go again next spring.

• When the first frost blackens the leaves, cut back and compost all the old foliage and flower stems, leaving about 10cm of the bottom of the stem intact. Cut the material up well or put it through a shredder to compost it
• If your soil is heavy or your garden is in a cold area, lift the tubers from the ground with a garden fork and brush off the soil, then put a loop label around the piece of stem so that you know which is which
• Store them in trays in a dry, unheated place such as a garage or shed, then pot them up again in April
• In mild gardens on very well-drained soil, cut the plants down to ground level and cover the top of the plant with a 5cm layer of compost. Push a label into the soil so that you know where to look at for new growth in spring

The choice is yours!

More than 57,000 different dahlias are currently registered with the Royal Horticultural Society (RHS) and it can be difficult to know where to start when choosing which ones to grow. More than 100 have been given Award of Garden Merit (AGM) status by the RHS, meaning that they are reliable and easy to grow so this may be a good place to start. Twenty-two dahlias have just been added to the list of AGM dahlias this summer, including two-tone lilac and plum ‘Crème de Cassis’ and Anemone-flowered ‘Totally Tangerine’. Look out for a ‘trophy’ symbol on plant labels, denoting an AGM plant.

Categories of flower

Due to there being so many dahlias to choose from, they are helpfully separated into different categories based on flower type. There are 15 in total: single-flowered, anemone-flowered, collerette, waterlily, decorative, ball, pompon, cactus, semi-cactus, miscellaneous, fimbriated, star, double orchid, peony and stellar.

If that seems rather overwhelming, take a look at this guide to the different types of dahlia to find out which ones will suit your garden style and needs.

Do dahlia flowers grow on trees?

While dahlias are perhaps thought of as a traditional plant, there is one that makes a perfect fit in an exotic, tropical style garden. The tree dahlia (Dahlia imperialis) is a giant dahlia, growing up to three metres tall, with chunky stems a bit like bamboo, and leaves that are similar to elder. It makes a spectacular foliage plant in its own right to pair with jungly Fatsia japonica or Tetrapanax papyrifera but if your garden is warm and sheltered you may also get nodding, parasol-like pink flowers with an orange centre at the end of summer. Or you can grow it indoors in a well-lit spot to encourage flowering.

Stake your claim!

There are dwarf dahlias that only grow to around 45cm tall, such as ‘Orange Nugget’ and ‘Yellow Happiness’ but taller types, especially those with large, heavy flowers will give their very best show if well staked. An easy way to keep them perky and upright is to grow them in blocks and put a stout stake or cane in each corner of the block. Wind string around the block to support the plants and secure more strings higher up as the plants grow.

Make free food to extend the blooming season

Deadheading is of course an important part of caring for dahlias and extending the flowering season. But they are also hungry plants, needing regular feeding throughout summer to flower well all season and into autumn. From spring, cut up young comfrey leaves and leave them to steep with a little water in a bucket or sterile container, then cover with a lid. Leave for a few weeks, then drain off the liquid into bottles and dilute with water, one-part comfrey food to 10-parts water, then top up with fresh leaves. Keep doing this through spring and summer (comfrey is a rapid grower!) and you’ll have a lovely organic, potassium-rich plant food ‘on tap’ to give to your dahlias. Add a little to your watering can each time you water the plants.

• How to make a comfrey liquid fertiliser