How to grow dahlias

Val Bourne / 25 June 2014

Dahlias are sun-loving, frost-tender Mexican plants that deliver months of flowers.



Dahlias are flamboyant Mexican beauties and I wouldn't be without them because they flower continually from July until the frosts intervene as long as you deadhead them. They also thrive in dry summers.

Dahlias come in many forms that include simple single, spiky cactus, tight pompom and soft waterlily. Colours range from pastel through to bright and on to sooty black. And the foliage can vary too, from green, to khaki to almost black.

Dahlias make great cut flowers and they flower on and on until the first frosts. They also carry their own water reservoir, in the shape of a tuber so they can withstand hot sun, once roots are established. Heights range from 30cm to 3m or more.

Where to plant

Give them a sunny position and good soil, although their tuberous roots allow them to tolerate drought. Always stake as you plant.

How to grow

The traditional way to raise dahlias is from tubers. Plastic packets from warm garden centres often fail and may be incorrectly named. Use a dahlia specialist and order in January or earlier.

Start your dahlia tubers off in early March under glass in a warm position. Plant them in large pots of loam-based John Innes 2, or use wooden fruit or tomato trays. Keep your tubers moist rather than wet.

They will only begin to sprout in warmth and the slightest hint of frost will check their growth and blacken their foliage. Use thick fleece on cold nights, if frost threatens, and do not plant outside until the first week of June. Harden them off for at least a week before planting, by placing them somewhere sheltered outside. This toughens the foliage, deterring slugs.

Stake as you plant. Use three canes, about one metre in height, and create an equilateral triangle. Cap them with protective tops and then tie two separate lines of string round the canes - one higher than the other, then stand back and wait for the floral fireworks.

Caring for dahlias

Water well in the first half of summer so that your plants establish themselves. Feed with a potash-rich plant food, either home-made comfrey tea or liquid tomato feed, once buds appear.

Deadheading

Once flowering, deadhead every few days. The pointed seed heads (which feel soggy to the touch) can look very similar to the bun-shaped buds when you’re a novice. So get your eye in before you start.

Storing

After a run of mild winters gardeners can be lulled into keeping the tubers underground. However it’s wise to lift your tubers and store them, once they become frosted. Wash the roots and store in a cool, frost-free place.

Taking cuttings

Ready-grown cuttings are commercially available, but they don’t tend to arrive until May. I like my dahlias to flower by July so I find it best to take my own cuttings.

Look for new shoots reaching about three inches in length (75mm). Fill three-inch plastic pots with John Innes no 1 and water them with a fine rose.

Using a sharp knife, cut away the shoots just above the base where they join the crown. Do not damage the crown. Discard any hollow-stemmed cuttings.

Trim the cutting just below the lowest pair of leaves and remove the leaves carefully. Dampen the end of the cutting and dip into hormone rooting powder. Insert the cutting into the pot, making a hole with a dibber, and firm with your fingers. Label them. One pot can house several cuttings from the same variety.

Place in a warm propagator if possible, away from direct sunlight.

Cuttings should root within twenty days. Once new leaves appear pot them up individually into John Innes 2. Stop your young plant once only, by pinching out the growing tip. This will make plants bushier.

Pests and problems

Slugs and snails are the main problem and the darker-leaved dahlias are most prone.

Viruses can effect dahlias, causing the lower leaves to become mottled, but plants usually grow away from the problem.

Clean and dry tubers before storing in cool, frost-free place. Check regularly for mould.

Plant with...

The warm-red ‘Bishop of Llandaff’ doesn’t need staking when supported by a cloud of the stiff-stemmed purple Verbena bonariensis, and it’s a vibrant combination.

The yellow daisies of Rudbeckia fulgida var. sullivantii ‘Goldsturm’ are greatly enhanced by sultry, almost-black dahlias like ‘Chat Noir’, ‘Karma Choc’, right, and ‘Nuit d’Été’.

Warmly-tinted dahlias in oranges, warm-yellows and brighter orange-reds mix well with tropical cannas and crocosmias. These colours also work with blues especially Agastache ‘Blue Fortune’, or a deep-blue agapanthus like ‘Northern Star’. This agapanthus also works with dark dahlias, or it will contrast against the red and yellow collarette ‘Pooh’.

Dahlias come all in all heights and hues, so many of them are tall enough to grow with taller grasses and the cactus dahlias make a real impact. Tall deep-purple and pinks work well with asters. 

Classic dahlias

‘Bishop of Landaff’ AGM
Superb, dark-black divided foliage with warm-red peony flowers held well above the leaves. Bred in 1928 by I. Treseder of Wales, but selected by the then Bishop. One of the greatest dahlias in the world and it has a strong constitution. (1m/ 3ft)

‘David Howard’ AGM
A soft-orange miniature decorative with bronzed leaves. Raised as a seedling from ‘Bishop of Llandaff’ by the modest Norfolk nurseryman David Howard in the late 1950s. On leaving for National Service, his last words were look after my dahlia. I’m so glad they did! (1m/3ft)

‘Franz Kafka’  
A perfectly-formed purplish pink pom-pom of great substance and beauty, with a ball of perfectly sculpted petals. A huge hit with the the Dahlia Panel and a wonderful cut flower. Dutch. (up to 1m/ 3ft)

‘Ann Breckenfelder’ AGM
This Dutch dahlia, named after an enthusiastic supporter of the Dahlia Society of Wisconsin, this sunny red and yellow collarette is a stronger grower than similar red and yellow collarettes. The wispy inner circle of yellow petals, slightly streaked in red, is surrounded by a ruff of orange-red outer petals marked in yellow on the backs. (1.2m/ 4ft)

‘Arabian Night’
Readily available, but the dark-red flowers of this small decorative tend to turn red as they age. A reliable dahlia, but somewhat despised by dahlia enthusiasts. (1.5m/ 5ft)

‘Sam Hopkins’
Another dark, sultry-red small decorative but without dark foliage, see right. This visitor favourite at RHS Wisley has almost-black flowers with a hint of green.

‘Knockout’ AGM
One of Keith Hammet’s best introductions. A rugged performer, bred in New Zealand, that performs in cool summers, forming a bush of black foliage below clear-yellow single flowers darkly middled.  Part of the Mystic Series.

‘Kilburn Rose’
Another day-glow combination, this vivid-pink waterlily has strong dark stems and good green foliage. British bred by Graham Hill, this was dazzling dahlia on the trail and one of my personal favourites.  

‘Blyton Softer Gleam’ AGM
A small, ball dahlia with rounded flowers in golden yellow washed in butterscotch-orange. Bred by Les Stothard of Blyton in Lincolnshire for exhibition, but excellent in the garden too. Part of a series.   

‘Waltzing Matilda’
This warm, flame-orange and coral pink whirligig has slightly ragged flowers that won’t please a dahlia exhibitionist, but in the garden the dark foliage, buds and simple flowers will please bees and gardeners alike. (90 cm/3ft) dutch.

'Ken's choice'
'Ken's Choice' was one of the stars of an RHS Wisley Trial back in 2005, and this newly-bred dahlia gained an AGM.

The opinions expressed are those of the author and are not held by Saga unless specifically stated.

The material is for general information only and does not constitute investment, tax, legal, medical or other form of advice. You should not rely on this information to make (or refrain from making) any decisions. Always obtain independent, professional advice for your own particular situation.