Growing dahlias – everything you need to know

September 1st, 2019
Posted In: Gardening know how

Steven Edney, award-winning head gardener and owner of the No Name Nursery has been growing dahlias all his life. And he comes from a family of professional dahlia growers.

So I asked him to answer the top questions about growing dahlias.

How to choose which dahlias to grow?

The answer is simple, says Steven. Just choose the dahlias you love the look of.

Dahlias for pollinators - Dahlia 'Lovely Eyes Loulou'

Steven developed the pollinator-friendly Dahlia ‘Lovely Eyes Loulou’ himself and named it after his daughter. if you’re growing dahlias for pollinators, the ‘single’ and semi-double flowers are best. It’s hard for bees to get into very elaborate double flowers.

‘But if you’re particularly interested in growing dahlias for bees and other pollinators, then the singles and semi-doubles are best,’ he says. ‘You need a dahlia where the eye of the flower is exposed so that the pollinator can land on it and get to the nectar.’

Which dahlias are best as cut flowers?

If you’re looking to cut a dahlia to use in a vase, Steven says that the decorative dahlias are better. ‘For me, the cactus types are my favourites – they look very good in a vase. There’s nothing quite like them. But the informal decoratives also work very well and then the ‘dinner plate’ dahlia are amazing. You could use them on their own with a bit of foliage and just bask in their glory.’

Growing dahlias for cutting - Cactus dahlia 'Rip City'

Cactus dahlias are brilliant if you’re growing dahlias for cutting. This is Dahlia ‘Rip City’. Cactus dahlias have very narrow petals that often curve inwards. They’re also ‘double blooms’ – they have double the amount of petals, which cover the eye of the flower.

He says that the single dahlias, although brilliant for pollinators and as garden plants, don’t work as well as cut flowers. ‘There are 14 different flower types in dahlias and it’s only the singles that don’t work as well for cutting.’

Dahlia 'Sylvia' is good for cutting

Dahlia ‘Sylvia’ would be a good choice if you’re growing dahlias for cutting.

Best dahlias for pollinators

But these single-flowered dahlias are best as garden plants and for pollinators, rather than as cut flowers.

Can dahlias grow in pots?

Steven says that dahlias can be very successful in pots. ‘The important thing is to remember that the bigger the dahlia, the bigger the pot should be. Firstly that will help stop them drying out on hot days. Secondly, it will help prevent them blowing over in the wind.’

Dahlias grow well in pots

Dahlias growing in pots at the Salutation Hotel (now closed)

Claus Dalby is a Danish gardening influencer, author and broadcaster. He is famous for his magnificent displays of dahlias in pots. Read Claus’s tips for growing dahlias in containers here.

Claus Dalby on dahlias in pots

Claus Dalby, the famous Danish gardener and broadcaster, with his dahlias in pots.

Can dahlias be grown indoors?

This is a surprising question – Steven has never been asked this before. ‘Dahlias can’t be grown as a house plant, no,’ he says. ‘The light levels are just too low. But if you have a conservatory, that might work.’

Can dahlias grow in shade or do they need direct sun?

Dahlias come from Mexico, so they’re generally used to bright sunlight and warm weather. Most dahlias will be best in full sun. But some varieties will be better in the shade than others. ‘Some of the wild species dahlias evolved as forest edge plants,’ explains Steven. ‘They would only have had a few hours of direct sunlight a day.’

Dahlia 'Yellow Hammer' does well in shade

Dahlia ‘Yellowhammer’. Steven says that this has had barely more than an hour of full sun a day and it has done very well.

Every year, Steven trials a few dahlias in the shade. He’s tried three this year, but only one has performed well. ‘Dahlia Yellow Hammer’ has done really well in a very shady spot. And it’s had barely a couple of hours of full sun a day.’

If you’re buying dahlias from a specialist dahlia grower, they’ll probably know which ones do better in shade.

When to plant dahlias out

You need to plant dahlias out after the risk of frost is over. ‘Here in South East England, that’s usually around the middle to end of May,’ says Steven. ‘Although you can plant dahlias up to the end of June. And at the other end of the spectrum, I occasionally plant dahlias out in a very sheltered part of the garden as early as April.’

Stunning dahlias at the Salutation - Dahlia Pink 'Pat and Perc'

Dahlia ‘Pink Pat and Perc’ – a showy but wildlife-friendly dahlia.

Start planting your dahlias in pots in March, so when you plant them out in May, they are already decent-sized plants. The pots need to be kept in a greenhouse, potting shed or frost-free area as a frost will kill the emerging foliage.

‘If we get surprised by a late frost once we’ve planted them out,’ says Steven, ‘then we’ll cover them with a layer of fleece overnight.’

A sheltered spot for dahlias at the Salutation

A sheltered spot for dahlias.

How to plant dahlia tubers

Steven digs a hole the size of the dahlia roots (either in its pot or just the roots) – no larger. And he doesn’t add compost or fertiliser to the hole. ‘We put the dahlia in the hole, and water really well. Then we add a layer of garden compost around the plant and leave it.

Steven advises against too much watering after that. ‘People often water too much in the early summer, but when the plant is smaller it doesn’t need much water. It’s in high summer that a dahlia may need extra watering, when the flowers are big and the weather is often very dry.’

He says that it would be better to concentrate on weeding around the dahlia in early summer and not to consider watering (unless you have an exceptionally dry spell) until the plant is much bigger.

Dahlia 'Hadrian's Sunlight' at The Salutation

Dahlia ‘Hadrian’s Sunlight’ – charming yellow dahlia with black leaves.

‘And use home-made garden compost or well rotted manure to mulch around the plants,’ he adds. ‘People often have a bag of commercial potting compost hanging around and decide to use that for mulch, but that’s not what it’s for.’

How to keep dahlias over winter

‘Here in South East England, we keep most of our dahlias in the ground over winter,’ says Steven. ‘And eight out of ten years, they’ll be fine.’ He covers them with mulch (see this post on ‘Don’t dig up your dahlias – here’s what to do instead.)

Dahlias at The Salutation

Another single dahlia. Steven breeds dahlias and often grows them by taking seeds from them. ‘Dahlias are octoploids,’ he says. ‘So you don’t always know how the seed is going to turn out.

Steven explains that it’s not just about how cold your winter gets, but what your soil is like. ‘Here in South East England, we have quite light sandy soil which drains away easily,’ he explains. ‘So the dahlia tubers aren’t sitting in cold damp soil. If you’ve got heavier clay soil, which gets very damp, then dahlias won’t over-winter so well.’

What plants do dahlias grow well with?

Steven loves to tap into the dahlias’ exotic look by pairing them with exotic looking plants, such as cannas. ‘They’re also plants which like the same conditions as cannas,’ he says. ‘It’s a good idea to put plants which like the same conditions together, rather than trying to look after plants in groups differently.’

The Middlesized Garden has a post on’ Everything you need to know about growing cannas’.

Plant dahlias with cannas

Dahlia ‘Western Pirate’ with Canna ‘Wyoming’ and sedum.

What to grow with dahlias

Dahlias with Ricinus communis, the castor oil plant. Its exotic looking leaves sit well with dahlias.

He also advises pairing dahlias with other ‘short day’ plants. These are plants, such as salvias, which flower more as the days shorten. There is an expert guide to growing salvias here.

What to grow with dahlias

Partner dahlias with other plants that flower later in the summer, such as salvias and agastache.

Steven also likes dahlias with grasses, such as Miscanthus.

Grow dahlias with grasses, such as Stipa tenuissima

Dahlia ‘Babylon Bronze’ and Feathergrass (Stipa tenuissima) in the Middlesized Garden.

There are more tips on pairing dahlias with other plants in this post on how to make your garden last longer with dahlias.

Do dahlias need alot of water?

‘If you’re growing exhibition dahlias, they’ll need alot of water. You have to think about the flower size. It’s not so much about the plant as about the flower. The big showy flowers need vast amounts of water. The smaller the dahlias, the smaller the flowers – the single flowers and dahlias up to a metre tall – the more drought-tolerant they are.

Drought-tolerant dahlias

Singe flowered dahlias and those less than a metre tall are more drought tolerant.

Here we don’t water our dahlias regularly, unless they really need it. But I’ll try to plant the bigger dahlias in a damper spot.’

Do dahlias need a lot of fertiliser?

Showy big exhibition dahlias are more likely to need extra fertilising, explains Steven. He also says that most plants will grow bigger and give you more prolific flowers if you give it extra fertiliser.

‘But we don’t add any extra fertilisers to our dahlias at The Salutation. That might mean we have fewer flowers or smaller flowers than some other growers, but I’m not comfortable with how that might affect the wider environment.

Growing dahlias at The Salutation

One of the gardens Steven runs. It’s lush, although he doesn’t use extra fertilisers. He mulches his soil with garden compost and well rotted manure, however.

Growing dahlias – how to deal with slugs and snails

‘This is the most asked-question,’ says Steven. ‘And there’s no magic bullet. You need to look at the bigger picture.’

Firstly, he runs his gardens to be as wildlife-friendly as possible, so that there are lots of birds, frogs, newts and hedgehogs to help eat the slugs and snails.

How to protect dahlias from slugs and snails

This is a dahlia in the Middlesized Garden. I feel I may not have got the balance quite right, although I do follow wildlife-friendly policies. I don’t suppose a snail would dare do this at The Salutation!

Steven surrounds his dahlias with a wool product. And he also uses slug pellets based on ferric phosphate, because they are organically approved and won’t harm the wildlife if they eat the slugs and snails.

‘But people don’t realise that ferric phosphate based slug pellets work differently from the old metaldehyde ones. They didn’t work if it rained, but ferric phosphate slug pellets need some water to activate them. So people often say that they’re not working, but I tell them that if it doesn’t rain – or at least have a heavy dew – within about 24 hours of scattering them, then you need to water.’

Read more about Steve’s philosophy of controlling slugs, snails and earwigs on dahlias organically here.

Steve and his partner Louise Dowle have also started a nursery to grow plants in the ground in a garden environment. They have created one of the longest herbaceous borders in the UK so that they can experiment. Read more about it and see this outstanding perennial border here.

Wildlife friendly anti slug products

Vitax make wool-based anti-slug pellets which you can buy from Amazon. Please note that links to Amazon are affiliate, see disclosure. As an Amazon Associate I earn from qualifying purchases, but it doesn’t affect the price you pay. Other links are not affiliate.

Ferric phosphate slug pellets are available from Sluggo and Solabiol

And if you’re interested in wildlife-friendly gardening, I’m often asked for recommendations. I’ve put together some useful lists on the Middlesized Garden Amazon store, with the gardening books, tools and products that I use. This is the Wildlife Friendly Gardening list of useful products.

See Steven’s interview and more of The Salutation gardens in video:

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10 comments on "Growing dahlias – everything you need to know"

  1. Patti says:

    I live in Washington State, Zone 8. This is my first time growing dahlias and they are beautiful! We had our first frost (31°F) earlier this week and then the rest of the week has been in the mid 40’s at night. The problem is I thought we were going to have more freezing nights so I cut them back. I don’t know what to do now. I cannot leave them unprotected, but if I cover them will they die? I used a cloche jar with bubble wrap for cover and pine needles. Is that too warm? I didn’t water them, it always rains here!
    Thank you

    1. Dahlias will survive in the ground over winter in some Zone 8 areas but not in others. How much rain you get also affects it as do other micro-climate issues. For example, dahlias planted near a sunny wall often have a better chance than those planted in a slightly more shady position. Don’t bother with cloche jars or bubble wrap, just pile garden compost or well rotted garden manure on top to protect the crown. Ultimately this will improve your soil and it will protect the dahlias if they’re going to survive. Then it’s just a question of waiting till next spring to see whether they have survived. If they don’t survive in the ground over winter, treat them as annuals and buy new every year (many people do this) or dig them up, shake off the soil and keep them protected (ie wrapped in newspaper) in a dark, cool, frost-free place. Hope that helps!

  2. Colin says:

    My first time having Dahlias this year. Grown in pots. They are spectacular.
    I live in Scotland, Glasgow to be precise.The pots are large and heavy so cant be moved.Should I remove and store them?

    1. Yes, I would, unless you have a very sheltered garden and the pots are close to the house. There are no guarantees, but if your pots are directly against the house wall, there’s a good chance that the warmth of the house in winter will protect them. And you could wrap horticultural fleece around them too, if you don’t mind the look of it. But otherwise dig them out of the pots, brush off the soil and store them somewhere cool, dark and frost-free.

  3. Rachel Firth says:

    I have some pale spots on my dahlia leaves. Not mildew I think – there is nothing to rub off, at least. Any suggestions?

    1. It’s difficult to say but the first thing to check is whether your dahlia is in overly damp conditions. They really don’t like very damp areas. If you have a high rainfall then it may be better to keep them in large pots, which will drain well. I hope that helps.

  4. Lorette Stoltzfus says:

    Why do my dahlia flowers turn brown?

    1. Once they’ve flowered, they’ll turn brown or fade and die, then the petals drop off. You should cut them off once they start to turn brown or start, so you get more flowers. I hope that helps, and that there isn’t a specific disease on your dahlias.

  5. Shirley Lodge says:

    For the last three years, whilst I have lived in this house, I have left my dahlias in the soil to overwinter and all has been well, so far. I want to move a few of them and would like to know if you think it is better to do it now (Nov) or wait until the Spring? It is to another place in the same garden. I would appreciate your comments.

    1. I moved dahlias last year in November from one part of the garden to the other and the dahlias didn’t survive. I think the best thing would be to dig them up now, store them over the winter if you can, then replant in spring, either starting them off early in pots or going straight into the ground once the frosts are over. Hope that helps.

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