Growing dahlias – everything you need to know
Steven Edney, award-winning head gardener at & The Salutation Hotel & Gardens has been growing dahlias all his life. And he comes from a family of professional dahlia growers.
So I went to The Salutation (now closed) to find out the answers to the top questions about growing dahlias. And I photographed some of the gorgeous dahlias growing at The Salutation in August and September.
How to choose which dahlias to grow?
Steven says that, overall, you can just choose the dahlias you love the look of.
‘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.’
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.’
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.’
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.’
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.’
You start planting your dahlias in pots in March, so when you plant them out in May, they are already decent-sized plants. ‘If we get surprised by a late frost, then we’ll cover them with a layer of fleece overnight.’
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.
‘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.)
One of the most-viewed videos on The Middlesized Garden YouTube channel is also about Don’t Dig Up Your Dahlias. I’ve had comments from people who live all over the world. Generally if you’re living in a part of the world that is ranked as a similar climate to South East England or USA hardiness Zone 9, then you should be able to leave your dahlias in. I get a few comments from people who have slightly colder winters who also manage it.
Steven explains that it’s not just about how cold your winter gets, but what your soil is like. ‘Here at The Salutation, 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.’
He also advises pairing dahlias with other ‘short day’ plants. These are plants, such as salvias, which flower more as the days shorten.
Steven also likes dahlias with grasses, such as Miscanthus.
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.
Here at the Salutation 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 – 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 The Salutation as a wildlife-friendly garden, so that there are lots of birds, frogs, newts and hedgehogs to help eat the slugs and snails.
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.’
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.
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:
The Salutation Hotel & Gardens is now closed.
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