Want your dahlias to flower for longer? Try these 10 genius tips

As we watch those impressive dahlia blooms come through, here’s how to keep these needy plants happy and encourage bigger flowers, according to gardening experts

Dahlias in full bloom are a sight to behold. Prettying up containers, performing well in borders; they bring bright colour and handsome foliage to every type of outdoor space. With such beautiful flowers, it’s no wonder that many of us are keen to know how to keep our dahlias blooming for longer.

Dahlia 'Cornel Brons' in flowerCredit: Alex Manders/Shutterstock

So, what if come July, or even August, your dahlias are not flourishing? Saga quizzed top gardening experts who have highlighted the main care needs of dahlias, common reasons why they may struggle to produce flowers and how to encourage a longer flowering period, with bigger blooms…

1. Check you’ve planted them properly

They shouldn’t be too deep or too shallow

This may be unlikely if you’ve been planting dahlias for years, but it could be worth checking the planting depth if yours aren’t performing as they did last year, according to Peter Ivanov, a gardening and plant expert at Fantastic Gardeners. “Planting dahlias too deep or too shallow can impede flowering.

“These plants should be planted with the crown – the place where the stem meets the tuber – about 5-10cm (2-4in) below the soil surface, depending on the plant. Otherwise, if the tubers are planted too deep, they may focus on producing foliage rather than blooms. In contrast, if they’re planted too shallow, they may be more susceptible to drying out.”

2. Give them room to thrive

Sufficient space will encourage bigger flowers

Particularly if you’re planting in pots, make sure you give your dahlias room to grow into their best-looking selves. “Give your dahlias the space they need to thrive and produce bigger blooms,” says Robert Silver, founder of the ProGardeningBlog. “Ample spacing allows for optimal airflow, light penetration and nutrient uptake. The results will be robust growth and the potential for larger, show-stopping flowers.”

Dahlias come in all shapes and sizes, so read the recommended spacing on the plant label or seed packet, if available. With some varieties, you may need to support them with stakes or flower cages as they grow.

Credit: Suttons

3. Make sure they have sunlight

Dahlias will do best with six to eight hours of direct sunlight per day

Dahlias want a warm, sunny spot for optimal health, and a lack of direct sunlight can cause them not to bloom. This is an important consideration when planting yours out, especially if you are putting them in the ground and not able to move them around.

“Dahlias thrive in full sun and typically require at least six to eight hours of direct sunlight per day,” says Ivanov. “If your plants aren’t blooming properly, it could be because they aren’t receiving enough sun.”

4. Be consistent with their watering schedule

Don’t give them too little or too much

Dahlias need a consistent and balanced watering schedule for good development, and incorrect watering can cause them not to bloom. “You can be ‘too kind’ when the plants are young and overwater them,” says Peter Freeman, buyer and product development manager at Thompson & Morgan.

Dahlias are susceptible to root rot and fungal diseases (more of this below) but they will also need a little more water in hot spells, so it’s a fine balance to strike.

“Both under-watering and over-watering can hinder dahlia blooms,” says Ivanov. “These flowers require consistent moisture but also shouldn’t be watered excessively because it can cause root rot. On the other hand, underwatering can stress them.”

Ivanov tells us the best way to keep dahlias watered: “Water your dahlias deeply once or twice a week, making sure that the soil is evenly moist but not waterlogged. Adding mulch around the plants can also help retain moisture and regulate soil temperature.”

Dahlia Gallery RivieraCredit: Suttons

5. Give them a nutrient boost

Balanced fertilisers will help with bud formation and blooms

Some plants need a nutrient boost in preparation for, or when in their flowering season. It’s always worth checking the product label for this information.

“To promote big blooms in dahlias, it is important to provide them with nourishment,” says Silver. “This can be achieved through regular feeding during the growing season. Continuous feeding ensures a steady supply of essential nutrients, fostering healthy growth and enabling the plants to produce larger and more impressive blooms.”

Ivanov highlights how nutrition is key for blooms, too: “Inadequate nutrition can affect dahlias’ flower production. These plants benefit from a balanced fertiliser high in phosphorus, which promotes blooming, and a lack of it can lead to poor bud formation. To address this, apply a slow-release or water-soluble fertiliser specifically formulated for flowering plants. Follow the instructions on the bottle.”

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6. Deadhead regularly

This encourages new blooms

Much like with roses, deadheading dahlias is best practice for healthy plants and to encourage more blooms. “When they produce flower buds, be sure to deadhead assiduously and they should keep going right through to the first frosts,” says Guy Barter, chief horticulturist of the RHS.

“Remove faded blooms by using secateurs to cut the stem just above a set of healthy leaves or buds,” says Ivanov. “This prevents the plant from investing energy in seed production and instead promotes the formation of more flower buds.”

7. Pinch them out

This will help lateral branches to form, for more flowers

Pinching is another technique that encourages more blooms and plant growth, which will then lead to more flowering potential.

“When the main stem of your dahlia reaches a height of around 30 to 45cm, pinch off the top couple of centimetres,” says Ivanov. “This will encourage the growth of lateral branches, resulting in more flower-bearing stems. Repeat this process when new shoots reach the same height.”

8. Prune when needed

Not pruning smaller side buds and shoots can waste a plant’s energy

General pruning and disbudding will also help encourage blooms. What’s more, they might be bigger. “Pruning and disbudding can redirect the plant’s energy towards producing larger blooms,” says Ivanov. “Once your dahlia plants have grown several sets of leaves, you can selectively remove smaller side buds or lateral shoots, focusing the plant’s energy on fewer but larger central buds. This results in larger, showier flowers.

“In addition, pruning excessive foliage can improve air circulation and light penetration, benefiting flower development.”

Dahlia 'Creme de Cassis' in flowerCredit: Alex Manders/Shutterstock

9. Protect them from pests

Keep a close eye on unwelcome visitors

We’re not the only ones with our eyes on dahlias. Pests like slugs, snails and aphids are also attracted to their foliage and buds, which can impact blooming potential. And if you’re really unlucky, they might actually eat your plants. “To prevent this, regularly monitor your plants and take appropriate measures to control pests,” says Ivanov.

Aphids can be picked off by hand, hosed off with a jet of water, or washed off with soapy water. Alternatively, you could use a chemical treatment. You can deal with snails just as you would get rid of slugs.

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10. Monitor for disease

Dahlias are vulnerable to common viral infections

If your dahlias aren’t blooming and seem generally stunted in growth, or if you spot unhealthy-looking foliage, it could be they are suffering from a viral infection or other disease.

“Diseases such as powdery mildew and botrytis can impact flower production,” Ivanov tells us. “Proper sanitation, good air circulation and appropriate fungicidal treatments, if needed, can help mitigate them.”

Silver highlights the threat of viral infections. “Dahlias are susceptible to various viral infections, such as mosaic viruses. These diseases cause mottled or distorted foliage, stunted growth and reduced flowering. Unfortunately, there is no cure for viral infections in plants.

To prevent viruses, you need to control aphid populations, as they can transmit viruses,” he says. Additionally, you can remove and destroy infected plants to stop any disease spreading to healthy plants.

Camille Dubuis-Welch

Written by Camille Dubuis-Welch she/her

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Camille is a freelance writer based in north London with her cat and two friends. She has been writing on lots of interesting subjects over the past few years, starting out with a travel blog and online fashion column when she was studying English Language and Italian at the University of Manchester. Cam has been in love with everything interior design and garden-related since before she can remember. She previously worked for Yankee Candle, as well as Groupon, and is the former deputy editor of realhomes.com where she got to collaborate with some very inspiring DIYers and focus on small-space improvements.