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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.
It's no surprise that they were chosen as the official flower of the Paris Olympics. More than 50,000 of the new 'Parc Floral de Paris' variety were planted around the city.
With a bit of luck, dahlias will keep flowering right into October or even beyond. All you have to do is follow a few tips to keep your dahlias blooming for longer.
Saga Magazine quizzed top gardening experts who have shared common reasons why they may struggle to produce flowers and how to encourage a longer flowering period, with bigger blooms.
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.”
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.
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.”
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.”
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.”
Phostrogen All Purpose Plant Food, 800g, RRP £4.19
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.”
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.”
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.”
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.
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.”
Additionally, you can remove and destroy infected plants to stop any disease spreading to healthy plants.
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