A Complete Guide on How to Grow Dahlias

Dahlias are magnificent flowering plants that add a vibrant burst of colour to any garden. With their diverse range of shapes, sizes, and colours, dahlias have become a favourite among gardeners. If you’re looking to cultivate these beautiful blooms in your own garden, this comprehensive guide will provide you with all the information you need on how to grow dahlias. We will also look at choosing the right varieties of dahlias for your garden, watering, feeding and fertilising, and much more.

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Choosing the Right Varieties of Dahlia

Dahlias are available in all sorts of shapes and sizes. For instances, you can get dwarf varieties, tree dahlias (dahlia imperials), and modern dahlias, all of which are the most popular plants on the market. Tree dahlias can grow the largest by up to 5m tall.

Gardeners residing in colder regions often opt to dig up their dahlia tubers as the first frosts approach in autumn, subsequently bringing them indoors for winter storage and protection.

Other types of dahlias that you can grow include:

• Dinner Plate Dahlias: Known for their enormous, dinner plate-sized blooms, varieties like ‘Café au Lait,’ ‘Kelvin Floodlight,’ and ‘Thomas Edison’ are highly sought after for their impressive flower size and stunning colours.

• Cactus Dahlias: With their unique, spiky petal arrangement, cactus dahlias add a touch of drama to gardens. Varieties such as ‘Chat Noir,’ ‘Bora Bora,’ and ‘Café Baudelaire’ showcase captivating cactus-like blooms in vibrant shades.
• Decorative Dahlias: These dahlias display fully double blooms with broad petals. Varieties like ‘Bishop of Llandaff’ (deep red), ‘Karma Choc’ (almost black), and ‘Blyton Softer Gleam’ (soft pink) are admired for their classic charm and robust growth.
• Pompon Dahlias: Featuring small, ball-shaped blooms, pompon dahlias offer a delightful burst of color. ‘Mignon Single’ (orange), ‘Witty’ (yellow), and ‘Franz Kafka’ (deep purple) are popular choices for their compact habit and profusion of rounded flowers.
• Waterlily Dahlias: These dahlias exhibit elegant, fully double blooms that resemble waterlilies. ‘Rip City’ (deep red), ‘Wizard of Oz’ (pink), and ‘Babe’ (peach) are admired for their intricate petal layers and romantic appearance.
• Single Dahlias: Single-flowered dahlias, such as ‘Bishop’s Children’ and ‘Yellow Hammer,’ are valued for their attractiveness to pollinators, making them excellent choices for wildlife-friendly gardens.
• Bishop Dahlias: This group includes varieties like ‘Bishop of Oxford’ and ‘Bishop of Auckland,’ known for their stunning dark foliage that contrasts beautifully with their vibrant blooms. They offer an added dimension to Dahlia displays.

Where to Buy Dahlias in the UK

For gardeners looking to grow dahlias in the UK, your best bet would be to purchase dahlia tubers, which are usually available from February onwards. Bearing in mind that there are different varieties of dahlias, you need to make sure that you have the right potting equipment to suit the sized dahlias that you want.

Online websites including Crocus[i], Thompson & Morgan,[ii] and Sarah Raven[iii] are the best resources to purchase these fine plants from.

How to Grow Dahlias

Now that you have chosen a dahlia set that is right for you, you next need to learn how to grow dahlias, since they can prosper under different growing conditions compared to other garden plants.

Preparing the soil:

Dahlias thrive in well-drained, fertile soil. Prior to planting, it’s crucial to prepare the soil by removing any weeds and incorporating organic matter such as compost or well-rotted manure. This will improve the soil structure and provide essential nutrients for healthy dahlia growth.

Where to Grow Dahlias

When learning how to grow dahlias, they thrive in a protected location that receives ample sunlight, making a south or west-facing border the perfect choice. Interestingly, dahlias are not particularly demanding when it comes to soil type.

To achieve optimal results, you should cultivate them in fertile soil that is moist yet well-drained. Also, enhance the soil with organic amendments, such as well-rotted manure, can provide beneficial nutrients. In the case of heavy clay soil, incorporating some grit into the planting hole will improve drainage. For those growing dahlias in containers, use high-quality, peat-free multi-purpose compost.

Planting dahlias indoors:

Dahlias are suitable to be grown in indoor environments. Simply plant them in a pot that contains multipurpose compost during the spring.

Like the tubers mentioned earlier, when planting dormant ones, they need to face the right way upwards. Some gardeners consider them to be shaped like sausages. They should be positioned together just below the compost surface. These need to be kept free from frost, and make sure that they get enough sunlight and water.

Planting dahlias outdoors:

Plant dahlias after the last frost date, typically in late spring. Choose a sunny location in your garden that receives at least six hours of direct sunlight each day. Dig a hole that is wide and deep enough to accommodate the tuber, placing it with the sprout facing upwards. Cover the tuber with soil, leaving a slight depression to hold water around the plant.

You can also use a general-use fertiliser which will aid in the vigorous growth of the dahlias. Make sure to follow the instructions labelled on the product.

Keep control of your garden by learning how to remove box blight.

Caring for Dahlias

After your dahlia has grown to at least a length of 20cm tall, you can encourage the side shoots to flower by pinching the main growth tips. Within the centre of the plant is the main shoot which grows upright. Equip a knife and remove it. From here, provoke new growth by tying into the stake each week.

Every fortnight, use efficient high potash feed (e.g., tomato feed) to feed the plant, and water thoroughly once a week.

Watering and mulching

Regular watering is essential for dahlias, especially during dry spells. Aim to keep the soil consistently moist but not waterlogged. Applying a layer of organic mulch, such as straw or wood chips, around the base of the plants will help retain moisture, suppress weed growth, and maintain a more even soil temperature.

Supporting dahlia plants

As dahlias grow, they may require support to prevent their tall stems from toppling over. Install stakes or a trellis system at during planting to provide support. As the plants grow, gently tie the stems to the support structure using soft garden twine, ensuring not to constrict their growth.

Feeding and fertilising

To promote healthy growth and abundant blooms, dahlias benefit from regular feeding. Use a balanced, slow-release fertiliser when planting and follow up with additional applications throughout the growing season. Alternatively, opt for a liquid fertilizer, such as a seaweed-based solution, applied every few weeks during active growth.

Deadheading and pruning

To encourage your dahlias to grow better, regularly deadhead faded flowers by removing them just above a pair of healthy leaves. This practice redirects the plant’s energy toward producing new blooms rather than setting seeds. Additionally, pinch out the tips of young dahlia plants when they reach about 12 inches tall to encourage bushier growth and more flower-bearing stems.

Managing pests and diseases

While dahlias are generally resilient plants, they can still fall victim to pests and diseases. Keep a close eye on your plants and promptly address any signs of trouble. Common pests to watch out for include:

• Slugs: These pests love to feast on young shoots, so make sure to apply adequate protection to keep slugs out.
• Aphids: These bugs can colonise and gather on plants which can cause them to decay. Try growing your dahlias alongside these fly-repellent plants to keep greenflies and blackflies away from your dahlias.
• Powdery mildew: This substance can appear in dry conditions, so in scenarios of long dry summers, make sure to water your dahlias more regularly, but do not overwater them.
• Earwigs: These will feast on foliage, flowers, and buds. You can use a garden cane to disrupt the soil and create an earwig trap. Watch their movement during the day and you can get rid of them when they retreat inside the pot.

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Overwintering dahlias

In cold weather, dahlias are not hardy and therefore not very tolerant to harsh elements. They need to be protected during dormant seasons. Before the first frost, carefully dig up the tubers, taking care not to damage them. Remove excess soil and let the tubers dry in a cool, dry location for a few days. Once dry, store them in a well-ventilated container filled with dry peat moss, vermiculite, or sawdust.

Propagating dahlias

Dahlias can be delicate when growing them in your garden, but fortunately, their tubers are a good size that can be easily divided during the spring.

1. To propagate dahlias, start by gently placing the tubers onto a tray containing shallow compost and allowing them to sprout in a greenhouse or a sunlit room.
2. Maintain a warm and moist environment by regularly tending to the compost. As soon as the tubers begin to develop shoots, carefully divide them using a clean and sharp knife.
3. Ensure that each divided section possesses at least one shoot and a corresponding set of roots.
4. Proceed to pot these divisions individually and nurture them until the risk of frost has completely subsided before planting them outdoors.

Enjoy watching your dahlias grow with our vast selection of wooden benches.

Pruning and training

Thinning Out Stems:

• Dahlias possess a robust growth habit, often leading to overcrowded stems. To enhance the overall appearance of your dahlias, it may be advantageous to thin them out during the months of June and July. Follow these guidelines for a more balanced display:
• Aim to retain a selection of seven to ten sturdy stems per plant, ensuring adequate spacing between them.
• Identify spindly or overcrowded shoots that you wish to remove, and firmly pull them to break them off from the plant.
• For giant-flowered dahlias, it is recommended to limit the number of stems to three or five per plant. This approach will result in fewer but significantly larger blooms, creating a striking visual impact.

Pinching Out Shoot Tips:

• Once the main stem reaches approximately 40cm (16in) in height, implement a pinching technique that encourages the production of side-shoots, fostering a bushier and well-shaped plant with an increased flower yield. Follow these steps:
• Pinch out the tip of the main stem just above a pair of leaves. This action will stimulate branching at this point, leading to a more compact and lush plant.
• If your goal is to cultivate giant flowers or flowers for cutting purposes, remove the smaller flower buds located behind the central larger bud. This practice will divert the plant’s energy towards the development of larger, higher-quality flowers. However, if you are growing dahlias solely for border displays, this step is not essential.

Common Dahlia Issues:

Lack of Blooming in Dahlias:

The most frequent cause of failed blooms in dahlias is underwatering. Ensure the soil remains consistently moist and water generously, particularly during dry periods or if they are grown in containers. Additionally, position the dahlias in a spot that receives full sun, preferably in a sheltered location.

Failure of Dahlia Tubers to Sprout:

One common mistake is overwatering newly planted tubers, which can lead to rot instead of sprouting. After initially watering the compost when planting the tuber, refrain from watering again until the first signs of sprouts emerge.

Wilting Dahlias:

Dahlias have high water requirements once they enter the peak growth phase. Maintain moist soil around the plants, which may entail watering every few days during periods of drought.

Stunted Growth of Dahlias:

Some dahlia varieties naturally exhibit compact growth. Consult the information provided on the seed packet or conduct research on the specific variety to determine its expected final size. If growth appears stunted, evaluate the growing conditions. Cold snaps or late frosts can negatively impact growth. Ensure that your dahlias are situated in a sunny location, as they thrive in abundant sunlight.

Pave the Way for Growing Beautiful Dahlias

Learn to grow dahlias to their full potential require attention to key factors such as proper watering, sunlight exposure, and protection from pests. By maintaining moist soil, providing ample sunlight, and implementing effective slug deterrents, you can enjoy a stunning display of dahlias in your garden. With these guidelines, you are well-equipped to embark on a successful journey of growing vibrant dahlias.

FAQs

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Sources

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[i] Crocus. (2023). Dahlia plants. [online] Retrieved from: https://www.crocus.co.uk/search/_/search.dahlia/ [accessed 26/06/23]

[ii] Thompson & Morgan (2023). Dahlia Tubers – Flower Bulbs. [online] Retrieved from: https://www.thompson-morgan.com/flowers-plants-shrubs/flower-bulbs/dahlia-tubers

[iii] Sarah Raven. (2023). Dahlias – Bulbs. [online] Retrieved from: https://www.sarahraven.com/bulbs/dahlias [accessed 26/06/23]