1. Introduction

Dahlias are revered for their vibrant blooms, diverse colours, and wide range of shapes and sizes, making them a beloved choice among gardeners and floral enthusiasts. This guide explores the world of dahlias, one of my favourite and delves into their botanical details, growing requirements, care tips, and various uses.

Whether you’re a seasoned gardener or a beginner looking to add a pop of colour to your garden, dahlias offer endless possibilities for creativity and beauty.

2. Plant Profile

• Botanical Name: Dahlia spp.
• Common Names: Dahlia
• Family: Asteraceae
• USDA Zone: USDA zones 8-11 (varies by species)
• Origin: Native to Mexico and Central America

3. Description

Dahlias are herbaceous perennials that produce an abundance of showy flowers in a wide array of colours, shapes, and sizes. The flowers can range from small, single blooms to large, dinner-plate-sized blossoms, with petals that may be rounded, pointed, or quilled. Dahlias are known for their lush, dark green foliage, which provides an attractive backdrop to the vibrant blooms.

4. Growing Conditions

Sunlight

Dahlias thrive in full sun, requiring at least 6 to 8 hours of direct sunlight per day for optimal growth and blooming.

Temperature

They prefer moderate temperatures between 60°F to 70°F (15°C to 21°C) but can tolerate warmer conditions.

Soil

Well-draining, fertile soil with a pH of 6.5 to 7.0 is ideal for dahlias. Amend heavy clay soils with organic matter to improve drainage.

Watering

Keep the soil consistently moist but not waterlogged, especially during hot, dry weather. Water at the base of the plants to avoid wetting the foliage.

Humidity

Dahlias prefer moderate humidity levels but can tolerate drier conditions once established.

5. Winter Care

In regions with mild winters, dahlias may survive outdoors if adequately mulched to protect the tubers from freezing temperatures. In colder climates, dig up dahlia tubers after the first frost and store them indoors in a cool, dry location until spring.

6. Planting

• Planting Time: Plant dahlia tubers in the spring after the danger of frost has passed, once the soil has warmed up to around 60°F (15°C).
• Planting Method: Dig a hole slightly larger than the tuber and place it horizontally in the hole, with the eye facing upwards. Cover the tuber with soil and water thoroughly. Space dahlia plants 1 to 3 feet apart, depending on the mature size of the variety.

7. Care and Maintenance

Watering

Water dahlias regularly, especially during dry spells, to keep the soil consistently moist. Avoid overhead watering to prevent fungal diseases.

Fertilising

Apply a balanced fertiliser high in phosphorus (such as a 5-10-10 NPK formula or 10-20-20) every 4 to 6 weeks during the growing season to promote healthy growth and flowering.
Do not over-fertilize, especially with nitrogen, or you risk small or no blooms, weak tubers, or rot

Pruning

Pinch out the growing tips of young dahlia plants when they reach 12 to 18 inches tall to encourage bushier growth and more blooms. Deadhead spent flowers to prolong blooming.

Pest and Disease Control

Keep an eye out for common pests such as aphids, slugs, and snails, and treat infestations promptly with insecticidal soap or organic remedies. Prevent fungal diseases by providing good air circulation and avoiding overhead watering.

Diseases: Botrytis blight; southern blight; crown gall; powdery mildew; smut; virus; wilt.
Pests: aphids, caterpillars, earwigs; nematodes; slugs and snails; thrips; spider mites

8. Propagation

Dahlias can be propagated from tubers, seeds, or cuttings.

• Tubers: Divide established clumps of tubers every 2 to 3 years in the spring to maintain vigour and prevent overcrowding.
• Seeds: Collect seeds from mature dahlia flowers and sow them indoors in early spring.
• Cuttings: Take stem cuttings from healthy plants during the growing season.

How to grow dahlias from seed

Bedding dahlias are available as packeted seed mixes and are easy to sow indoors in spring:

1. Follow the steps outlined in our guide to sowing seeds indoors
2. When the seedlings are large enough to handle, prick out individually into modules or small pots
3. Apply liquid feed every two weeks
4. In late May or early June, gradually acclimatise the plants to outdoor conditions by hardening off, before planting into their flowering positions

Once they flower, you can select your favourite plants and save their tubers to grow next year.

How to take dahlia cuttings

1. Pot up dormant tubers and start them into growth in warm conditions (e.g. a heated propagator on a windowsill) in late winter to early spring
2. Select healthy, vigorous shoots and take basal or softwood cuttings
3. As your new plants grow, pot them on into slightly larger pots
4. Keep them indoors until late May or early June, then gradually acclimatise them to outdoor conditions by hardening them off. Then plant into their flowering positions

Dividing clumps of dahlia tubers

1. Start the dormant tubers into growth indoors in early spring (February) in a shallow tray of peat-free potting compost
2. When the shoots are about 2.5cm (tin) tall, lift the clump of tubers and divide it into several pieces, making sure each has both roots and shoots
3. Pot up each section into a separate container and grow on in a bright, frost-free place until late May or early June
4. Gradually acclimatise the plants to outdoor conditions by hardening off, then plant into their flowering positions

9. Flowering

Dahlias typically bloom from midsummer to the first frost, with peak flowering occurring in late summer and early autumn. The flowers come in a wide range of colours, including white, yellow, orange, pink, red, and purple, with variations in shades and patterns.

10. Uses

Ornamental

Dahlias are prized for their ornamental value and are commonly grown in flower beds, borders, containers, and cutting gardens. They make excellent cut flowers for bouquets and floral arrangements.

Medicinal

While not traditionally used for medicinal purposes, some dahlia species may have potential medicinal properties that warrant further research.

Culinary

Dahlia flowers are edible and can be used to garnish salads, desserts, and cocktails. Ensure the flowers are pesticide-free and only consume them in moderation.

11. Using Dahlias in the Garden

A. Companion Plants

• Beneficial Plants: Pair dahlias with other sun-loving annuals and perennials such as zinnias, marigolds, salvia, and cosmos. These plants complement dahlias’ vibrant blooms and attract pollinators.
• Avoided Plants: Avoid planting dahlias near plants that require frequent watering or have invasive root systems, as this may compete with dahlia growth.

B. Garden Design Ideas

• Using Dahlias as a Focal Point: Create focal points in garden beds or containers by planting dahlias in groups or as standalone specimens. Mix and match different colours and varieties for visual interest.
• Using Dahlias in Mass Plantings: Plant dahlias en masse in flower borders or cutting gardens for a striking display of colour and texture. Choose complementary colours and heights for a cohesive look.

C. Landscaping Ideas

• Using Dahlias in Naturalistic Plantings: Incorporate dahlias into cottage gardens, meadow plantings, or wildflower borders for a relaxed and informal aesthetic. Mix them with grasses, daisies, and other cottage garden favourites.
• Using Dahlias in Formal Plantings: Dahlias can also be used in formal garden designs, such as geometric beds or formal knot gardens, where their structured growth habit and vibrant blooms add elegance and sophistication.

12. Conclusion

Fun Facts

• Dahlias are named after the Swedish botanist Anders Dahl, who played a significant role in classifying and cataloguing these plants in the 18th century.
• Dahlias are the national flower of Mexico and are celebrated during the country’s annual Dia de la Dahlia (Dahlia Day) festival.
• There are over 42 different species of dahlias, with thousands of cultivars and varieties bred for their unique colours, forms, and characteristics.
• Dahlias were originally cultivated by the Aztecs for their edible tubers, which were roasted and eaten as a food source.
• Some dahlia flowers can reach impressive sizes, with dinner-plate dahlias measuring up to 12 inches (30 cm) in diameter.
• Dahlias are a favourite among pollinators such as bees, butterflies, and hummingbirds, making them valuable additions to wildlife-friendly gardens.
• The tubers of certain dahlia species contain compounds with potential medicinal properties, including anti-inflammatory and antioxidant effects.
• Dahlias have a long history of hybridisation and selective breeding, resulting in an incredible diversity of flower shapes, colours, and patterns available to gardeners today.
• In Victorian flower language, dahlias symbolise dignity, elegance, and inner strength, making them popular choices for floral arrangements and bouquets with symbolic meanings.
• Dahlias have inspired artists, poets, and gardeners for centuries, with their bold colours and intricate forms often depicted in paintings, literature, and garden designs.