Height 25cm-1.5m (10in – 5ft)
Planting distance 45-120cm (1-½ – 4ft)
Flowers mid summer till the first severe frost
Any well-drained garden
Sunny site or light shade
Tubers available mid winter until late spring
Dahlias come in a range of forms, sizes and colours unmatched by any other garden plants. Their glorious flower heads appear from late summer until mid autumn or the first severe frosts, filling the garden with colour when most other plants are past their best.
Garden dahlias are grown from tubers or cuttings, but dwarf bed-ding dahlias are grown from seed as well so they are treated as true annuals.
The bold flower heads and strong colours of dahlias make them outstanding plants for garden decoration and cutting. Depending on their size, they can be grown in a bed of their own, in perennial borders, open shrubberies or along walls. They prefer a sunny bed but will tolerate shade. As they are only half-hardy, the tubers should be lifted in autumn and stored in a frost-free place over winter. They are easy to grow if you merely want bright splashes of colour in late summer and autumn. If you’re aiming for the perfect exhibition flower head, however there’s a host of refining techniques.
Border dahlias are organized into nine divisions or groups, deter-mined by the shape of the flower heads.
Single-flowered dahlias have blooms up to 10cm (4in) across with a single outer ring of florets and a central disc. The plants are 30-50cm (12-20in) tall and should be grown 30-45cm (1 – 1-1/2ft) apart. Varieties in this group can be cultivated in beds or mixed borders as they don’t require support. Theirare abundant and they will last a long time if dead-headed regularly. Popular varieties include ‘Cosmos’ (pink), ‘Nellie Geerling’s’ (red), ‘Yellow Hammer’ (yellow) and ‘Princess Marie Jose’ (pink).
Anemone-flowered dahlias have blooms up to 10cm (4in) across, resembling anemones: double flowers with flat outer florets surrounding a densely packed group of shorter, tubular florets – often of a different colour. The plants reach 25-45cm (10-18in) high and should be grown 30-45cm (1-1/2ft) apart in beds, borders, tubs or deep window-boxes. This is one of the rarer “groups; it includes several varieties in pastel shades. Varieties available are ‘Comet’ (maroon), ‘Jazz’ (scarlet) and ‘Thalia’ (lavender-pink).
Collerette dahlias also have blooms 10cm (4in) across. They are single-flowered with an inner ring or collar (often of another colour) and a central disc. This group includes varieties reaching 75-100cm (30-40in) high. Set the plants 60-75cm (2 – 2-1/2ft) apart. Collerette dahlias have especially strong stems, making them favourites among flower arrangers. Popular varieties are ‘Grand Due’ (red and gold), ‘Can-Can’ (pink and yellow), ‘Claire de Lune’ (yellow), and ‘Kaiserwalzer’ (fiery red and yellow).
Peony-flowered dahlias have blooms up to 12cm (5in) across. These consist of two or more rings of flat ray florets and a central disc. The plants reach 100cm (40in) high and should be grown 60-75cm (2 – 2-1/2ft) apart. Only a few varieties are available. They ‘ include ‘Gerrie Hoek’ (pink), ‘Bishop of Llandaff’ (scarlet) and ‘Orange Flora’ (orange).
Decorative dahlias have double blooms consisting of broad flat ray florets without a central disc. They form a large group which, like both the cactus and the semi-cactus groups, is sub-divided into sections according to flower size.
GIANT varieties are 1.2-1.5m (4-5ft) high with flowers 25cm (10in) or more wide; plant 1.2m (4ft) apart.
LARGE varieties are 1-1.5m (3-½ – 5ft) tall with blooms 20-25cm (8-10in) wide; plant 1.2m (4ft) apart.
MEDIUM varieties reach 1-1.2m (3-½ – 4ft) high and have blooms 15-20cm (6-8in) across; plant 90cm (3ft) apart.
SMALL varieties are 1-1.2m (3-½ – 4ft) tall with blooms only 10-15cm (4-6in) across; plant 75cm (2-1/2ft) apart.
MINIATURE varieties are 90cm-1.2m (3-4ft) tall with blooms up to 10-15cm (4-6in) across; plant 75cm (2-1/2ft) apart.
Decorative dahlias have an extensive colour range. They are good for both exhibiting and cutting. Popular varieties are ‘Arabian Night’ (maroon-red), ‘Eveline’ (white, lilac tint), Tnca Blaze’ (golden yellow), ‘Patty’ (coral pink overlaid with salmon-pink), and ‘Trendy’ (rose and yellow).
Ball dahlias have fully double, ball-shaped blooms sometimes flattened on top. Reaching 90-120cm (3-4ft) high, they are suitable for growing in mixed borders, for cutting and for exhibiting. Plant the tubers 75cm (2-1/2ft) apart. The varieties are sub-divided into two groups: small ball with blooms 10-15cm (4-6in) wide, and miniature ball with blooms up to 10cm (4in) across. Popular varieties are ‘Rokesley Rocket’ (scarlet), ‘Nettie’ (primrose yellow), ‘Opal’ (pink and white) and ‘Alltami Melody’ (mauve-pink).
Pompon dahlias have flowers similar to ball varieties, but they are more globular and much smaller – only 5cm (2in) across. The free-flowering plants reach 90-120cm (3-4ft) high and should be grown 60cm (2ft) apart. Their selling point is their long-lasting cut flowers. Popular varieties are ‘Moor Place’ (purple), ‘Andrew Lockwood’ (lilac) and ‘Stoneleigh Cherry’ (red).
Cactus dahlias have fully double blooms with pointed ray florets. This is another group divided into sections, determined by the size of the blooms.
GIANT varieties reach 1.2-1.5m (4-5ft) high with blooms over 25cm (10in) across; plant 1.2m (4ft) apart. Their flowers don’t appear until early autumn.
LARGE varieties are also 1.2-1.5m (4-5ft) high with slightly smaller blooms at 20-25cm (8-10in) across; plant 1.2m (4ft) apart.
MEDIUM varieties are 1-1.35m (3-½ – 4ft) high with blooms 15-20cm (6-8in) across; plant 90cm (3ft) apart.
SMALL varieties are 1-1.2m (3-½-4ft) high with blooms 10-15cm (4-6in) across; plant 75cm (2-1/2ft) apart.
MINIATURE varieties are 90-120cm (3-4ft) high with blooms up to 10cm (4in) across; plant 75cm (2-1/2ft) apart. All cactus varieties are easy to grow well and make impressive cut flowers. Popular varieties are ‘Golden Explosion’ (yellow-golden), ‘Rokesley Mini’ (white) and ‘Star’s Favourite’ (rose-pink).
Semi-cactus dahlias have flowers similar to the cactus varieties, but the ray florets are wider. They are divided into the same sections as the cactus group, determined by flower size. Semi-cactus dahlias are excellent for exhibiting. Popular varieties include ‘Reginald Keene’ (orange and flame) and ‘Hamari Sunset’ (orange-yellow).
Dahlias will grow in any well-drained soil enriched with compost, manure or other organic material. Rake in some bone-meal at planting time. Plant the tubers in; leave any that have sprouted until late spring. If it is cold and wet, delay planting until the weather improves. Dig holes 10-15cm (4-6in) deep for the tubers, insert stout supporting stakes 30cm (1ft) shorter than the final height of the , and then place the in the hole and cover with soil. As the stems grow, tie them loosely to the stakes. Water well after planting.
Three or four weeks after planting, pinch out the tips on the main stems to encourage strong sidegrowths. To grow large flowers on long stems suitable for cutting or exhibiting, disbud regularly and dead-head as necessary.
A week after frosts have blackened the leaves in autumn, cut down the stems to 15cm (6in) above ground. Using a spade, make a cut one spade’s depth around each plant, 30cm (1ft) from the main stem. Holding the stems, gently ease the tubers from the soil with a fork. Take care not to damage the point at which the stem joins the tuber – this is where new growth begins. Discard any broken and rotting tubers. Place the tubers upside down and under cover for a week to drain off water that has accumulated in the hollow stems.
Put tubers in shallow boxes of dry compost, keeping the crowns free. Store in a frost-free place at 5-8°C (41-43°F). Inspect the tubers every few weeks for signs of shrivelling or disease. Place any shrivelled tubers in a bucket of water overnight, then dry thoroughly before returning to storage.
Division is the simplest method for propagating dahlias. In early spring, begin watering stored dahlia tubers, but avoid watering the crowns. Two to three weeks later the eyes on the crowns of the tubers should swell. Divide the tubers with a sharp knife, making sure each division has an undamaged eye. Dust the cut parts of the tubers with flowers of sulphur to prevent fungal attack. Then plant out when weather permits.
If planting outdoors has to be delayed, the tubers may be potted up singly in potting compost and kept in a frost-proof frame until conditions improve.
Pests and diseases
Slugs sometimes attack young plants in wet weather and aphids may infest dahlias at any stage of growth. Earwigs often hide in the flowers and eat the leaves. Grey mould may be troublesome on flower stalks and tubers in a wet summer.