Dahlia

Cultivation notes

Dahlias are easy to grow but do need winter protection in most parts of the UK. They are tolerant of a wide range of soil types and situations but best planted in fertile, well-drained soil in full sun when danger of frost is over, usually May to early June. Planting could be carried out slightly earlier in southern counties or later in the north of England and Scotland.

• Incorporate plenty of organic matter (one or two buckets per sq m/sq yd) such as well-rotted manure into the soil prior to planting and use a general purpose fertilizer, according to the manufacturer’s instructions
• Plant tubers in their flowering position. Tubers should be planted 10-15cm (4-6in) deep
• Stake as required and pinch out shoots to promote bushiness (see pruning and training below)
• Keep well watered and once flowers appear feed with a high potash liquid feed every two weeks from July to early September

Overwintering

Cut plants to the ground after the first frosts have blackened the foliage. In mild regions and on well-drained soils, leave the tubers in the ground and cover with a 7.5-15cm (3-6in) deep layer of bark chips or garden compost to protect them from frost. In colder areas or on heavy soils, lift and store the tubers (see below) and replant the following spring.

Lifting and storage

1. Cut down foliage and use a fork to carefully prise plants out of the soil
2. Dry off naturally and then clean away any soil clinging to the tubers. Trim stems to 15-20cm (6-8in). If the tubers have been washed, position them upside down in a cool place for a few weeks to dry off
3. Trim off any fine roots
4. Place tubers in shallow wooden boxes or open trays and pack with a peat-free compost or dry sand, just covering the tubers but leaving the crown exposed
5. Store in a dry, cool, frost-free place. If stored in a garden shed cover with newspaper if a hard frost is predicted
6. Inspect tubers regularly during winter for rotting and discard any that are unhealthy

Pruning and training dahlias

• Insert canes on planting and tie in as growth develops
• Pinch out growing tips once plants reach a height of about 40cm (16in) to encourage branching
• For giant blooms restrict the number of flowering stems to three to five per plant; for smaller blooms allow seven to 10 flowering stems per plant
• To produce a long-flowering display and strong stems, remove the two pairs of flower buds developing in the leaf axils below the terminal bud
• Deadhead as flowers fade
• Bedding dahlias need no staking or disbudding; just pinch out the growing tip to encourage bushiness and deadhead regularly

Propagation

• Start tubers into growth in early spring indoors. Divide the tubers when shoots are 2-3cm (about an inch) tall by separating them into portions ensuring each section has both roots and shoots. Pot each section into a separate container and grow on
• Alternatively, take basal shoot cuttings from sprouted tubers; dip into hormone rooting powder and plant in a free draining compost. Using a propagator, maintain the temperature at 21°C (70°F) until rooted, usually in around two weeks. Pot on rooted cuttings in individual 9cm (3½in) containers and soilless compost
• Bedding dahlias are easily grown from seed sown indoors. When seedlings are large enough, prick out into modules or small containers and liquid feed every two weeks

Cultivar Selection

Dahlia merckii: A tough species (Botanical Group) dahlia, multi-branched, with mid-green leaves composed of toothed leaflets and profuse broad-rayed lilac-pink flowers with yellow disc florets that open in loose clusters above the foliage. Height up to 1.5m (5ft)
D. ‘Twyning’s After Eight’ AGM: A long-flowering stylish (Single-flowered Group) introduction with profuse ivory-white single flowers held on strong stems above almost black foliage. Height 90cm-1.2m (3-4ft)    
D. ‘Bishop of Llandaff’ AGM: Popular peony-flowered (Miscellaneous Group) dahlia with almost black foliage and semi-double deep-red blooms. Height 1m (3¼ft)

Links

RHS Dahlia Trials
Dahlia Registrar (held by the RHS)
National Dahlia Society: for information on UK dahlia classification 2009, show schedules, history of the dahlia, species dahlias
RHS AGMs
RHS Plant Selector
RHS Plant Finder

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'Twyning's After Eight' is a single-flowered dahlia with dark leaves, ideal for borders.

'Brantwood' is another single-flowered dahlia.

'Fascination' is a small waterlily or dwarf bedding dahlia. It has dark, blackish-brown foliage and is of a diminutive height, ideal for the front of the border.

'Gwyneth' is a small waterlily dahlia. Its long stems make it a great cut flower and a useful cultivar for the show bench.

'Taratahi Ruby' is a small waterlily dahlia. Don't be put off by its strong red hues; these fit perfectly with the vibrant tints of autumn. This variety is also a prolific bloomer, producing a display between July and the first frosts.

'Hillcrest Royal' is a medium cactus-flowered dahlia. The intense pinky red blooms mix surprisingly well with other border plants, such as persicaria and tender salvias. The neat form has helped ensure its popularity on the show bench too.

'Ken's Choice' is a small ball dahlia. The quilled petals form neat patterns in the flower, and the back petals do not drop as quickly as some cultivars.

'Kiwi Gloria' is a semi-cactus form. It has stout stems and a tidy form, which are good for the show bench, but the cultivar doesn't make sturdy tubers so can be difficult to overwinter.

Problems

Dahlias can be prone to a number of pests, diseases and disorders;

• Pests include aphids, capsid bug, earwigs, caterpillars, slugs and glasshouse red-spider mite
• Powdery mildew can be a problem but is easily controlled with a fungicide
• Can be prone to mosaic virus and tomato spotted wilt virus
• Storage rots are common. Make sure tubers are properly dried and healthy
• Poor flowering can be due to overfeeding with high nitrogen fertilisers, not enough direct sunshine or too dry at the roots
• Dense clusters of distorted leafy shoots, often close to ground level, are leafy gall