Growing delphiniums

Delphiniums, also known as larkspur, are tall perennials with spikes of colourful flowers. Every traditional cottage gardener aspires to a stand of handsome hybrid delphiniums in June and July. Consequently, these giants have held their popularity for over a hundred years despite being demanding plants to grow.

The reason is simple. Delphiniums add vertical presence and they come in a seductive range of colours few flowers aspire to. This includes every shade of blue, cream, white, mauve and pink - colours that fit perfectly into the summer colour palette.

Read our guide to creating a cottage garden

When to plant delphiniums

Delphinium seeds

Sow delphinium seeds in late January in seed compost.

Delphinium plants

Young plants should be planted outside from May onwards.

Where to plant delphiniums

Delphiniums need sun to prosper and they also need fertile, enriched soil that is airy and well-drained. 

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How to plant delphiniums

If growing delphiniums from seed, plant in late January in seed compost and cover with black polythene and keep at 16 C. Germination should take 10 days.

Prick out (by handling leaves) into 9cm pots. Harden off and plant outside in May.

Delphiniums need fertile soil, so incorporate well-rotted garden compost into the ground when planting.

Those on very heavy soil should incorporate some grit into the bottom of the planting hole.

Find out about other blue flowers for your garden

Taking delphinium cuttings

The best varieties of delphinium have been named by raisers and they will need to be propagated from cuttings taken in March and April. Often it is only possible to get one cutting per year.

These soft-stemmed cuttings should be four inches in length (7-10 cm). Remove the lower leaves and use fungicide if you wish.

Plunge into a 50% mixture of compost and perlite, or pure perlite, and place somewhere warm (out of the sun) to speed root growth. 14 C is ideal and a heated propagator will help.

Pot up into John Innes no 2, once rooted, and then harden off and plant outside in May or later.

When delphiniums bloom

Delphiniums bloom in early summer, but if you cut them back as soon as the flower starts to fade you could get a second display at the end of August.

When to divide delphiniums

Divide large clumps of delphiniums in March and April, replanting into freshly enriched ground.

Read our guide to dividing perennials

When to cut delphiniums

If you cut delphiniums back hard after flowering delphiniums sometimes give you a second flush.

Delphiniums can also make good cut flowers.

Delphiniums come in a wide range of colours, meaning there is a delphinium for every garden style.

Staking delphiniums

Delphiniums are top heavy plants and they need staking firmly once they reach a foot in height. This can be done with tall, semi-circular metal plant supports or bamboo canes, or twiggy hazel supports. For this reason delphiniums are often teamed with roses and tall herbaceous campanulas and hardy geraniums, so that they emerge from the behind. 

Don't tie individual stems in, just support them with framework.

Feeding delphiniums

Mulch delphiniums with well-rotted compost after flowering.

A dusting of blood, fish and bone is also beneficial in spring.

Pests and diseases

Slugs Slugs are partial to delphinium foliage and they devour it early in the year, mainly because it’s the first soft-leaved herbaceous plant through the ground. As an organic gardener I cannot use slug pellets. But a September application of nematodes in the area close to delphiniums decimates the population in autumn and helps greatly.

Grit, liberally sprinkled through the shoots, will also help. If you can get the shoots away undamaged, to height of a foot, that is half the battle.

Read our guide to controlling slugs and snails

Mildew This water-stress disease can be prevalent in dry years but plants recover. It often affects purple varieties more.

Fasciation This produces flattened stems and it may be caused by a growth check due to cold weather. Again plants do recover - it’s a just temporary condition.

Grow with…

Grow delphiniums with traditional shrub roses like the creamy hybrid musk 'Penelope'. Place dark varieties close to the foamy white flowers of Crambe cordifolia or use them as a warm up act behind herbaceous phloxes.

Find out how to grow border phloxes

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Delphinium varieties

'Celebration' - a pale-cream with a dark eye (220 cm)
'Langdon’s Royal Flush' - a magenta-pink (200 cm)
'Min' - a dark-eyed lavender blue (160 cm)
'Langdon's Pandora' - a black-eyed mid-blue (240 cm)
'Fenella' - a black-eyed gentian blue (165 cm)
'Blue Nile' - a white-eyed dark blue (180 cm)
'Black Knight' - a deep indigo delphinium (150cm)
'Bruce' - a deep-violet with a dark eye (210 cm)
'Magic Fountain' series - compact delphiniums with dense flowers in white, blue and purple (100cm)
'King Arthur' series - tall plants with white-eyed purple flowers (150cm)

Good seed strains of delphinium

Although the best plants are named and sold by specialist nurseries there are some good seed strains available. They are usually labelled Delphinium hybridum and you generally get ten to twelve seeds for between two and three pounds. There has been an RHS seed trial and 'Pagan Purples' stood out well as did 'Double Innocence' (a green-white from Jelitto Seeds). The Centurion Series also did well.

AGM winners

'Centurion Sky Blue' (from Plants of Distinction)
'Rosy Future' (from Mr Fothergill’s)
'Sweetheart' (from D’Arcy Everest)

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