How to grow perennial delphiniums

What is a delphinium?

Delphiniums are tall

Perennials are plants that live for multiple years. They come in all shapes and sizes and fill our gardens with colourful flowers and ornamental foliage. Many are hardy and can survive outdoors all year round, while less hardy types need protection over winter. The term herbaceous perennial is used to describe long-lived plants without a permanent woody structure (they die back to ground level each autumn), distinguishing them from trees, shrubs and sub-shrubs.

• ​The Belladonna group have loosely branched flowers with many sidespikes, they flower in early and late summer. The upright, branching plants that can reach 1-1.2m (3-4ft) high and 45cm (18in) wide
• The Elatum group bear the tallest flower spikes, which can be up to 2m (6⅔ft) high, flower in early to midsummer and often again in autumn if cut back. The clump forming plants grow 1.5-2m (5ft-6⅔ ft) high by 60-90 cm (2-3ft) wide
• Pacific hybrids are similar to the Elatum group but not so large overall, reaching 1.8m (6ft) and 75cm (30in) wide. These are short-lived perennials and so often best grown as
  Annuals are plants that complete their life cycle in one growing season. They are generally easy to grow from seed or can be bought as young plants from garden centres. Annuals are ideal for growing in summer containers and filling gaps in borders. Some examples of annuals include sunflowers, cosmos, sweet peas and zinnia.

  annuals or
  Biennials are plants that complete their life cycle over the course of two years. In the first year, biennials typically produce leaves and roots but no flowers. In the second year, they flower and produce seeds before dying. Some common biennials include foxgloves (Digitalis), honesty (Lunaria annua) and viper's bugloss (Echium vulgare).

  biennials

Choosing the right delphinium

With so much choice, it helps to narrow down your options by considering the following:

Height  — delphiniums come in a range of sizes; from towering perennials such as Delphinium ‘Faust’ that are up to 2m (6⅔ft) tall and so good at the back of borders, to the smaller Magic fountain series at 1m (3⅓ft) tall that are ideal for near the front of borders and in containers. Taller perennials need staking with twiggy sticks/canes or sturdy metal supports the stems can grow through.

Width  — don’t forget

Perennials are plants that live for multiple years. They come in all shapes and sizes and fill our gardens with colourful flowers and ornamental foliage. Many are hardy and can survive outdoors all year round, while less hardy types need protection over winter. The term herbaceous perennial is used to describe long-lived plants without a permanent woody structure (they die back to ground level each autumn), distinguishing them from trees, shrubs and sub-shrubs.

Flower colours and forms — delphinium flowers come in a range of pastel or rich shades of blue, mauve, purple, pink, white and occasionally red. Choose colours that work well with your other plants, either complementing or contrasting. Flowers can be either single or double. Belladonna delphiniums are particularly good for cut flowers.

To browse photos and descriptions of many delphinium cultivars, or to track down specific cultivars go to RHS Find a Plant. You can also search by height, flower colour, growing position and RHS Award of Garden Merit, to help narrow down your choices.

Around 40 delphinium cultivars have an RHS Award of Garden Merit, which means they performed well in RHS trials, so are a reliable choice.
 

How and what to buy

Delphinium plants and seeds are widely available in spring and summer in garden centres and online from mail-order suppliers, including RHS Plants:

• Plug plants are available in spring from mail-order suppliers. This is a cheaper way to buy, especially in larger quantities, but the choice of cultivars is limited. Additionally, you need to look after these tiny plants carefully for several months before they are large enough to plant into their final position
• Young and more mature delphinium plants are sold in containers – 9cm (3½in) or larger – ready for planting out
• Seed suppliers sell a range of seed mixes, but be aware that not all cultivars can be propagated from seed
• Specialist growers  such as Blackmore & Langdons, Home Farm Plants and Dishforth Nursery Gardens sell a range of plants and seeds

Buying: garden centre plants

Buying: garden centre plants

Buying: mail order plants

Buying: mail order plants

Where to plant

All delphiniums like well-drained soil in full sun. Flowering wil be poor in shade and plants will not thrive in soil that is poorly drained or permanently wet in winter. You should also make sure they are sheltered from winds, as the tall flower spikes can easily bend over or break.

When to plant

Plant delphiniums in borders and containers in spring and early summer, as they become available in garden centres. However, you can plant any time as long as you water in dry spells and don't plant when the soil/compost is waterlogged or frozen.

How to plant

In borders:

RHS guide to planting perennials

RHS guide to planting perennials

In containers: 

• For short-term planting, use a peat-free multipurpose potting compost
• For permanent planting, use a loam-based compost, such as peat-free John Innes No 2 with added grit to improve drainage

RHS guide to container maintenance

RHS guide to container maintenance

RHS guide to planting up containers

RHS guide to planting up containers

Watering

• Water newly planted delphiniums as needed during their first summer to keep the soil around the roots moist until they are well established
• Even when well established, perennial delphiniums may need extra watering in summer with the aim of keeping the soil damp but not soggy
• Avoid plants in containers dry out completely, watering as needed to keep the compost moist but not waterlogged. In winter, protect them from excessive rain by moving them into a cold greenhouse or into the rain shadow at the base of a wall. Keeping them drier will improve their tolerance to cold

• See our guide to recycling and collecting water​

Feeding

Delphiniums planted in the ground shouldn't need feeding, however plants grown in containers will benefit from being fed as they have less acess to nutrients. In summer, feed with a high potassium fertliser such as tomato feed. Simply follow the manufacturer's instructions on the pack. 

How and what to feed plants

How and what to feed plants

Mulching

To help keep moisture at the roots and benefit the health of your plants, spread a 5cm (2in) layer of mulch around the outside of delphinium clumps in spring. Use well-rotted organic matter, such as garden compost or farmyard manure.

Overwintering

Perennial delphinium are hardy (RHS hardiness rating H5-H6) even in a severe winter, as long as they are grown in free-draining soil and a sunny spot. 

Plants in containers are more susceptible to cold, as their roots are less insulated than in the ground.

To help delphiniums in containers survive the winter:

• Move containers to a sheltered spot over winter, where they won't be exposed to really harsh weather
• Protect them from excess winter rain by standing them in the lee of a wall or in a coldframe or greenhouse. Roots in cold, damp compost are more susceptible to rotting

Caring for older plants

Training and support

• Medium to tall plants, of 1.2m (4ft) or more, need support. Push at least three twiggy stems into the ground, evenly spaced around the clump. Alternatively use metal supports with a grid through which the stems can grow, putting them in place before new growth reaches 30cm (1ft) high
• For supporting a particularly tall flower stems, you can insert a strong cane into the ground that is the same height as the eventual flower spike. Tie the stem to the cane with soft twine as it grows

Pruning and deadheading

• To encourage particularly tall flower stems on mature clumps, thin out the young shoots in spring, leaving just five to seven of the strongest
• Cut down faded flower spikes of perennial delphiniums at ground level to encourage a further flush of flowers
• ​You can also cut back the old leaves once they die down, after the first frosts, to keep the border tidy
• Be aware that delphinium foliage may irritate skin, so wear gloves when pruning or handling plants

Potentially harmful garden plants

Potentially harmful garden plants

You can propagate delphiniums by dividing mature clumps, taking cuttings or sowing seeds. But be aware that all parts of delphiniums are poisonous and contact with foliage may irritate your skin, so wear gloves when handling plants and seeds. 

Potentially harmful garden plants

Potentially harmful garden plants

By division

The easiest way to propagate perennial delphiniums is by division in spring: 

• Divide mature clumps into two to four pieces, discarding the woody centre
• ​The new plants should flower the same year​

From cuttings

Take basal stem (softwood) cuttings of perennial Elatum and Belladonna delphiniums in late spring:

• Use pencil-thick 8–10cm (3–4in) long shoots, which shouldn't yet be hollow, as they would be prone to rotting
• Root in peat-free cuttings compost 
• Keep at 15°C (60°F) and pot up individually when rooted, usually after about ten days
• Plant out in early summer

From seeds

Raise new plants of delphiniums such as the Pacific Hybrids and Magic Fountains Series from seed:

• Sow seeds in pots at 13°C (55°F) in spring
• Seedlings should appear in 14 days
• New plants should flower in about 18 months
• The seedlings may not be exactly the same as the parent plant

Seed: collecting and storing

Seed: collecting and storing

Delphiniums are relatively trouble free. Slugs and snails can devour young growth in spring, so are the main thing to control.

You may occasionally see some of these pests on the foliage: caterpillars, leaf miners, tortrix moth caterpillars and delphinium moth caterpillars.

Leaves can be affected by powdery mildews, which cause white powdery patches on the leaves. This disease is often worse if the plants are under environmental stress, for example those growing in shade or in very dry conditions. Brown or black blotches on the leaves may be due to delphinium black blotch. Plants can sometimes develop viruses which can cause discolouration and distortion of the growth.

If you are a member of the RHS, you can use our online Gardening Advice Service via MyRHS, for all your gardening questions.