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What is a delphinium?

Delphiniums are tall perennials that produce vivid flower spikes in early summer. Most are perennials, thriving in borders for years, and they fall into three main groups based on their looks:

• 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 (6â…”ft) high, flower in early to midsummer and often again in autumn if cut back. The clump forming plants grow 1.5-2m (5ft-6â…” 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
  

  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.

  or 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 perennial

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.

delphiniums grow wide as well as tall, forming clumps with a spread of 45cm (18in) or more. Those in the Elatum Group can reach 90cm (3ft) wide, so make sure you have room for them - delphiniums grow best when not crowded by other plants

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.

Positioning — all delphiniums need full sun to perform well. Tall perennial types also need a sheltered spot, out of strong winds. They are usually best at the back of borders or grouped with other large plants such as ornamental grasses.

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 :

• Plug plants
  

  Seedlings or young plants grown singly in small modules, with the advantage that they can be transplanted with minimal root disturbance. Bedding plants and young veg plants are often sold as plug plants of various sizes, with smaller ones requiring more aftercare. They usually need to be potted up and grown on indoors until large enough to plant outside.

  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 , and sell a range of plants and seeds

Buying: garden centre 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

Can refer to either home-made garden compost or seed/potting compost:

• Garden compost is a soil improver made from decomposed plant waste, usually in a compost bin or heap. It is added to soil to improve its fertility, structure and water-holding capacity. Seed or potting composts are used for growing seedlings or plants in containers - a wide range of commercially produced peat-free composts are available, made from a mix of various ingredients, such as loam, composted bark, coir and sand, although you can mix your own.

is waterlogged

Describes soil or potting compost that is saturated with water. The water displaces air from the spaces between soil particles and plant roots can literally drown, unless they are adapted to growing in waterlogged conditions. Waterlogging is common on poorly drained soil or when heavy soil is compacted.

or frozen.

How to plant

In borders:

Planting delphiums is just the same as planting any perennial

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.

plant. See our 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 planting up containers

RHS guide to container maintenance

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
  

  Can refer to either home-made garden compost or seed/potting compost:

  
  • Garden compost is a soil improver made from decomposed plant waste, usually in a compost bin or heap. It is added to soil to improve its fertility, structure and water-holding capacity. Seed or potting composts are used for growing seedlings or plants in containers - a wide range of commercially produced peat-free composts are available, made from a mix of various ingredients, such as loam, composted bark, coir and sand, although you can mix your own.

  moist but not waterlogged
  

  Describes soil or potting compost that is saturated with water. The water displaces air from the spaces between soil particles and plant roots can literally drown, unless they are adapted to growing in waterlogged conditions. Waterlogging is common on poorly drained soil or when heavy soil is compacted.

  . 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

To grow well, plants need a wide range of nutrients in various amounts, depending on the individual plant and its stage of growth. The three key nutrients usually taken up from the soil are nitrogen, phosphorus and potassium. Plants also need carbon, oxygen and hydrogen to make carbohydrates — their main energy source. Carbon and oxygen come from the air as carbon dioxide, while hydrogen is drawn from water absorbed by the roots. Other important soil nutrients include magnesium, calcium and sulphur. Most garden soils contain enough of these nutrients to support healthy plant growth.

. In summer, feed with a high potassium fertliser such as tomato feed

A potassium-rich liquid feed to encourage flowering and fruiting in various plants, not just tomatoes. Take care to follow the instructions on the packet.

. Simply follow the manufacturer's instructions on the pack.

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

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.

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

Plants will need digging up and dividing in early spring every 2-5 years. The aim is to improve the soil, replant the most vigorous sections, and to increase the number of plants you have. Also see Propagation below.

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
  

  A cane is a slender, straight, length of woody plant material, usually bamboo. Canes are primarily used as plant supports. The fruiting stems of blackberries, raspberries and hybrid berries (such as loganberries and tayberries) are also known as canes, so these crops are often referred to as cane fruit.

  into the ground that is the same height as the eventual flower spike. Tie the stem to the cane with soft twine
  

  Twine is a soft, fine loosely woven string treated with preservatives for outdoor use. Used for tying in climbers and other plants to supports.

  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

You can propagate delphiniums by dividing mature clumps, taking cuttings

A method of growing new plants from parts of an existing plant, such as sections of root, stem, leaf or bud. When prepared correctly and planted in the right conditions, they can produce roots and eventually become independent plants. There is a wide range of different methods for taking cuttings, depending on the plant and time of year.

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

By division

The easiest way to propagate perennial

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.

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
  

  Can refer to either home-made garden compost or seed/potting compost:

  
  • Garden compost is a soil improver made from decomposed plant waste, usually in a compost bin or heap. It is added to soil to improve its fertility, structure and water-holding capacity. Seed or potting composts are used for growing seedlings or plants in containers - a wide range of commercially produced peat-free composts are available, made from a mix of various ingredients, such as loam, composted bark, coir and sand, although you can mix your own.

• 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
  

  A seedling is a young plant grown from seed.

  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

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.