Lexi Webster
Delphiniums are a beautiful addition to any garden, with their tall, stately spikes and trademark blue flowers. They are a clump-forming herbaceous perennial and a member of the Ranunculaceae or buttercup family. They can grow up to 2m tall, but there are also dwarf varieties that only reach about 60cm. Delphiniums are slow to spread and are usually grown from seedlings or seeds, best planted in early autumn. They like a moist, humus-rich soil with good drainage and shelter from strong winds. They should be planted in full sun with some shade from the hot afternoon sun. They are heavy feeders and require fertilisation during the spring and flowering season.
| Characteristics | Values |
|---|---|
| Height | 20cm-2m tall depending on species |
| Climate | Delphiniums prefer moist, cool summers and cannot tolerate dry summers |
| Soil | Well-drained, fertile, neutral to slightly alkaline (pH 6.0-7.5) |
| Sunlight | Full sun with shelter from strong winds |
| Spacing | 70cm apart |
| Support | Stakes, trellis, or wire mesh |
| Pests | Snails, slugs, thrip, aphids, cyclemen mites, nematodes |
| Diseases | Powdery mildew, grey mould, rust, aster yellows, blight, leaf spot, downy mildew, mould, crown and root rot, viruses, smut, wilts |
| Fertilisation | Liquid organic fertiliser, liquid seaweed fertiliser, worm castings tea, high-potash fertiliser, Yates Thrive Roses and Flowers Liquid Plant Food |
| Watering | Keep well-watered, water generously during germination |
| Mulching | Organic mulch such as bark chips, sugar cane, or pea straw |
| Flowering | Spring to summer, late spring with proper care |
| Colours | Blue, pink, yellow, red, white, purple, green |
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What You'll Learn
Soil and fertiliser
Delphiniums prefer a neutral to slightly alkaline soil pH of 6.0 to 7.5. Before planting, it is important to prepare the soil by mixing in 2 to 4 inches of compost or aged manure, such as well-rotted cow manure, to ensure the soil is fertile. If the soil is acidic, add lime or wood ashes to increase alkalinity. The soil should also be well-drained, so raise the garden bed slightly in damp areas.
To promote healthy growth, feed delphiniums with fertiliser during the growing season in spring and summer. Apply a high potash fertiliser every week or a liquid feed every 4-5 weeks. Liquid organic fertilisers, such as liquid seaweed fertiliser and 'worm castings tea', are recommended. Top dressing with fertiliser can also be beneficial. After germination, mulch with an organic mulch, such as bark chips, sugar cane, or pea straw, to retain moisture in the soil and prevent it from drying out.
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Planting and spacing
Delphiniums are slow-growing perennials that can reach heights of up to 2 metres, though dwarf varieties are also available, growing to around 20-60cm. The taller varieties will require staking to support their hollow stems, which can fall over or break in strong winds.
Delphiniums should be planted in the spring, in fertile soil with good drainage and a neutral to slightly alkaline pH of 6.0 to 7.5. The soil should be loosened and mixed with 2-4 inches of compost or aged manure. A handful of blood and bone can also be added. It is best to prepare the soil a few weeks before planting. Delphiniums grow well in areas with cool, moist summers and do not flower well in dry summers. They require around 7 hours of sun per day, preferably in the morning in hotter climates.
When planting delphinium seeds, cover them lightly with soil and press down. Water generously and cover with newspaper or cardboard until the seedling is established and grows to around 5cm above the ground. Feed every week or so with a high-potash fertiliser or liquid fertiliser. After germination, remove the newspaper or cardboard and mulch with an organic mulch such as bark chips, sugar cane or pea straw. Feed every 1-2 weeks with liquid plant food.
The spacing of delphiniums is debatable, as the closer they are together, the taller they will grow. However, taller plants can be weak and fall over. It is recommended to plant them around 70cm apart and stake them with bamboo canes.
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Support and staking
Delphiniums are tall plants with hollow stems that cannot withstand strong winds. They require support, especially when they are about 12 inches tall.
Delphiniums do not need a trellis, but they do require a stake for support. Some growers use a wire mesh for support, but stakes are easier to use. Bamboo canes are a good option for staking. They should be placed at planting time to avoid disturbing the roots of mature plants.
The closer the plants are together, the taller they grow. However, taller plants can be weak and fall over, so staking is necessary.
Delphiniums are susceptible to snails and slugs, especially when they are small seedlings.
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Watering and mulching
Delphiniums require moist soil, so be sure to water them generously, especially during dry periods in summer. They grow well in areas with cool, moist summers but do not fare well in dry summers. They are short-lived perennials, lasting two to three years in the garden.
To prepare the soil, it is recommended to dig in some well-rotted compost and cow manure, add a handful of blood and bone and dig over again. Leave this mixture for a few weeks before planting.
Delphiniums also benefit from mulching, which helps to keep the soil moist and cool and prevents the soil from drying out quickly. Mulch around the base of the plant to provide the cool, moist conditions they thrive in. You can use an organic mulch such as bark chips, sugar cane or pea straw.
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Common pests and diseases
Delphiniums are susceptible to a range of pests and diseases. Good garden hygiene, regular inspections, and providing good airflow and drainage can help keep your delphiniums healthy.
Common Pests
The main pests are snails and slugs, especially with the small seedlings. Slugs and snails feed on the leaves and stems, leaving holes in the foliage. You can pick them out by hand, or use baits to trap and kill them. Beer is one of the most commonly used baits to trap slugs. You can pour some beer onto a plate and keep it next to the delphiniums. Slugs will climb in and drown in the beer.
Thrips and aphids can also be an issue. Insecticidal soap can be effective in treating aphids or plant lice, which cause distorted, curly leaves.
Common Diseases
Delphiniums are also susceptible to several diseases. Powdery mildew, caused by fungi that grow in moist, humid conditions, is one of the most common. Grey mould and rust are other frequent issues.
Waterlogging is another serious problem, which can cause root rot. Once root rot sets in, it is difficult to save the plant. It is important to dispose of the affected parts and avoid planting any seedlings in the same spot, as the disease can spread through the soil.
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