How to grow delphinium
Overview
Delphinium elatum is a tall, clump-forming herbaceous perennial grown for its spectacular spires of blue, purple, pink, or white flowers. It is a classic border plant that adds vertical structure and is often used to fill gaps at the back of beds. Key fact: all parts of the plant are toxic if ingested.
Varieties
- Pacific Giants — Mixed colours, tall 1.5–2 m spikes, reliable in cool and temperate zones.
- Magic Fountains — Compact 1–1.2 m, good for smaller gardens and less staking.
- New Millennium — Improved strain with strong stems and deep blues, bred for Australian conditions.
Soil Preparation
Delphinium prefers deep, fertile, well-drained soil with a pH of 6.5–7.5. Before planting, dig in plenty of well-rotted manure or compost. On heavy clay, raise beds or add gypsum to improve drainage. A light dressing of lime in autumn can help if soil is acidic.
Growing Guide
Plant delphinium in a position with full sun and rich, well-drained soil. Space plants 60–90 cm apart to allow for their tall, upright habit. Set the crown just below soil level and water in well. Tall varieties need staking early in spring; insert stakes at planting time to avoid damaging roots later. In cool climates, delphinium thrives with steady moisture and a long, cool growing season; in temperate zones, provide afternoon shade and extra mulch to keep roots cool.
Sowing
Sow delphinium seed in late spring or early summer in a seed tray of moist, fine compost. Cover lightly with vermiculite and keep at 15–20°C; germination takes 14–21 days. Transplant seedlings when large enough to handle, spacing 60 cm apart. Direct sowing is possible but less reliable; seeds need light to germinate.
Care
Feed with a balanced liquid fertiliser every 2–3 weeks during active growth. After the first flowering, cut back spent flower stalks to the base to encourage a second flush in late summer. In autumn, cut the whole plant down to 10 cm and mulch for winter. Divide clumps every 3–4 years to maintain vigour.
Companion Planting
Good companions at the back of a border include foxgloves, lupins, and echinacea, which share similar height and sun requirements. Avoid planting near low-growing groundcovers that may be smothered. Roses and peonies also pair well, providing contrast in form and colour.
Common Pests & Risks
Slugs and snails are the main pests, especially on young shoots in spring. Use iron-based pellets or beer traps. Aphids can cluster on new growth and flower buds; hose off or apply a soft soap spray. Crown rot may occur in wet, poorly drained soil; improve drainage and avoid overwatering.
Harvesting
Delphinium is grown for cut flowers. Harvest flower spikes when the lowest florets on the stem are fully open and the upper ones are still in bud. Cut early in the morning and place immediately in water. For longest vase life, remove lower leaves and use a floral preservative.
Troubleshooting
Yellow leaves: overwatering or poor drainage; reduce water and improve soil. Brown leaf edges: heat stress or low humidity; provide afternoon shade and mulch. Stunted growth: lack of nutrients or root competition; feed with liquid fertiliser and remove weeds. Flower spikes flopping: insufficient staking or wind damage; stake early and tie loosely. Powdery mildew on leaves: poor air circulation; space plants wider and avoid overhead watering.