How-to Guide
How to Grow Delphiniums
The short answer
Perennial delphiniums (Delphinium elatum hybrids) need full sun to perform well and free-draining soil; the RHS rates them hardy through severe winters (H5-H6) provided they never sit in waterlogged winter ground. Plant in spring or early summer, keep new plantings consistently moist their first year, and protect spring shoots from slugs and snails, which devour young growth.
Two cuts define the season: when a spike fades, cut it down at ground level to encourage a further flush of flowers; after the first frosts, cut back the old dying foliage for winter. Stake anything heading past 4 feet before the growth reaches about 30 cm — after that it's too late.
Site, soil, and planting
Full sun and well-drained soil are the two non-negotiables; delphiniums perform poorly in shade and won't survive permanently wet winter soil. The RHS advises planting in spring and early summer, though container plants can go in almost year-round if soil isn't frozen or waterlogged. Water new plantings consistently through their first summer; thereafter keep soil damp but not soggy in summer. Plants in the ground need no feeding; container plants benefit from a high-potassium (tomato-type) feed in summer.
Staking
- 1Stake early — Put supports in place before growth reaches 30 cm (12 in). Plants expected to reach 1.2 m (4 ft) or more need support.
- 2Choose a method — Push at least three twiggy stems into the ground evenly spaced around the clump, or set a metal grid support over the crown for stems to grow through.
- 3Tie tall spikes — Individual show-height stems can be tied to canes with soft twine as they extend.
Cutting back for a second flush
When a flower spike has faded, cut it down at ground level — the RHS notes this encourages a further flush of flowers later in summer. In autumn, once foliage dies down after the first frosts, cut back the old leaves to keep the border tidy and remove slug shelter.
⚠ All parts of delphiniums are poisonous, and contact with the foliage may irritate skin — the RHS advises wearing gloves when handling plants and seeds. If ingestion by a pet or child is suspected, consult poison-control/ASPCA resources.
Slugs and other problems
- Slugs and snails devour young growth in spring — protect emerging shoots; this is the make-or-break delphinium problem.
- Powdery mildew, especially in shade or dry conditions.
- Delphinium black blotch (brown-black leaf spots) and occasional viruses causing distorted growth.
- Leaf miners and tortrix moth caterpillars.
Propagation
- Division: in spring, split mature clumps into two to four pieces, discarding the woody centre; divisions flower the same year.
- Basal cuttings: take pencil-thick 8-10 cm shoots in late spring; they root in about ten days in cuttings compost at 15°C.
- Seed: sow at 13°C in spring; seedlings emerge in about 14 days and flower in roughly 18 months.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- Royal Horticultural Society — Delphinium (perennial) Growing Guide — Full sun and free-draining soil; hardy H5-H6; plant spring/early summer; stake plants over 1.2 m before growth reaches 30 cm; cut faded spikes at ground level for a further flush; cut back old leaves after first frosts; slugs/snails devour young growth; divide mature clumps in spring discarding woody centre; basal cuttings; all parts poisonous, wear gloves