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Delphinium: sowing, planting and success

Delphinium: sowing, planting and success

Daniel Platteau

In short: the perennial delphinium is a tough plant in Belgium (it withstands −15 to −20 °C in free-draining soil), but it has two enemies: slugs in spring and wind in June. To sow it, one rule really matters: fresh seed. Sown at harvest (August) or kept in the fridge and sown in spring at around 13-15 °C, it comes up in two to three weeks. Expect about 18 months before the first real flowering. Plant in sun, 40-60 cm apart, stake early, and cut faded spikes to the ground for a second flush in late summer. The whole plant is poisonous: gloves recommended.

The tall blue spikes of delphiniums are part of the image of the English garden. People think they are difficult. In fact the plant is hardy and long-lived; what fails is mostly the sowing (seed too old) and the young shoots (eaten by slugs). This article follows the whole journey, from seed to second flowering, based on advice from the Royal Horticultural Society (RHS), a germination trial published by a New Zealand breeder, and a year of filmed follow-up by the YouTube channel ART - Le pouvoir des plantes.

Delphinium or larkspur: what are we talking about?

The word "delphinium" comes from the Greek delphis, dolphin: the flower bud, with its spur, recalls its shape. In English, the spur also gave the name "larkspur". The genus belongs to the buttercup family, Ranunculaceae, like columbines, anemones and hellebores.

In the garden you meet two very different kinds of plants:

  • Perennial delphiniums, mostly derived from Delphinium elatum and its hybrids. These are the tall spikes, one to two metres high, that come back every year.
  • Annual larkspurs, long classified in a separate genus, Consolida. Current botanical databases place them in Delphinium: the garden larkspur is now called Delphinium ajacis (POWO, n.d.). They are sown in place, in spring or autumn, and die after flowering.

This article is about the perennials. The RHS sorts them into three groups (RHS, n.d.):

Group Height Flowering Feature
Elatum 1.5 to 2 m, up to 90 cm wide early to midsummer, often again in autumn if cut back the tallest, densest spikes
Belladonna 1 to 1.2 m early and late summer light habit, many side spikes, little need for staking
Pacific (Pacific Giants) up to 1.8 m summer short-lived, often grown as biennials

There are also dwarf seed series such as Magic Fountains (about 1 m), which need no staking in an open spot and do well in a large pot (RHS, n.d.). Around forty delphiniums hold the RHS Award of Garden Merit, a label given after garden trials.

Delphinium, larkspur, pied-d'alouette in French, ridderspoor in Dutch: it is the same genus. Annual larkspurs (formerly Consolida) are now Delphinium too.

Why sow rather than buy?

A packet of seed costs barely more than a single potted plant, and gives dozens. Sowing is therefore well worth it if you want a group of five or seven plants, the number the RHS recommends for impact (plant in threes or fives).

It has one limit you should know: seed from a hybrid does not faithfully reproduce the parent plant (RHS, n.d.). If you collect seed from a deep blue 'Faust', you will get plants of varying colours and heights. Often pleasant, but not a copy. To multiply a particular variety, use cuttings or division (see below). Seed series (Pacific Giants, Magic Fountains) are selected to give fairly uniform plants.

The golden rule: fresh seed

All serious sources stress the same point: delphinium seed quickly loses its ability to germinate at room temperature. The RHS recommends always using fresh seed, as old seed may germinate erratically (RHS, n.d.). Enthusiasts' associations advise keeping seed in an airtight box in the fridge until sowing.

In practice, two strategies work:

  1. Sow at harvest, in August-September. That is what the ART video shows: seed collected around 15 August, when the pods turn brown and split, sown straight away, up on 1 September.
  2. Keep the seed in the fridge and sow in spring, in pots, at about 13 °C (RHS, n.d.). Seedlings appear in about two weeks and flower about 18 months later.

A packet bought at a garden centre has often spent months on a warm shelf. Buy it early, put it in the fridge, and sow plenty: germination of a shop packet is rarely complete.

Light or darkness? What a trial says

A persistent myth says delphinium seed needs total darkness to germinate. Terry Dowdeswell, a New Zealand delphinium breeder, tested this in 1998 with seed from a hybrid and from Delphinium × belladonna 'Bellamosum' (Dowdeswell, n.d.). Identical batches were sown in four places: near a bright window, in a closed cupboard, on a poorly lit shelf and in the fridge.

Location Hybrid 'Bellamosum'
Bright light (no direct sun) 88 % 85 %
Dark cupboard 86 % 86 %
Office shelf 92 % 83 %
Fridge 84 % 78 %

The differences between light and darkness are negligible. The only clear difference is speed: in the fridge, the seed germinated much more slowly. The author's conclusion: temperature matters more than light. It is a simple trial, without statistical replication, but it agrees with common practice: seeds are covered with a thin layer of compost or vermiculite, mainly to keep them moist.

The RHS advises germinating seed at around 13 °C. Yet in Dowdeswell's trial the seed germinated well at 25 °C by day and 16 °C at night: a normally heated house is fine, but there is no point putting the trays on a radiator. A few days in the fridge in a damp paper towel (stratification) is often recommended for older seed; it is not needed for fresh seed, as the ART video shows.

Sowing step by step

In place or in a tray (late summer). Loosen the soil, add a thin layer of seed compost, firm and water before sowing. Sow broadcast or in rows, cover lightly with compost, firm again so the seed makes contact, then water with a fine rose. Protect from birds and cats: a simple mesh frame or an upturned tray will do. In strong sun, shade netting prevents drying out.

In pots (spring). Sow on moist seed compost, on the surface, and cover with a thin layer of vermiculite or sieved compost. Keep cool and in the light (13-15 °C), never letting it dry out.

Pricking out. Wait until seedlings have four true leaves. Lift them gently with a stick: at this stage they have only two or three roots. Pot them into small pots of potting compost, peat pots, or plant them straight into the ground 40-50 cm apart. Water at once: this is the most fragile stage.

In the video, ART compares plants left in the ground with plants potted up for winter. On 14 March, the ones in the ground are greener and bushier; the potted ones are well rooted but a little weak. The following year, the delphiniums never moved, in a humus-rich raised bed, are the finest in the garden. Practical lesson: if you have the space, leave the seedlings in the ground, thinned to 40-50 cm.

Getting through the first winter

Once established, the perennial delphinium is very hardy: the RHS rates it H5-H6, meaning it survives winters of −15 or even −20 °C, provided it is in sun and in soil that drains well (RHS, n.d.). So it is not cold that kills a delphinium in Belgium, it is standing water.

Young plants sown in late summer are more fragile:

  • In pots, move them into a cold greenhouse or a well-ventilated tunnel when frosts set in. A pot freezes faster than the open ground.
  • Water sparingly in winter: excess moisture rots the crown. According to ART, this is the main risk of the season.
  • Watch for slugs from autumn and again in March: one damp night can wipe out a row of young plants.

Plants in the ground can stay where they are. Our seasonal advice for protecting the garden is in December in the garden.

Planting a delphinium

The RHS recommends planting in spring or early summer, or at other times if you water in dry spells, never in frozen or waterlogged soil (RHS, n.d.).

  • Aspect: full sun. In shade, flowering is poor.
  • Shelter: tall varieties need shelter from strong winds.
  • Soil: well drained, fertile. Avoid soils that stay wet in winter. On heavy soil, plant on a mound or add grit.
  • Spacing: at least 45 cm, up to 90 cm for large Elatum types.
  • Mulch: 5 cm of well-rotted compost or manure around the clumps in spring.
  • In a pot: a loam-based compost (such as John Innes No. 2 in the UK) with added grit for long-term growing, and a high-potash feed (such as tomato feed) in summer.

In open ground, the RHS considers feeding generally unnecessary.

Thinning, staking, cutting back

Thin the shoots. On a mature clump, the RHS advises keeping only the five to seven strongest shoots in spring, when they are about 7 cm tall (RHS, n.d.). The remaining spikes will be taller and denser. On a young plant, two or three shoots are enough.

ART does something else with the young plants: when planting them out in March, the tips are pinched out so they branch rather than flower straight away. The two practices do not contradict each other: pinching strengthens a young plant in its first year, thinning concentrates the vigour of an established clump.

Stake early. Above 1.2 m, a delphinium needs support. Push in at least three stakes around the clump, or put a metal grid in place before the shoots reach 30 cm (RHS, n.d.). For very tall spikes, use a cane the final height of the flower spike, tied with soft twine. A June thunderstorm can flatten an unstaked group in an hour: ART notes it in the video, on the eve of a forecast storm.

Cut back to flower again. Cut faded spikes down to ground level: the plant often produces a second flush in late summer or autumn (RHS, n.d.). After the first frosts, remove the old leaves.

Cuttings and division: to keep a variety

Seed brings surprises. To reproduce a variety exactly, there are two methods (RHS, n.d.):

  • Basal cuttings, in late spring: take pencil-thick shoots from the base of the clump, 8 to 10 cm long, still solid (not hollow). Root them in cuttings compost at about 15 °C; they root in around ten days. Plant out in early summer. This works for the Elatum and Belladonna groups.
  • Division, in early spring, every two to five years: cut the clump into two to four pieces, discarding the woody centre. Divisions often flower the same year.

Slugs, mildew and other troubles

  • Slugs and snails: problem number one according to the RHS. They devour young shoots in spring. In ART's video, several second-year plants are eaten at the edge of the bed, while one plant surrounded by phacelia survives. Evening hand-picking, traps, pellets approved for organic growing (ferric phosphate): do not leave young shoots unwatched.
  • Powdery mildew: white felt on the leaves, made worse by shade and drought. Water at the base in dry weather and space the plants.
  • Black blotch (delphinium black blotch, a bacterial disease): brown to black patches on the leaves; remove affected leaves.
  • Caterpillars and leaf miners: occasional, rarely serious.
  • Viruses: discoloured or distorted leaves. Pull the plant out, it will not recover.

ART also observes forgotten seed pots flowering in September after a heatwave, and explains it by stress: a plant that suffers "starts to flower". It is a plausible garden observation (many plants speed up their cycle under difficult conditions), but we found no study demonstrating it for delphiniums.

A beautiful poisonous plant

All parts of the delphinium are poisonous, and handling the leaves or seed may irritate the skin: the RHS advises wearing gloves (RHS, n.d.). Its toxicity comes from norditerpenoid alkaloids, including methyllycaconitine, which block transmission between nerves and muscles. In the western United States, wild larkspurs are a major cause of cattle deaths on mountain rangelands (Welch et al., 2015).

In the garden, the real risk is low: the plant is bitter and nobody eats it by accident. But do not plant it where young children play, keep it out of animal hay, and wash your hands after pruning. In case of ingestion, call a poison centre (Belgium: 070 245 245).

How long does it take from sowing a perennial delphinium to its first real flowering?

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Answer: About 18 months

According to the RHS, a delphinium sown in spring flowers about 18 months later. Sown in August at harvest, it flowers the following summer.

Does delphinium seed need total darkness to germinate?

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Answer: No, light makes little difference; freshness and temperature matter more

In a trial by Terry Dowdeswell, seed germinated at 85-88 % in light and in a cupboard alike. Fresh seed and cool temperatures (13-15 °C) make the difference.

What should you do after a delphinium's first flowering to get a second flush?

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Answer: Cut the faded spikes down to ground level

The RHS advises cutting faded spikes at ground level: the plant often flowers again in late summer or autumn.

Sources

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