Delphinium
Delphinium elatum
Stately blue spire for the back of the border — a tall draw for bumblebees.
- Type
- Flower · Perennial
- Botanical family
- Mixed / other
- Foliage color
- Green
- Flower color
- Blue
- Sunlight
- Full sun
- Watering
- Moderate
- Soil
- Rich, deep, well-drained
- Feeding
- A feed in spring, backed up with a liquid feed every two weeks.
- USDA zones
- 3a–7b
- Frost tolerant
- Yes
- Flowering
- June
- Pruning
- Cut it back after the first flush and it will often flower again.
- Sow indoors
- 56 days before the last spring frost
- Plant out
- at the last spring frost
- First harvest
- 60 days after the last spring frost
- Harvest ends
- 45 days before the first fall frost
- Mar–Jun Spring Growth
- Jun–Jul Flowering
- Jul–Oct Growing
- Oct–Dec Leaf Fall
- Jan–Mar Leaf Fall
- Plant spacing
- 19.7 in
- In-row spacing
- 17.7 in
- Between rows
- 23.6 in
- Square-foot density
- 1 per square
- Root system
- Fibrous
- Working depth
- 12–24 in
- Can reach
- 3 ft down where the soil allows
- Spread
- 24 in–3 ft across, about 1–1.5× the width of the plant
- Shallowest pot
- 12 in of soil
A dense mass of roots through the topsoil with a few running deeper after water. Work the bed to a spade’s depth before planting and it will find everything it needs without much help.
A fleshy crown at the surface that slugs find in March before anything else is awake, which is the single reason most delphiniums fail. Ring it with grit and check it nightly as the shoots come.
The habit is Delphinium’s own; the figures are what that habit usually comes to at this size, not a measurement of this species. Roots follow the soil either way: a compacted pan, a high water table or heavy clay stops them well short of any of it, and deep open ground lets them go further.
- Cats and dogs
- Recorded toxic
Recorded as toxic to cats and dogs by ASPCA.
The ASPCA files this genus under the common name Larkspur, not Delphinium, at genus level, so it covers our Delphinium elatum. Young growth is the more toxic stage.
- When
- Cut in cool morning in early summer; a second smaller flush comes in early autumn if plants are cut back hard after the first.
- What to look for
- Cut when one-quarter to one-third of the florets on the spike are open and the top buds show color. All-open spikes shatter in transit — delphiniums drop florets when past.
- How to harvest
- Cut the spike low with pruners; the stems are hollow, so either fill the inverted stem with water and plug it with cotton wool, or simply stand immediately in deep water. Handle upright — like snapdragons, tips bend upward if laid flat. (Foliage is toxic; wash hands.)
- How often
- After the main spikes finish, cut the whole plant to the ground, feed and water — most delphiniums return a second flush of shorter spikes in 6–8 weeks.
- Storing the crop
- Vase life 6–8 days; commercial growers use anti-ethylene treatment because ethylene (ripening fruit!) shatters florets — keep vases away from the fruit bowl. Individual florets and whole spikes press and air-dry well. Seed-save from brown pods; sow fresh or refrigerate (seed is short-lived).
Beneficial predators that hunt this plant's pests — grow their favourite flowers nearby to recruit them. companion plant
- Beneficial nematodes Steinernema & Heterorhabditis spp.Hunts Slugs & snails
Microscopic worms watered onto damp soil that hunt down soil-dwelling grubs and release a bacterium that kills them within days. Timing is the whole thing: warm, damp soil and a small target grub, which means late summer for most of what they are used against.
- Centipedes ChilopodaHunts Slugs & snails
Flat, fast, one pair of legs to a segment — and a predator, unlike the slow round millipede it is constantly confused with. It hunts slugs, cutworms, springtails and woodlice by night under mulch and stones. Learn the difference and leave this one alone.
- Frogs & toads AnuraHunts Slugs & snails
A resident toad works the beds every night for slugs, cutworms, and weevils. Recruit them with water — even a sunken basin pond — plus damp shade and an upturned-pot toad house; skip the slug pellets that poison their prey.
- Garden birds PasseriformesHunts Slugs & snails
Both things are true: birds strip your currants and a brood of blue tits takes several thousand caterpillars out of one fruit tree in a fortnight. Net what you must and welcome them everywhere else — a nest box near the apples is worth more than a spray.
- Ground beetle CarabidaeHunts Slugs & snails
Fast, glossy night hunters that patrol the soil surface for slugs, cutworms, and root-fly larvae. Give them daytime cover — thick perennial edges, stepping stones, a log — and skip the tillage that wrecks their burrows.
- Hedgehog Erinaceus europaeusHunts Slugs & snails
A European animal, and worth saying so plainly: there are no hedgehogs in North America, and the ally that does the same night shift here is the toad, the opossum or the ground beetle. Where they do occur, cut a gap in the fence and skip the pellets.
- Rove beetle StaphylinidaeHunts Slugs & snails
Slim, quick beetles with short wing cases that hunt in soil and litter, taking root-fly eggs and larvae, fungus-gnat larvae, and mites. Mulch and undisturbed ground keep them resident year-round.
- Salamanders & newts CaudataHunts Slugs & snails
Damp-loving hunters of slugs, worms, woodlice and beetle larvae that spend the day under a log or a stone and work the beds at night. Companions to the toads: they want the same damp shade, the same log pile, and the same absence of slug pellets.
- Snakes & lizards Colubridae & PhrynosomatidaeHunts Slugs & snails
A garter snake works the beds for slugs and voles and a fence lizard clears grasshoppers off a sunny wall. Almost every snake a gardener meets is harmless and useful; give them a log pile, a warm flat stone and a bit of cover and they stay.
Roses, Phlox, Geraniums, Lupins
Good neighbors
Among the tallest of border flowers — but every part is toxic if eaten.
Green-leaved flowers, like this one.
The Cottage Garden Companion — the gardener's herbarium at cottagegardencompanion.com