Ornamental · Ranunculaceae
Delphinium
Delphinium elatum
Tall spires of true blue that reward staking, steady moisture, and a careful hand around children and pets.
Full sun
Every 2–4 days
USDA 3–7
3–7 ft · 0.9–2.1 m
Overview
Grow a delphinium and watch it tower over almost everything else in the border. The flower spikes rocket to 6 feet (180 cm) or more in blues, purples, whites, and pinks that pull every eye in the garden. This buttercup-family perennial thrives where summers stay cool, so it really hits its stride in cottage and country plantings through the cooler zones.
It asks for a bit of work, and it pays that back in spades. Delphiniums are short-lived perennials that run 3 to 5 years, and the tall hollow stems will snap in wind and rain unless you stake them early, so get ahead of that. Feed them well, dig in rich, moist, well-drained ground, keep the roots cool, and they will deliver one of the most thrilling shows of the early summer garden.
Plant it with one big caveat in mind: every part of a delphinium is poisonous. The very diterpenoid alkaloids that make larkspur a killer of range cattle run through the garden forms too, loaded heaviest in the young growth and seeds. Pick a spot well out of reach of children, pets, and grazing animals, and then enjoy the climb.
Care
The short version: full sun and cool, rich, evenly moist soil, sturdy staking set early, and a balanced feed driving it through the growing season.
Light
Pour on full sun for the strongest stems and the boldest bloom, with a touch of afternoon shade where summers run hot. Leave them in heavy shade and they stretch, flop, and grab mildew far more readily, so bright is the way to go.
Water
Keep that soil cool and evenly moist and never let it dry out while the plant is charging through growth. Water at the base every 2 to 4 days in warm spells, since soggy ground and wet leaves both throw the door open to disease.
Soil
Dig in fertile, well-drained ground loaded with organic matter so it holds moisture but never turns to muck. Better still, delphiniums shrug off pH worries and run happily from 6.5 to 8.0, even leaning alkaline.
Temperature
These plants laugh off brutal cold, surviving winters down to -40°F (-40°C), but heat trips them up. Growth and flowering stall once summers blast past the mid-80s, near 84°F (29°C), which is why they chase cool-summer regions.
Humidity
Plain outdoor humidity covers them, so skip chasing a number. Airflow is what really counts, since still, muggy air around crowded plants is what fires up powdery mildew and rot.
Feeding
Delphiniums eat like hungry plants, throwing up growth at a clip. Feed a balanced fertilizer every month through the growing season to power that thick foliage and those soaring flower spikes.
Pruning
Stake the hollow stems early, before wind and rain can fold them flat. Deadhead the spent spikes back to the basal leaves to spark a second flush, then cut the whole plant back once the main show winds down.
Size & spacing
Plants surge to 3 to 7 feet (91 to 213 cm) tall in bloom, so hand each one 2 to 3 feet (61 to 91 cm) of room. That spacing keeps air ripping through and beats back disease on these mildew-prone plants.
When to plant
Delphiniums race on a cool-season clock. Start seed indoors in late winter, push young plants outside in spring once hard frost clears, and the spires launch from early to midsummer. Trim the spent spikes and they often fire off a second round in late summer.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: Late winter to early spring, 8 to 10 weeks before last frost
- Plant: Spring after danger of hard frost passes
- Harvest: Early to midsummer, sometimes reblooming in late summer if deadheaded
Hardiness
USDA hardiness zones: 3–7
Temperature: down to -40°C
Troubleshooting
Most delphinium trouble comes straight from wet soil, weak stems, or pests swarming the soft new growth. Spot the early signs and you can jump in fast, usually before a plant is lost. Find the match below and act while it still turns things around.
Plant keeling over as its base turns black and rots
A plant that suddenly flops with a blackened base has crown rot, which strikes when the soil stays wet or the crown gets buried too deep. Dig out and destroy the worst plants quick, before it runs through the bed. Then fix the drainage and replant with the crown sitting right at the soil line, never below it.
Fix:
- Remove and destroy severely affected plants
- Improve soil drainage
- Avoid planting the crown too deeply
White powdery dust creeping over stems and foliage
That dusty white coat is powdery mildew, a fungus that pounces when plants sit crowded, shaded, or starved of moving air, and it tends to break out late in the season as nights cool. Open up the spacing and grow them in full sun to push back. Then strip off the worst-coated leaves to slow it down fast.
Fix:
- Improve air circulation and spacing
- Grow in full sun, which improves resistance
- Remove and discard affected foliage
Towering bloom spikes that kink, topple, or snap off
The stems are hollow, so one strong gust or a heavy downpour can fold or snap them, especially out in the open. Stake each plant early, before the spikes shoot up and turn top-heavy. Drop delphiniums into a sheltered corner and you knock most of the wind right out of the problem.
Fix:
- Stake plants early in the season
- Site in a location sheltered from strong wind
Warped new shoots, a sticky film, and leaves full of holes
Several pests pile onto delphiniums at once. Aphids drain the new shoots and leave a sticky film, slugs and snails rip ragged holes and trail slime, and leaf miners and mites toss in their own damage. Hand-pick the slugs after dark, blast or treat the aphids, and yank any leaves the miners have tunneled.
Fix:
- Hand-pick slugs and snails
- Wash off aphids or treat as needed
- Remove mined leaves
Toxicity
Here is the part to take dead seriously: treat the whole plant as poisonous. Every part packs diterpenoid alkaloids that block the nerves driving the muscles, and the young shoots and seeds carry the heaviest dose. It can sicken or kill cats, dogs, horses, rabbits, birds, and livestock, and it threatens people too, so keep it well away from children, pets, and grazing animals.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Severe
A cat that bites any part can drool hard, quit eating, and turn constipated, then slide into muscle tremors, stiffness, and weakness. Push it further and the cat may collapse and convulse, with death following from heart or breathing failure.
Dogs Severe
A dog usually starts with drooling, colic, and constipation, then trembling muscles, stiff movement, and a wobbly, weak gait. Larger doses can throw the dog into convulsions and end in fatal cardiac or respiratory collapse.
Horses Severe
Horses run the classic neuromuscular script of muscle tremors, weakness, and dropping to the ground. They rarely eat enough to be poisoned unless pasture runs thin and they turn to the plant.
Rabbits Severe
The same paralysing alkaloids charge through every part of the plant, so a rabbit should never get to graze it. Even a small bite can spark tremors and weakness, so keep it well clear of the hutch and run.
Livestock Severe
Cattle take the hardest hit, and larkspur kills more range cattle in the western states than almost any other plant. Affected animals stagger, lose coordination, tremble, and bloat, then drop with depressed breathing, and many die.
Birds Severe
The toxic alkaloids charge throughout the plant, so seeds and foliage spell real danger for birds. Keep pet and backyard birds away from every part of it.
People Severe
No part of this plant is food, and the young shoots and seeds carry the heaviest dose. Eat it and you invite a burning mouth, nausea, a slowed and irregular heartbeat, and muscle paralysis, so keep children well away.
Keep plants and especially seeds away from children and pets; keep grazing livestock off ranges where larkspur grows, particularly in spring and early summer when alkaloid levels are highest.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
Poison Control: (800) 222-1222
Canada
Poison Centres: 1-844-764-7669
Varieties
Most garden delphiniums spring from a handful of well-known strains, split mainly by height and stem strength. These are the names worth chasing down.
Pacific Giant (Pacific Hybrids) series
These classic tall border hybrids climb to about 5 to 6 feet and load up dense flower spikes in blue, purple, white, and pink.
Aurora series
A more compact, sturdier strain bred for stronger stems and dependable rebloom, and it comes in blue and white.
Summer Blues
An especially tall pick that can shoot up to around 7 feet.
How to propagate
Multiply delphiniums two ways, by seed or by splitting a clump you already grow, though neither moves fast and named hybrids only stay true when you divide them.
SeedModerate
Start indoors in late winter to early spring; fresh seed germinates best · Plants typically flower the first or second season after sowing
Sow seed indoors in late winter to early spring, and grab the freshest seed going, since it burns off its vigor fast. Most plants flower in their first or second season, which keeps things moving. Just note that hybrids rarely come true from seed, so keep named cultivars rolling by division instead.
DivisionModerate
Early spring as new growth emerges · One season to re-establish
Lift and split mature clumps in early spring, right as the new shoots punch up, and handle those brittle crowns with care. Each division needs about a season to dig back in. Splitting every few years also recharges a plant that is naturally short-lived, lasting only 3 to 5 years.
Myths
Larkspur and perennial delphinium are the same plant, so you can swap one for the other.
Not so. Perennial delphinium covers Delphinium elatum and its hybrids, while annual larkspur sits in a separate genus, Consolida, and the two grow with a different habit and life cycle. Both still carry the same dangerous alkaloids, so neither gets a free pass.
The worst delphinium can do is a short bout of tummy trouble, nothing genuinely dangerous.
Far from it. The plant is loaded with potent diterpenoid alkaloids that paralyse the muscles and can kill. It is a notorious killer of cattle on western rangelands, which puts it miles past a passing tummy ache.