Ornamental · Ranunculaceae
Delphinium
Delphinium elatum
Tall spires of true blue that want 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
Want serious height at the back of the border? A delphinium in bloom is hard to beat. The spikes shoot up to 6 feet (180 cm) or more in blues, purples, whites, and pinks you just don't see on much else. It's part of the buttercup family, and since it loves a cool summer, it's right at home in cottage and country gardens through the cooler zones.
Here's the deal, though: it does ask for a little fussing. Delphiniums are short-lived perennials that usually stick around just 3 to 5 years, and those tall hollow stems will snap in wind or rain if you don't stake them early. They're big eaters too, so they want rich, moist, well-drained soil and a cool root run, and in return they put on one of the best shows in the early summer garden.
One thing you really can't skip: every part of a delphinium is poisonous. It's the same diterpenoid alkaloids that make larkspur deadly to range cattle, and the garden types have them too, strongest in the young growth and seeds. So pick a spot where kids, pets, and grazing animals simply can't get to it.
Care
The short version: full sun and cool, rich, evenly moist soil, sturdy stakes put in early, and a balanced feed all through the growing season.
Light
Delphiniums want full sun for the sturdiest stems and the best flowers, but they'll take a bit of afternoon shade where summers get warm. Give them too much shade, though, and they'll stretch, flop, and catch mildew way more easily.
Water
Keep that soil cool and evenly moist, and don't let it dry out while the plant's growing hard. Water at the base every 2 to 4 days when it's warm, because soggy ground and wet leaves both roll out the welcome mat for disease.
Soil
Go for fertile, well-drained ground worked full of organic matter, so it stays moist without turning into a swamp. The nice thing? They're not fussy about pH and handle anything from 6.5 to 8.0, even on the alkaline side.
Temperature
These are tough as nails in the cold, shrugging off winters down to -40°F (-40°C), but real heat's another story. Growth and flowering taper off once summers climb past the mid-80s, around 84°F (29°C), which is why they do best where summers stay cool.
Humidity
Plain old outdoor humidity is fine, so there's no number to chase here. Airflow's the thing that matters, since still, muggy air around crowded plants is what kicks off powdery mildew and rot.
Feeding
Delphiniums are hungry plants that put on growth in a hurry. Feed a balanced fertilizer once a month through the growing season and you'll keep all that leaf and those tall spikes well fed.
Pruning
Get the hollow stems staked early, before wind and rain can fold them over. Once flowers fade, cut the spent spikes back to the basal leaves to nudge a second flush, then cut the whole plant back when the main bloom's done.
Size & spacing
Plants hit 3 to 7 feet (91 to 213 cm) tall in bloom, so give each one 2 to 3 feet (61 to 91 cm) of elbow room. That spacing keeps the air moving and holds disease down on these mildew-prone plants.
When to plant
Delphiniums work on a cool-season clock. Start seed indoors in late winter, move the young plants outside in spring once the hard frost's gone, and you'll get spires from early to midsummer. Snip off the spent spikes and they'll often give you 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
When a delphinium acts up, it's usually down to wet soil, weak stems, or pests piling onto the soft new growth. The good part? The early signs are easy to spot, so you can usually jump in before you lose a plant. Find what matches below and act while it still does some good.
Droopy plant that's going black and mushy at the base
If a healthy-looking plant suddenly wilts and goes black at the base, that's crown rot, and it shows up when the soil stays wet or the crown got buried too deep. Dig out and toss the worst plants fast so it doesn't spread. 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
Leaves and stems wearing a powdery white dusting
That dusty look is powdery mildew, a fungus that moves in when plants are crowded, shaded, or short on moving air, and it usually turns up late in the season once nights get cool. Don't sweat it too much. Space the plants out, grow them in full sun, and pull off the worst-coated leaves to slow it down.
Fix:
- Improve air circulation and spacing
- Grow in full sun, which improves resistance
- Remove and discard affected foliage
Flower spikes leaning over, flopping, or breaking off
Why do they flop? The stems are hollow, so one strong gust or a heavy rain can fold or break them, especially out in the open. Stake each plant early, before the spikes shoot up and get top-heavy. Tucking delphiniums into a sheltered corner takes a lot of the wind out of the problem too.
Fix:
- Stake plants early in the season
- Site in a location sheltered from strong wind
Curled new shoots, tacky leaves, and holes chewed all over
A few pests like to work delphiniums all at once. Aphids suck on the new shoots and leave a sticky film, slugs and snails chew ragged holes and trail slime, and leaf miners and mites pitch in their own damage. Pick the slugs off after dark, wash or treat the aphids, and pull any leaves the miners have tunneled.
Fix:
- Hand-pick slugs and snails
- Wash off aphids or treat as needed
- Remove mined leaves
Toxicity
Let's be clear about this one: treat the whole plant as poisonous, because it is. Every part holds diterpenoid alkaloids that mess with the nerves running the muscles, and the young shoots and seeds pack the most. It can sicken or kill cats, dogs, horses, rabbits, birds, and livestock, and it's dangerous to people too, so keep it well away from kids, pets, and grazing animals.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Severe
A cat that nibbles any part can start drooling hard, lose interest in food, and get constipated, then move into muscle tremors, stiffness, and weakness. If it's bad, the cat might collapse and convulse, and death can follow from heart or breathing failure.
Dogs Severe
With a dog, watch for drooling, colic, and constipation first, then trembling muscles, stiff movement, and a wobbly, weak walk. Bigger doses can tip the dog into convulsions and end in fatal heart or breathing failure.
Horses Severe
Horses show the usual neuromuscular picture: muscle tremors, weakness, and finally going down on the ground. They hardly ever eat enough to get poisoned unless the pasture runs thin and they turn to the plant.
Rabbits Severe
Those same paralysing alkaloids are in every part of the plant, so a rabbit should never get to graze it. Even a little can trigger tremors and weakness, so keep it well clear of the hutch and run.
Livestock Severe
Cattle catch the worst of it, and larkspur kills more range cattle in the western states than just about any other plant. Affected animals stagger, lose coordination, tremble, and bloat, then go down with labored breathing, and a lot of them die.
Birds Severe
The toxic alkaloids run all through the plant, so seeds and foliage are a real danger to birds. Keep pet and backyard birds away from any 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're looking at a burning mouth, nausea, a slowed and irregular heartbeat, and muscle paralysis, so keep the kids 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 come from just a few well-known strains, sorted mostly by how tall they get and how strong their stems are. Here are the ones worth knowing by name.
Pacific Giant (Pacific Hybrids) series
These classic tall border hybrids get to about 5 to 6 feet and pack dense flower spikes in blue, purple, white, and pink.
Aurora series
This one's a more compact, sturdier strain bred for stronger stems and dependable rebloom, and you'll find it in blue and white.
Summer Blues
An especially tall pick that can stretch up to around 7 feet.
How to propagate
Want more delphiniums? You've got two options, seed or splitting a clump you already have, though neither's fast and named hybrids only stay true if 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 you can, because it loses its punch fast. Most plants flower in their first or second season. One catch, though: hybrids hardly ever come true from seed, so it's smarter to keep named cultivars going 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 push up, and go easy on those brittle crowns. Each division takes about a season to settle back in. Splitting every few years has a bonus too, since it freshens up a plant that's naturally short-lived and only lasts 3 to 5 years.
Myths
Larkspur and perennial delphinium are the same plant, so you can swap one for the other.
Not really. Perennial delphinium covers Delphinium elatum and its hybrids, while annual larkspur is a whole different genus called Consolida, and they grow with a different habit and life cycle. Both carry the same dangerous alkaloids, though, so don't let one feel safer than the other.
Delphinium isn't all that poisonous, it'll just give you a bit of a stomach ache at worst.
It's a lot more than that. The plant's loaded with potent diterpenoid alkaloids that paralyse the muscles and can kill. It's a notorious killer of cattle on western rangelands, which is a far cry from a passing tummy ache.