Ornamental · Ranunculaceae
Delphinium
Delphinium elatum
Tall spires of true blue that demand 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
Few border plants stand as tall as a delphinium in full bloom. The flower spikes climb to 6 feet (180 cm) or more in shades of blue, purple, white, and pink that are hard to match anywhere else in the garden. It belongs to the buttercup family and thrives in cool summers, which is why it does its best work in cottage and country gardens through the cooler zones.
This is a plant for a gardener willing to fuss a little. Delphiniums are short-lived perennials, often lasting only 3 to 5 years, and the tall hollow stems snap in wind and rain unless you stake them early. They are heavy feeders that want rich, moist, well-drained ground and a cool root run, and they reward that care with one of the most striking displays in the early summer border.
One thing to respect before you plant it: every part of the delphinium is poisonous. The same diterpenoid alkaloids that make larkspur a killer of range cattle are present in the garden forms too, strongest in the young growth and seeds. Site it where children, pets, and grazing animals can't get at it.
Care
The short version: full sun and cool, rich, evenly moist soil, with sturdy staking set early and a balanced feed through the growing season.
Light
Give delphiniums full sun for the strongest stems and the best bloom, though they take a little afternoon shade where summers run warm. Too much shade leaves them stretched, floppy, and far more prone to mildew.
Water
Keep the soil cool and evenly moist and never let it dry out while the plant is growing hard. Water at the base every 2 to 4 days in warm spells, since soggy ground and wet leaves both invite disease.
Soil
Plant in fertile, well-drained ground worked full of organic matter, which holds moisture without staying waterlogged. Delphiniums are easy about pH and handle anything from 6.5 to 8.0, even on the alkaline side.
Temperature
These are cold-hardy plants that survive winters down to -40°F (-40°C) but resent real heat. Growth and flowering fade once summers push past the mid-80s, near 84°F (29°C), which is why they favor cool-summer regions.
Humidity
Ordinary outdoor humidity suits them, so there's no number to chase. Airflow matters far more, since still, muggy air around crowded plants is what brings on powdery mildew and rot.
Feeding
Delphiniums are hungry plants that put on a lot of growth fast. Feed a balanced fertilizer monthly through the growing season to fuel the heavy foliage and those tall flower spikes.
Pruning
Stake the hollow stems early, before wind and rain can fold them. Deadhead the spent spikes back to the basal leaves to draw a second flush, then cut the plant back once the main flowering is done.
Size & spacing
Plants reach 3 to 7 feet (91 to 213 cm) tall in bloom, so give each one 2 to 3 feet (61 to 91 cm) of room. That spacing keeps air moving and holds disease down on these mildew-prone plants.
When to plant
Delphiniums run on a cool-season rhythm. Start seed indoors in late winter, set young plants out in spring once hard frost has passed, and the spires open from early to midsummer, often coming again in late summer if you cut back the spent spikes.
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 traces back to wet soil, weak stems, or pests on the soft new growth. Catch the early signs and you can usually step in before a plant is lost. Match what you see below, then act while it still helps.
Wilting plant with a blackened, rotting base
Crown rot sets in when the soil stays wet or the crown is buried too deep, and a healthy-looking plant can collapse fast once it takes hold. Dig out and destroy the worst plants before it spreads. Fix the drainage and set new plants with the crown right at the soil line, never below it.
Fix:
- Remove and destroy severely affected plants
- Improve soil drainage
- Avoid planting the crown too deeply
Dusty white film over the leaves and stems
This is powdery mildew, a fungus that settles in when plants are crowded, shaded, or short on moving air. It tends to show up late in the season once nights cool. Space the plants well, grow them in full sun, and strip 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
Tall flower spikes that bend, flop, or snap
The stems are hollow, so a strong gust or a heavy rain can fold or break them, especially in an open spot. Stake each plant early, before the spikes shoot up and get top-heavy. Tucking delphiniums into a sheltered corner takes most of the wind out of the problem.
Fix:
- Stake plants early in the season
- Site in a location sheltered from strong wind
Twisted new growth, sticky leaves, and ragged holes
Several pests work on delphiniums at once. Aphids suck the new shoots and leave a sticky film, while slugs and snails chew ragged holes and trail slime, and leaf miners and mites add their own damage. Hand-pick the slugs after dark, wash or treat the aphids, and pull any leaves tunneled by miners.
Fix:
- Hand-pick slugs and snails
- Wash off aphids or treat as needed
- Remove mined leaves
Toxicity
Treat the whole plant as poisonous, because it is. Every part holds diterpenoid alkaloids that block the nerves controlling 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 is dangerous to 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 nibbles any part of the plant can drool heavily, go off its food, and grow constipated before muscle tremors, stiffness, and weakness set in. In a serious case the cat may collapse and convulse, and death can follow from heart or breathing failure.
Dogs Severe
Watch a dog for drooling, colic, and constipation, then trembling muscles, stiff movement, and a wobbling, weak gait. Larger doses can drop the dog into convulsions and lead to fatal cardiac or respiratory collapse.
Horses Severe
Horses show the classic neuromuscular picture of muscle tremors, weakness, and going down on the ground. They rarely eat enough to be poisoned unless their pasture runs short and they turn to the plant.
Rabbits Severe
The same paralysing alkaloids run through every part of the plant, so a rabbit should never be allowed to graze it. Even a small amount can trigger tremors and weakness, so keep it well clear of the hutch and run.
Livestock Severe
Cattle take the worst of it, and larkspur kills more range cattle in the western states than almost any other plant. Affected animals stagger, lose coordination, tremble, and bloat, then go down with depressed breathing, and many die.
Birds Severe
The toxic alkaloids are present throughout 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 of toxin. Eating it can bring on 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 come from a few well-known strains, sorted mainly by height and how sturdy their stems are. These are the ones worth knowing by name.
Pacific Giant (Pacific Hybrids) series
These classic tall border hybrids reach about 5 to 6 feet and carry dense flower spikes in blue, purple, white, and pink.
Aurora series
A more compact, sturdier strain bred for stronger stems and reliable rebloom, sold in blue and white.
Summer Blues
An especially tall selection that can stretch up to around 7 feet.
How to propagate
You can raise delphiniums from seed or split an existing clump, though neither is quick 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 reach for the freshest seed you can find, since it loses its punch fast. Plants usually flower in their first or second season. Just know that hybrids rarely come true from seed, so named cultivars are best kept going by division rather than seed.
DivisionModerate
Early spring as new growth emerges · One season to re-establish
Lift and split mature clumps in early spring, just as the new shoots push up, and handle the brittle crowns with care. Each division takes about a season to settle back in. Dividing every few years also refreshes a plant that is naturally short-lived, lasting only 3 to 5 years.
Myths
Larkspur and perennial delphinium are the same plant and you can swap one for the other.
Perennial delphinium covers Delphinium elatum and its hybrids, while annual larkspur belongs to a separate genus, Consolida. They grow with a different habit and life cycle, though both carry the same dangerous alkaloids.
Delphinium is only mildly toxic, more of a stomach upset than a real danger.
The plant holds potent diterpenoid alkaloids that paralyse the muscles and can kill. It is a notorious killer of cattle on western rangelands, not a passing tummy ache.