Ornamental · Ranunculaceae
Delphinium
Delphinium elatum
Tall spires of true blue on hollow stems, asking for 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
A delphinium in flower is mostly vertical line and dense color. Each stem ends in a tall flower spike that climbs to 6 feet (180 cm) or more, packed with bloom in blue, purple, white, and pink, the blues running to a clear, saturated tone few garden plants match. It is a member of the buttercup family, and below the towering spikes sits a mound of basal foliage from which the flowering stems rise.
Look closer at those stems and the plant's main weakness shows itself. Delphiniums are short-lived perennials that last only 3 to 5 years, and the tall stems are hollow, which is why they fold or snap in wind and rain without early staking. The plants are heavy feeders that want rich, moist, well-drained ground and a cool root run, building the bulk of leaf and stem that supports those weighty spikes.
The whole of that lush green and bright color is poisonous. Every part carries the same diterpenoid alkaloids that make wild larkspur deadly to range cattle, concentrated most in the young shoots and the seeds. Site it where children, pets, and grazing animals cannot reach any part of it.
Care
The short version: strong full sun, ground that stays cool, fertile, and steadily damp, stakes driven in before the hollow stems stretch up, plus a balanced monthly feed while the plant is growing.
Light
Give delphiniums full sun to build the thickest, most upright stems and the fullest spikes of bloom, with a little afternoon shade where summers run warm. In too much shade the stems thin and lengthen, leaning and flopping, and mildew settles on the leaves more readily.
Water
Keep the soil cool and evenly moist so the plant never dries out while it is putting on growth. Water at the base every 2 to 4 days in warm spells, since soggy ground and water sitting on the leaves both open the way to disease.
Soil
Plant in fertile, well-drained ground worked full of organic matter, which holds moisture around the roots without staying waterlogged. The plant is easy about pH and grows across 6.5 to 8.0, even where the soil runs alkaline.
Temperature
The crown is remarkably cold-hardy, surviving down to -40°F (-40°C), while the top growth resents real heat. Stems and spikes slow and fade once summers climb past the mid-80s, near 84°F (29°C), so the plant favors cool-summer regions.
Humidity
Plain outdoor humidity is enough, so there is no figure to chase. Moving air matters more, since still, muggy air settling among crowded leaves and stems is what brings on powdery mildew and rot.
Feeding
Delphiniums are hungry plants that throw up a lot of leaf and stem fast. A balanced fertilizer each month through the growing season feeds that dense green growth and the tall flower spikes it has to carry.
Pruning
Stake the hollow stems early, before wind and rain can crease or break them. Cut the spent spikes back to the low basal leaves to draw a second flush, then cut the whole plant down once the main flowering is finished.
Size & spacing
In bloom the spikes stand 3 to 7 feet (91 to 213 cm) tall, so each plant wants 2 to 3 feet (61 to 91 cm) of room. That gap lets air pass between the leafy clumps and holds down disease on these mildew-prone plants.
When to plant
Delphiniums follow a cool-season rhythm. Seed starts indoors in late winter, young plants go out in spring once hard frost passes, and the spikes open from early to midsummer. Cut the spent spikes back and a second, often shorter set of spikes frequently rises 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 shows up first on the plant itself: a film on the leaves, a darkening at the base, or a leaning, broken stem. The early signs are easy to read up close, and catching them usually saves the plant. Match what you see below, then act while it still helps.
Stems collapsing above a base gone soft, dark, and rotten
When the crown and stem base turn soft, dark, and decayed and the plant wilts above, the cause is crown rot, which sets in where the soil stays wet or the crown was buried too deep. Dig out and destroy the worst plants before it creeps further. 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
A floury white bloom coating leaf surfaces and stems
A pale, dusty coating spreading over the upper leaf surfaces is powdery mildew, a fungus that takes hold when plants stand crowded, shaded, or short on moving air, and it usually shows late in the season as nights cool. Wider spacing and full sun build resistance. Stripping off the worst-coated leaves slows the white film from spreading.
Fix:
- Improve air circulation and spacing
- Grow in full sun, which improves resistance
- Remove and discard affected foliage
Head-high flower spires folding, sagging, or snapping mid-stem
Because the stems are hollow, a strong gust or a heavy rain can crease, fold, or break the tall spikes, especially out in the open. Stake each plant early, before the spikes shoot up and grow top-heavy. A sheltered corner shields the stems and takes much of the wind out of the problem.
Fix:
- Stake plants early in the season
- Site in a location sheltered from strong wind
Puckered fresh growth, a tacky sheen, and tattered, hole-riddled leaves
Several pests feed on delphiniums at once, each leaving its own mark. Aphids cluster on the soft new shoots and coat them in a sticky film, slugs and snails rasp ragged holes and trail slime, and leaf miners tunnel pale trails while mites stipple the leaves. Hand-pick the slugs after dark, wash or treat the aphids, and pull any leaves the miners have mined.
Fix:
- Hand-pick slugs and snails
- Wash off aphids or treat as needed
- Remove mined leaves
Toxicity
Treat every part of this plant as poisonous, from the young shoots to the seeds. All of it holds diterpenoid alkaloids that block the nerves controlling the muscles, and the concentration runs highest in the new growth and the seed. It can sicken or kill cats, dogs, horses, rabbits, birds, and livestock, and it is dangerous to people, 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 into any part can drool heavily, refuse food, and grow constipated, then show muscle tremors, stiffness, and weakness. In a serious case the cat collapses and convulses, and death can follow from heart or breathing failure.
Dogs Severe
A dog first drools, colics, and turns constipated, then develops trembling muscles, stiff movement, and a wobbling, weak gait. Larger doses can bring on convulsions and end in fatal cardiac or respiratory collapse.
Horses Severe
Horses show the neuromuscular signs of muscle tremors, weakness, and finally lying down on the ground. They seldom eat enough to be poisoned unless pasture grows short and they begin to take the plant.
Rabbits Severe
The same paralysing alkaloids run through every leaf, stem, and seed, so a rabbit should never graze it. Even a small bite can set off tremors and weakness, so keep it well clear of the hutch and run.
Livestock Severe
Cattle suffer the most, 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 reach through the whole plant, seeds and foliage alike, making both 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 hold the heaviest dose of toxin. Eating it can bring 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 trace to a few well-known strains, told apart mainly by height and by how sturdy their stems are. These are the ones worth knowing by name.
Pacific Giant (Pacific Hybrids) series
These classic tall border hybrids rise to about 5 to 6 feet, their stems crowded with dense spikes of blue, purple, white, and pink.
Aurora series
A more compact, sturdier strain carried on thicker, stronger stems, with reliable rebloom and flowers in blue and white.
Summer Blues
An especially tall selection whose spikes can stretch up to around 7 feet.
How to propagate
Delphiniums can be raised from seed or split from an existing clump, though neither is quick and named hybrids stay true only through division.
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, choosing the freshest seed possible, since it loses its vigor quickly. Most plants flower in their first or second season. Hybrids rarely come true from seed, though, so named cultivars are best carried on by dividing the crown rather than sowing.
DivisionModerate
Early spring as new growth emerges · One season to re-establish
Lift and split mature clumps in early spring as the new shoots push up through the soil, handling the brittle crowns gently so they do not break apart. Each division takes about a season to settle back in. Dividing every few years also renews a plant that is naturally short-lived, lasting only 3 to 5 years.
Myths
Larkspur and perennial delphinium are the same plant, so one can stand in for the other.
They are distinct plants. Perennial delphinium covers Delphinium elatum and its hybrids, while annual larkspur belongs to a separate genus, Consolida, and the two grow with a different habit and life cycle. Both carry the same dangerous alkaloids, whatever they look like.
Delphinium is only mildly toxic, more a stomach upset than a real danger.
It is far more dangerous than that. 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.