Ornamental · Ranunculaceae
Delphinium
Delphinium elatum
Tall spires of true blue that bring height and old-world romance, asking for staking, steady moisture, and care around children and pets.
Full sun
Every 2–4 days
USDA 3–7
3–7 ft · 0.9–2.1 m
Overview
Few plants give a garden the vertical drama of a delphinium at its peak. Rising to 6 feet (180 cm) or more in blue, purple, white, and pink, it draws the eye upward and gives a border the kind of architecture a designer plans around. A member of the buttercup family that thrives where summers stay cool, it feels made for the cottage and country setting, the perfect partner to a brick wall, a picket fence, or a softly planted path.
It does, however, suit the gardener who enjoys the ritual of tending. As short-lived perennials that often grace a garden for just 3 to 5 years, delphiniums fold neatly into a planting that's replanted and reimagined over time. The tall hollow stems want staking before storms arrive, and the plants are generous feeders happiest in rich, moist, well-drained ground, repaying that attention with one of the most romantic displays of the early summer season.
Style it with intention, because every part of a delphinium is poisonous. The same diterpenoid alkaloids that make larkspur a hazard to range cattle live in the garden forms too, strongest in young growth and seeds. In a home shared with children, pets, or grazing animals, place it where it can be admired from a distance rather than touched.
Care
The short version: full sun and cool, rich, evenly moist soil, with sturdy staking set early and a balanced feed carried through the growing season.
Light
Plant delphiniums in full sun to keep the stems upright and the bloom at its fullest, with a little afternoon shade welcome where summers turn warm. In deeper shade they stretch and lean, losing the poised vertical line that makes them so striking in a border.
Water
Keep the soil cool and evenly moist, never letting it dry out while the plant is in active growth. Water at the base every 2 to 4 days in warm weather, since both soggy ground and wet foliage invite the diseases that spoil the display.
Soil
Give them fertile, well-drained ground enriched with organic matter, the sort of soil that holds moisture without staying waterlogged. They ask little of pH, content anywhere from 6.5 to 8.0, even on the alkaline side, which makes siting them refreshingly simple.
Temperature
Remarkably cold-hardy, delphiniums survive winters down to -40°F (-40°C), yet they wilt at the thought of real heat. Growth and flowering fade once summers climb past the mid-80s, near 84°F (29°C), which is why they grace cool-summer gardens most generously.
Humidity
Ordinary outdoor humidity suits them, so there is no figure to fuss over. Airflow is the real consideration, since still, muggy air around crowded plants is what brings on powdery mildew and rot.
Feeding
Delphiniums are hungry plants that build their stature quickly. A balanced fertilizer applied monthly through the growing season sustains the full foliage and those tall, statement-making flower spikes.
Pruning
Stake the hollow stems early, before wind and rain can fold the season's centerpiece. Deadhead the spent spikes back to the basal leaves to encourage a second flush, then cut the plant back once the main flowering is over.
Size & spacing
Reaching 3 to 7 feet (91 to 213 cm) tall in bloom, each plant earns 2 to 3 feet (61 to 91 cm) of room in the design. That generous spacing keeps air moving and disease at bay on these mildew-prone plants.
When to plant
Delphiniums follow the slow, pleasing tempo of the cool season. Seed goes in indoors in late winter, young plants move to the border in spring once hard frost has passed, and the spires open from early to midsummer, often returning for a second act in late summer when the spent spikes are trimmed.
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 setbacks come down to wet soil, weak stems, or pests on the soft new growth, and catching the early signs keeps a centerpiece from becoming a gap. Read what you see below, then act while a small intervention still does the most good.
Wilting plant with a blackened, rotting base
A plant that suddenly wilts with a blackened base has crown rot, which sets in where the soil stays wet or the crown sits too deep. Lift and discard the worst plants before it travels through the bed. Correct the drainage and replant with the crown right at the soil line, never below it, to keep a border whole.
Fix:
- Remove and destroy severely affected plants
- Improve soil drainage
- Avoid planting the crown too deeply
Dusty white film over the leaves and stems
A dusty pallor on the foliage is powdery mildew, a fungus that takes hold when plants stand crowded, shaded, or starved of moving air, and it tends to arrive late in the season as nights cool. Open up the planting and keep it in full sun. Strip away the worst-coated leaves to slow it and protect the look.
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 spikes lean and break because the stems are hollow, no match for a strong gust or heavy rain in an open spot. Stake each plant early, before the spikes rise and turn top-heavy, so the vertical line holds. A sheltered corner, tucked beside a wall or hedge, takes much 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 court delphiniums at once, dulling an otherwise pristine planting. Aphids feed on the new shoots and leave a sticky film, slugs and snails chew ragged holes and trail slime, and leaf miners and mites add their share. Hand-pick the slugs after dark, wash or treat the aphids, and remove any leaves the miners have tunneled.
Fix:
- Hand-pick slugs and snails
- Wash off aphids or treat as needed
- Remove mined leaves
Toxicity
Beauty and caution sit side by side here, so treat the whole plant as poisonous. Every part holds diterpenoid alkaloids that block the nerves controlling the muscles, with the heaviest dose in young shoots and seeds. 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 nibble of any part can leave a cat drooling heavily, refusing food, and constipated, with muscle tremors, stiffness, and weakness coming on after. A serious case may bring collapse and convulsions, and death can follow from heart or breathing failure.
Dogs Severe
A poisoned dog tends to drool, colic, and grow constipated first, then develop trembling muscles, stiff movement, and a wobbling, weak gait. A larger dose can bring convulsions and end in fatal cardiac or respiratory collapse.
Horses Severe
In horses the signs follow the neuromuscular pattern of muscle tremors, weakness, and finally going down on the ground. Poisoning is uncommon unless pasture grows short and the animals begin to graze the plant.
Rabbits Severe
Every part carries the same paralysing alkaloids, so grazing must never be an option for a rabbit. A small amount alone can set off tremors and weakness, which is reason enough to keep it well away from the hutch and run.
Livestock Severe
Cattle suffer most, and larkspur claims more range cattle in the western states than nearly any other plant. The animals stagger, lose coordination, tremble, and bloat, then go down with depressed breathing, and many do not recover.
Birds Severe
With the toxic alkaloids spread through the whole plant, both seeds and foliage put birds at real risk. Pet and backyard birds belong well away from any part of it.
People Severe
None of this plant is food, and its 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 children belong 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
A handful of well-known strains supply most of the delphiniums in cultivation, distinguished mainly by height and the strength of their stems. These are the names worth knowing as you plan a planting.
Pacific Giant (Pacific Hybrids) series
The classic choice for a grand border, these tall hybrids rise to about 5 to 6 feet with dense spikes in blue, purple, white, and pink.
Aurora series
For a tidier, more dependable display, this compact and sturdier strain offers stronger stems and reliable rebloom in blue and white.
Summer Blues
When height is the whole point, this especially tall selection can stretch up to around 7 feet.
How to propagate
There are two paths to more delphiniums, raising them from seed or dividing a clump already in the garden, 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 you can find, since it loses its vigor quickly. Plants usually flower in their first or second season. Bear in mind that hybrids rarely come true from seed, so a treasured named cultivar is best carried on 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, handling the brittle crowns with a gentle hand. 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 not interchangeable. 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, so neither earns a place within easy reach.
Delphinium is only mildly toxic, more a stomach upset than a real danger.
The reality is far graver. The plant holds potent diterpenoid alkaloids that paralyse the muscles and can kill. It is a notorious killer of cattle on western rangelands, not the passing tummy ache its beauty might suggest.