Ornamental · Ranunculaceae
Larkspur
Consolida ajacis
Tall blue spires that set the tone for a cool spring garden, lovely to live with yet poisonous in every part.
Full sun
Every 3–7 days
30–75°F · -1–24°C
USDA 2–11
Overview
Larkspur gives the early garden its vertical line, lifting tall spikes of blue through the cool weeks that bracket spring. An annual in the buttercup family and a close relation of the perennial delphiniums it is so often taken for, it reads as elegant and slightly old-fashioned, the plant of cottage borders and cutting patches. From June to August its spike-like racemes of spurred flowers carry the show, mostly in blue, with pink and white slipping in far less often.
Because larkspur is a cool-season hardy annual, it suits a garden that peaks in spring and early summer rather than one built for the dog days. Sown in place it grows fast, often to 2 to 4 feet, its height earning a spot at the back of a bed or in a row for the vase before heat draws the season to a close. Gardeners in cold zones sow before the last frost, while those in mild regions sow in fall for an early, effortless start.
For all the grace it lends a setting, larkspur is toxic from root to seed, and that shapes where it belongs at home. The same diterpenoid alkaloids that make it one of the deadliest plants on western cattle ranges also put pets, children, and people at risk, so it fits a garden kept out of reach of curious hands and mouths, not a play space or a pet run.
Care
The short version: give larkspur full sun, loose well-drained soil, and steady moisture, sow it early so it blooms before the heat, and deadhead to keep the spikes coming.
Light
Larkspur wants full sun to hold its spikes tall and straight, though it will accept a little partial shade in an eastern or dappled spot. Sun also keeps the air around the foliage dry, which is part of what keeps powdery mildew off.
Water
Keep the soil evenly moist but never soggy, watering roughly every 3 to 7 days as the weather and your site call for it. The roots rot in wet, waterlogged ground, so a freely draining bed matters as much as steady moisture.
Soil
Plant into loose, moderately rich, well-drained soil. Larkspur is easy about pH and takes to alkaline ground well, anywhere from 6.5 to 8.0, so long as water moves through freely.
Temperature
This is a plant of cool weather, at ease from about 30°F to 75°F (-1°C to 24°C). It carries through light frost without complaint but bows out once summer heat sets in, which is why an early start suits it.
Humidity
Ordinary outdoor humidity is all it asks, so there is no figure to chase. Air movement is the thing that matters, since still, damp air around a crowded planting is what invites powdery mildew.
Feeding
Work a balanced fertilizer into rich, organic soil at planting, and the plant is largely set. Larkspur is a light feeder, and good ground at the outset carries it through its short, quick life.
Pruning
Deadhead the spent spikes to keep the flowers coming over a longer stretch of the season. As the heat builds and the plant begins to fade, cut it back rather than leaving a tired stand to linger in the bed.
Size & spacing
Larkspur stands 2 to 4 feet (61 to 122 cm) tall on a slim footprint of 12 to 18 inches (30 to 46 cm), which makes it easy to slot into a border or a cutting row. Thin or space seedlings about 9 to 12 inches apart so air moves and the planting stays clean.
When to plant
Larkspur belongs to the cool half of the gardening year and has little patience for high summer, so its calendar sets the rhythm. Gardeners in cold climates sow indoors 8 to 10 weeks before the last frost or direct-sow in early spring, while milder regions sow outdoors in fall for a graceful head start. The aim, whichever route suits your setting, is bloom from June to August, before the heat closes the season.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: Northern climates: sow indoors 8 to 10 weeks before last spring frost
- Plant: Sow outdoors in fall in southern climates, or early spring before last frost in northern climates; space 9 to 12 inches apart
- Harvest: Blooms June to August in spike-like racemes
Hardiness
USDA hardiness zones: 2–11
Temperature: down to -46°C
Troubleshooting
Larkspur is an easy plant to live with, and its few troubles trace mostly to heat or crowding. Read the early signs on the leaves and time the sowing to the season, and you head off nearly all of them. Match what you see below, then step in while it still makes a difference to the display.
Plants wilt and fade out as summer heats up
Larkspur keeps to the cool half of the calendar as a cool-season hardy annual, and it takes its leave once real heat arrives, soonest in the deep South. A plant retiring in the heat is following its nature, not failing, and there is nothing to salvage. Plan around it instead: sow in fall or very early spring so the display peaks and closes before high summer, and offer afternoon shade where the season turns fierce.
Fix:
- Provide afternoon shade in hot-summer climates
- Sow in fall or very early spring so bloom finishes before peak heat
White powdery dust on the leaves and stems
A hazy white film reads as powdery mildew, the fungus that settles in when a planting grows too dense and the air around it goes still and damp. Give the stand more breathing room, water low at the base so the foliage stays dry, and clear away any leaf that has already picked up the bloom. Sited in full sun and open air, larkspur rarely lets it take hold.
Fix:
- Improve air circulation by thinning and spacing plants
- Avoid overhead watering
- Remove and discard affected foliage
Chewed, holed, or distorted leaves
When the foliage looks ragged, the culprits are usually aphids, slugs, snails, leaf miners, and mites. Turn a few leaves to catch clustered pests underneath, look for the shine of slime trails after dark, and note the pale winding tunnels the miners trace. Lift slugs and snails off by hand, send aphids and mites packing with a firm rinse of water, and edit out any leaf too far gone to look its best.
Fix:
- Hand-pick slugs and snails
- Wash off aphids or mites with a strong water spray
- Remove heavily infested leaves
Toxicity
For all its poise in a border, treat every part of this plant as poison. The flowers, leaves, stems, and above all the seeds carry diterpenoid alkaloids that block nerves and muscles, the very trait that makes larkspur a leading killer of range cattle. It is just as dangerous to cats, dogs, horses, rabbits, birds, and people, so it belongs only where no one, animal or human, could take a bite. Nothing on this plant is safe to eat.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Severe
For a household cat, no part of larkspur is safe, and the seeds and young growth hold the most toxin of all. The diterpenoid alkaloids work on nerve and muscle, so a poisoned cat may drool, cramp, tremble, stiffen, and weaken, and in the worst turn that gives way to convulsions, heart failure, and death when breathing stops. This is a plant for gardens closed to cats, not open ones.
Dogs Severe
A dog with the run of the border meets the same neuromuscular poison in any part it chews. The warning signs are heavy salivation, constipation, colic, muscle tremors, and a wobbly, weakening gait that can slide into recumbency, convulsions, and fatal respiratory paralysis. A veterinary visit should not wait for the picture to worsen.
Horses Severe
Horses are especially sensitive, and the alkaloids act quickly once taken in. The first signs are colic, constipation, more salivation than usual, and stiffness, moving on to recumbency and convulsions, with cardiac failure and death from respiratory paralysis in serious cases. Keep larkspur clear of any paddock or pasture edge.
Rabbits Severe
The alkaloids that fell far larger animals are no gentler on a rabbit. There is no part of the plant a rabbit can safely browse, and even a little risks tremors, weakness, paralysis, and death. Site larkspur well away from any hutch or run, and never as a planting a rabbit can reach.
Livestock Severe
On the range, larkspur stands among the deadliest plants for cattle, which graze it readily and are lost in numbers year after year. Stricken animals show muscle weakness and tremors, lose coordination, grow reluctant to walk, and go down, often with bloating, a rapid irregular heartbeat, and labored breathing that ends in collapse and death. Careful grazing management is the only real protection.
Birds Severe
Birds are not exempt from the alkaloids that poison mammals, and the seeds are the sharpest hazard to anything that pecks. The whole plant is best treated as off limits, since even a small amount can bring the tremors, weakness, and paralysis the toxin is known for. Keep it out of reach of the garden's visitors.
People Caution
No part of larkspur belongs on a plate, and people are poisoned by it too. Swallowing any of it can cause vomiting, muscular weakness, a weak pulse, and breathing trouble, so keep seeds and foliage away from children and reach for gloves before handling the plant.
Seeds and young foliage are most toxic; keep all plant parts away from children, pets, and grazing livestock and wear gloves when handling.
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 larkspur grown for the garden and the vase is chosen for the color and length of its spikes. A few of the named types you are likely to meet are gathered here.
Stiletta Indigo Blue
A cut-flower favorite that sends up tall, upright spikes crowded with indigo-blue blooms for the vase.
Cannes Deep Blue
A florist's choice, carrying deep blue, spurred flowers on long, sturdy stems made for arranging.
White-flowered forms
The uncommon white variants of a plant that usually arrives blue and only occasionally turns to pink or white.
How to propagate
Larkspur takes readily from seed, which is really the only practical route, as it resists having its roots disturbed.
SeedEasy
Sow outdoors in fall (mild climates) or early spring; seeds benefit from cool conditions to germinate · Germination in roughly 2 to 3 weeks under cool soil conditions
Larkspur is happiest sown straight into the spot where it will bloom, since it dislikes root disturbance and settles best undisturbed. Set the seeds outdoors in fall in mild climates or in early spring elsewhere, allow them the cool conditions they need to rouse, and look for germination in about 2 to 3 weeks. Thin the seedlings to 9 to 12 inches apart so each spike has the space to stand tall.
Myths
Larkspur becomes harmless to animals once it stops flowering.
The plant does soften in potency as it matures, yet it never becomes safe. The seeds stay highly poisonous, and livestock are still lost to larkspur late in the season, so a plant past bloom deserves the same wide berth as one in full flower.
Larkspur is a danger only to cattle, not to pets or people.
The diterpenoid alkaloids reach cats, dogs, horses, and humans just as readily as cattle. In people, ingestion can bring vomiting, muscular weakness, a weak pulse, and respiratory paralysis, which makes larkspur a concern for the whole household, not merely the herd.