Ornamental · Ranunculaceae
Larkspur
Consolida ajacis
Tall blue spikes for a cool spring garden, beautiful but poisonous in every part.
Full sun
Every 3–7 days
30–75°F · -1–24°C
USDA 2–11
Overview
Larkspur brings tall, airy spikes of blue to the cool end of the gardening year. It's an annual member of the buttercup family, related to the perennial delphiniums it's often mistaken for, and it earns its place with spike-like racemes of spurred flowers that rise above ferny foliage from June to August. Most plants bloom blue, with pink and white turning up far less often.
This is a cool-season hardy annual, happiest in the mild stretch between frost and real heat. Sow it where it's meant to grow and it shoots up fast, often reaching 2 to 4 feet, then fades once summer turns hot. Gardeners in cold zones start it before the last frost; those in mild regions sow in fall and let it overwinter for an early show.
For all its grace, larkspur is toxic from root to seed. 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 belongs where curious mouths can't reach it.
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 for the strongest, straightest spikes, though it will take a little partial shade. Sun also keeps the air around the leaves dry, which holds powdery mildew at bay.
Water
Keep the soil evenly moist but never soggy, watering roughly every 3 to 7 days depending on weather and soil. The roots rot in wet, waterlogged ground, so good drainage matters as much as steady moisture.
Soil
Plant in loose, moderately rich, well-drained soil. Larkspur is unfussy about pH and handles alkaline ground well, anywhere from 6.5 to 8.0, as long as water drains freely.
Temperature
This is a cool-weather plant, comfortable from about 30°F to 75°F (-1°C to 24°C). It shrugs off light frost but quits once summer heat settles in, which is why early sowing pays off.
Humidity
Ordinary outdoor humidity is fine, so there's no target to hit. Air movement is what counts, since still, damp air around crowded plants is what invites powdery mildew.
Feeding
Work a balanced fertilizer into rich, organic soil at planting and that's largely it. Larkspur isn't a heavy feeder, and good ground at the start carries it through its short, fast life.
Pruning
Deadhead spent spikes to push the plant into more bloom over a longer stretch. As the heat builds and the plant starts to decline, cut it back rather than letting it sit and sulk.
Size & spacing
Plants stand 2 to 4 feet (61 to 122 cm) tall and stay narrow at 12 to 18 inches (30 to 46 cm) across. Thin or space seedlings about 9 to 12 inches apart so air moves freely and mildew stays down.
When to plant
Larkspur runs on the cool half of the season and has no use for summer heat. Cold-climate gardeners sow indoors 8 to 10 weeks before the last frost or direct-sow in early spring, while milder regions sow outdoors in fall for an early start. Either way, the goal is bloom from June to August, before sustained heat shuts the plant down.
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 with a short list of troubles, most of them tied to heat or crowding. Read the early signs on the leaves and time your sowing well, and you head off nearly all of it. Match what you see below, then act while it still helps.
Plants wilt and fade out as summer heats up
Larkspur is a cool-season hardy annual, and it simply quits once sustained heat arrives, worst of all in the deep South. There's no saving a plant that's checking out from heat. The answer is timing: sow in fall or very early spring so the flowers finish before the worst of summer, and give afternoon shade in hot regions.
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
This is powdery mildew, a fungus that takes hold when plants stand crowded in still, humid air. Thin and space the plants so air can move, water at the base rather than over the top, and strip off any leaves already coated. Full sun keeps it from getting a foothold.
Fix:
- Improve air circulation by thinning and spacing plants
- Avoid overhead watering
- Remove and discard affected foliage
Chewed, holed, or distorted leaves
Several pests go after larkspur foliage, including aphids, slugs, snails, leaf miners, and mites. Check the undersides of leaves for clusters of pests, look for slime trails after dark, and watch for the pale tunnels that miners leave behind. Hand-pick slugs and snails, blast aphids and mites off with a sharp spray of water, and pull any leaf that's badly infested.
Fix:
- Hand-pick slugs and snails
- Wash off aphids or mites with a strong water spray
- Remove heavily infested leaves
Toxicity
Treat every part of this plant as poison. The flowers, leaves, stems, and especially the seeds carry diterpenoid alkaloids that block nerves and muscles, and larkspur is a leading killer of range cattle for exactly that reason. It's just as dangerous to cats, dogs, horses, rabbits, birds, and people, so keep it well away from anyone, animal or human, who might take a bite. Nothing on this plant is safe to eat.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Severe
Every part of the plant is dangerous to a cat, with the seeds and young growth the most potent. The diterpenoid alkaloids attack the nerves and muscles, bringing on drooling, colic, tremors, stiffness, and weakness that can collapse into convulsions, heart failure, and death from paralyzed breathing.
Dogs Severe
A dog that chews any part of larkspur faces the same neuromuscular poison. Watch for heavy salivation, constipation, colic, muscle tremors, and a wobbly, weakening gait that can give way to recumbency, convulsions, and fatal respiratory paralysis. Get to a vet without waiting for the picture to worsen.
Horses Severe
Horses are highly sensitive, and the alkaloids hit fast. Early signs include colic, constipation, increased salivation, and stiffness, followed by recumbency and convulsions, with cardiac failure and death from respiratory paralysis in serious cases.
Rabbits Severe
The same diterpenoid alkaloids that fell far larger animals are no safer for a rabbit. There is no portion of the plant a rabbit can nibble, and even a small amount risks tremors, weakness, paralysis, and death. Keep larkspur well clear of any hutch or run.
Livestock Severe
Larkspur is one of the deadliest range plants for cattle, which graze it readily and die in numbers each year. Poisoned 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.
Birds Severe
Birds are not spared the alkaloids that poison mammals, and the seeds pose the sharpest risk to anything that pecks at them. Treat the whole plant as off limits, since even a little can trigger the tremors, weakness, and paralysis the toxin is known for.
People Caution
No part of larkspur is food, 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 pull on gloves before you handle 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 sold for gardens and bouquets is grown for the color and length of its spikes. These are a few of the named types you'll come across.
Stiletta Indigo Blue
A cut-flower selection prized for its tall, straight spikes packed with indigo-blue blooms.
Cannes Deep Blue
A cut-flower type that carries deep blue, spurred flowers on long, sturdy stems built for the vase.
White-flowered forms
Uncommon white variants of a plant that usually blooms blue and only now and then turns pink or white.
How to propagate
Larkspur grows readily from seed, and seed is really the only practical route, since it dislikes 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
Start larkspur straight in the ground, since it resents transplanting and does best sown where it's meant to grow. Sow outdoors in fall in mild climates or in early spring elsewhere, give the seeds the cool conditions they need to wake up, and expect germination in about 2 to 3 weeks. Thin the seedlings to 9 to 12 inches apart so each spike has room.
Myths
Larkspur turns harmless to animals once it finishes flowering.
Toxicity does drop as the plant matures, but it never goes away. The seeds stay highly poisonous, and livestock still die from larkspur late in the season, so a plant past bloom is no safe plant.
Larkspur only threatens cattle, not pets or people.
The diterpenoid alkaloids poison cats, dogs, horses, and humans just as surely as cattle. Human ingestion can bring on vomiting, muscular weakness, a weak pulse, and respiratory paralysis, so this is a hazard for the whole household, not just the herd.