Ornamental · Ranunculaceae
Larkspur
Consolida ajacis
Tall spires of spurred blue over fine ferny leaves, striking to the eye yet poisonous in every part.
Full sun
Every 3–7 days
30–75°F · -1–24°C
USDA 2–11
Overview
Larkspur sends up tall, slender stems tipped with spike-like racemes of blue, each floret carrying the backward-pointing spur that gives the plant its name. The foliage below is fine and ferny, cut into narrow threadlike segments that soften the whole plant into a hazy green column. An annual in the buttercup family and a near-twin of the perennial delphiniums, it opens its spurred flowers from June to August, mostly in clear blue, with the occasional plant breaking to pink or white.
This is a cool-season hardy annual, and it grows quickly once the cool of the year settles in, the airy stems stretching to 2 to 4 feet before summer heat halts them. Sown where it will stand, it races up fast, then browns and slumps as the season turns hot. Gardeners in cold zones sow it before the last frost, while those in mild regions sow in fall and let the young ferny rosettes overwinter for an early spike.
For all the delicacy of those blue flowers and thread-fine leaves, larkspur is toxic from root to seed. The diterpenoid alkaloids that make it one of the deadliest plants on western cattle ranges load every part, from the soft new growth to the hard dark seeds, so it must be kept where curious mouths cannot reach it.
Care
The short version: full sun holds the tall blue spires upright, loose soil with free drainage and even moisture feeds the ferny growth, an early sowing brings the color on before real heat, and cutting browned spikes makes room for fresh ones.
Light
Larkspur wants full sun to hold its tall spikes straight and true, though it will take a little partial shade. Sun also keeps the ferny foliage dry, and dry leaves give powdery mildew no chalky foothold.
Water
Keep the soil evenly moist but never soggy, watering roughly every 3 to 7 days as the weather and soil shift. The fine roots rot in wet, waterlogged ground, so free drainage matters as much as steady moisture.
Soil
Set larkspur in loose, moderately rich, well-drained soil. It is easy about pH and takes to alkaline ground well, anywhere from 6.5 to 8.0, so long as water slips through freely.
Temperature
This is a cool-weather plant, comfortable from about 30°F to 75°F (-1°C to 24°C). Its stems and leaves shrug off a light frost but wilt and brown once summer heat settles in, which is why an early sowing pays off.
Humidity
Ordinary outdoor humidity suits it, so there is no figure to chase. Air moving through the leaves is what matters, since still, damp air around a crowded stand is what dusts the foliage with powdery mildew.
Feeding
Work a balanced fertilizer into rich, organic soil at planting and the feeding is largely done. Larkspur is a light feeder, and good ground at the start carries the fast-rising stems through their short life.
Pruning
Deadhead the spent blue spikes as they brown to push the plant into fresh bloom over a longer stretch. As the heat builds and the stems start to flop and fade, cut them back rather than leaving a tired plant standing.
Size & spacing
The slim stems rise 2 to 4 feet (61 to 122 cm) tall while the plant stays narrow at 12 to 18 inches (30 to 46 cm) across. Thin or space seedlings about 9 to 12 inches apart so air threads through the ferny foliage and mildew stays down.
When to plant
Larkspur runs on the cool half of the season and browns off in summer heat. 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 an early start. The aim is those blue spikes filling out from June to August, before sustained heat wilts and ends the plant.
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 written plainly on the leaves and stems and traceable to heat or crowding. Read the early signs in the foliage and time your sowing well, and you head off nearly all of it. Match what you see below, then act while it still helps.
Stems slump and foliage browns as the heat builds
The tall stems soften, the ferny leaves droop and brown, and the whole plant slumps, because larkspur is a cool-season hardy annual that gives out once steady heat arrives, worst in the deep South. A plant collapsing in the heat is spent and will not green up again. The remedy is timing, so a fall or very early spring sowing lets the spikes finish before the worst of summer, with afternoon shade easing hot regions.
Fix:
- Provide afternoon shade in hot-summer climates
- Sow in fall or very early spring so bloom finishes before peak heat
A chalk-white film creeping over leaf and stem
A pale, chalky bloom spreading across the green is powdery mildew, a fungus that settles onto crowded plants in still, humid air. Thin the stand so air slips between the stems, water low at the base to leave the leaves dry, and strip off any blade already filmed over. Under full sun the leaf surface stays dry and the fungus finds no purchase.
Fix:
- Improve air circulation by thinning and spacing plants
- Avoid overhead watering
- Remove and discard affected foliage
Chewed, holed, or distorted leaves
Ragged edges, punched holes, and warped blades mark the work of aphids, slugs, snails, leaf miners, and mites. Aphids and mites cluster in soft green knots on the leaf undersides, slugs and snails leave glinting slime trails after dark, and miners scribble pale, wandering tunnels inside the leaf tissue. Pluck the slugs and snails off by hand, drive aphids and mites away with a hard rinse of water, and cut away any blade too tattered to recover.
Fix:
- Hand-pick slugs and snails
- Wash off aphids or mites with a strong water spray
- Remove heavily infested leaves
Toxicity
Behind the delicate blue spikes and lacy leaves, treat every part of this plant as poison. The flowers, the ferny foliage, the slim stems, and above all the small hard seeds carry diterpenoid alkaloids that block nerves and muscles, the reason larkspur is a leading killer of range cattle. It is 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
From the ferny young leaves to the small dark seeds, no part of larkspur is safe for a cat, and the seeds and tender new growth are the most loaded with diterpenoid alkaloids. The toxin works on nerve and muscle, so a cat may drool, cramp, tremble, stiffen, and weaken, and in the worst cases sink into convulsions, heart failure, and death once breathing is paralyzed. A vet should see it fast.
Dogs Severe
A dog that bites into a stem, leaf, or seedpod takes in the same neuromuscular poison. The signs run from heavy, ropey salivation and constipation to colic, muscle tremors, and a wobbly, weakening gait that can buckle into recumbency, convulsions, and fatal respiratory paralysis. A vet visit should not wait for the picture to darken.
Horses Severe
Horses are keenly sensitive, and the alkaloids move quickly through them. The first signs are colic, constipation, a wet mouth from rising salivation, and stiffness, giving way to recumbency and convulsions, with cardiac failure and death from respiratory paralysis in serious cases.
Rabbits Severe
The same alkaloids that drop far larger animals are no gentler on a rabbit. Not one part of the plant, from soft leaf to hard seed, is safe to nibble, and even a small bite risks tremors, weakness, paralysis, and death. Larkspur belongs nowhere near a hutch or run.
Livestock Severe
Larkspur ranks among the deadliest plants a cow can find on the range, and cattle graze the leafy young shoots readily, dying in numbers each year. Poisoned animals go weak and tremble, lose their footing, balk at walking, and collapse to the ground, often bloated, their heartbeat fast and irregular and their breathing labored as they fail.
Birds Severe
Birds are not spared the alkaloids that poison mammals, and the hard little seeds are the sharpest danger to any beak that cracks them. The whole plant, spike to root, should be treated as off limits, since even a small taste can bring on the tremors, weakness, and paralysis the toxin drives.
People Caution
No part of larkspur, from petal to seed, is food, and it poisons people too. Swallowing any of it can bring on vomiting, muscular weakness, a thready weak pulse, and labored breathing, so the seeds and foliage stay away from children and gloves go on before the plant is handled.
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 gardens and bouquets is chosen for the color and length of its spikes. A few of the named types you will meet are described here.
Stiletta Indigo Blue
A cut-flower selection whose tall, ramrod-straight spikes are packed tight with saturated indigo-blue florets.
Cannes Deep Blue
A cut-flower type carrying deep blue, spurred blooms along long, rigid stems built to hold their line.
White-flowered forms
The uncommon pure-white variants of a plant whose flowers run mostly blue and only now and then soften toward pink or white.
How to propagate
Larkspur grows readily from its small dark seeds, which 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
The small dark seeds go straight into the ground where the plant will stand, since larkspur resents root disturbance and grows best undisturbed from the spot it sprouts. Sow them outdoors in fall in mild climates or in early spring elsewhere, give them the cool soil they need to break dormancy, and the first thread-fine seedlings push up in about 2 to 3 weeks. Thin those seedlings to 9 to 12 inches apart so each rising spike has room to stretch.
Myths
Larkspur becomes harmless to animals once it finishes flowering.
The alkaloids do thin as the plant ages, yet they never drain away entirely. The dark little seeds stay highly poisonous, and livestock still die from larkspur late in the season, so a plant past its last blue spike is no safer to trust.
Only cattle on the range need fear larkspur, while pets and people go untouched.
The diterpenoid alkaloids strike cats, dogs, horses, and humans as surely as they strike cattle. In people, swallowing it can bring vomiting, muscular weakness, a weak pulse, and respiratory paralysis, so the danger reaches the whole household, not just the herd on the range.