Ornamental · Ranunculaceae
Larkspur
Consolida ajacis
Tall blue spikes for the cool garden, elegant in the border yet poisonous in every part.
Full sun
Every 3–7 days
30–75°F · -1–24°C
USDA 2–11
Overview
Larkspur supplies tall, airy spikes of blue at the cool end of the growing year. It is an annual member of the buttercup family, Ranunculaceae, and a close relative of the perennial delphiniums for which it is often mistaken. From June to August it carries spike-like racemes of spurred flowers above finely divided, ferny foliage, and the species flowers predominantly blue, with pink and white arising far less frequently.
Its schedule follows its physiology: as a cool-season hardy annual, larkspur completes its cycle in the mild interval between frost and sustained heat. Sown in place, it grows quickly and often reaches 2 to 4 feet before high temperatures end its run. Growers in cold zones sow before the last frost, while those in mild regions sow in fall and overwinter the plants to secure an earlier flush.
For all its ornamental value, larkspur is toxic from root to seed. The diterpenoid alkaloids that make it one of the deadliest plants on western cattle ranges act by the same neuromuscular mechanism in pets, children, and people, so it belongs where curious mouths cannot reach it.
Care
The short version: give larkspur full sun, loose well-drained soil, and steady moisture, sow it early so bloom finishes before the heat, and deadhead to extend the flowering period.
Light
Larkspur performs best in full sun, which drives the strongest, straightest spikes, though it will accept limited partial shade. Sun also keeps the leaf surface and surrounding air dry, and that dryness is what suppresses powdery mildew.
Water
Maintain the soil evenly moist but never saturated, watering on the order of every 3 to 7 days as weather and soil dictate. Drainage matters because the roots rot in waterlogged ground, so free drainage protects the plant as much as consistent moisture does.
Soil
Grow larkspur in loose, moderately rich, well-drained soil. The species is indifferent to pH and tolerates alkaline ground well, across a range of 6.5 to 8.0, provided water moves freely through the profile.
Temperature
This is a cool-season plant, comfortable between about 30°F to 75°F (-1°C to 24°C). It withstands light frost but ceases growth once summer heat is established, which is precisely why early sowing improves the result.
Humidity
Ambient outdoor humidity is adequate, so there is no target value to meet. Air movement is the operative factor, because still, damp air within a crowded stand is what favors powdery mildew.
Feeding
Incorporate a balanced fertilizer into rich, organic soil at planting, and little further feeding is required. Larkspur is not a heavy feeder, and fertile ground at establishment sustains it through its short, fast life cycle.
Pruning
Deadhead spent spikes to redirect the plant's energy from seed set into further bloom, extending the flowering period. As heat mounts and the plant declines, cut it back rather than leaving it to deteriorate in place.
Size & spacing
Mature plants stand 2 to 4 feet (61 to 122 cm) tall on a narrow footprint of 12 to 18 inches (30 to 46 cm). Thin or space seedlings about 9 to 12 inches apart to keep air moving through the stand and hold mildew in check.
When to plant
Larkspur is keyed to the cool half of the season and cannot carry through summer heat. Growers 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 head start. The objective in every case is bloom from June to August, completed before sustained heat ends the plant's growth.
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 whose few problems trace mostly to heat or crowding, both of which the record ties to predictable outcomes. Reading the early leaf signs and timing the sowing correctly prevents nearly all of them. Match the symptom below, then intervene while intervention still changes the result.
Plants wilt and fade out as summer heats up
The physiology explains this: larkspur is a cool-season hardy annual and cannot sustain growth once temperatures hold high, an effect most pronounced in the deep South. A plant declining from heat has reached the end of its natural cycle and will not recover. The remedy is scheduling, not rescue, so sow in fall or very early spring to finish bloom ahead of peak heat, and supply afternoon shade to blunt the thermal load 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
That coating is powdery mildew, a foliar fungus that establishes where crowded plantings trap still, humid air against the leaf surface. Thin and respace the stand so air can carry that moisture away, water at the base to keep the canopy dry, and remove leaves already colonized so spores have less to spread from. Full sun works against the fungus by keeping the leaf surface dry.
Fix:
- Improve air circulation by thinning and spacing plants
- Avoid overhead watering
- Remove and discard affected foliage
Chewed, holed, or distorted leaves
This damage traces to a familiar set of foliage feeders: aphids, slugs, snails, leaf miners, and mites. Each leaves a diagnostic sign, so inspect the leaf undersides for aphid and mite colonies, look for the slime trails that betray molluscs after dark, and read the pale serpentine tracks that mark miner activity within the leaf. Hand-pick slugs and snails, dislodge aphids and mites with a forceful water spray that disrupts their feeding, and cull leaves too heavily infested to recover.
Fix:
- Hand-pick slugs and snails
- Wash off aphids or mites with a strong water spray
- Remove heavily infested leaves
Toxicity
Regard every part of this plant as poisonous. The flowers, leaves, stems, and above all the seeds contain diterpenoid alkaloids that block neuromuscular transmission, the mechanism that makes larkspur a leading killer of range cattle. The same compounds endanger cats, dogs, horses, rabbits, birds, and people, so the plant must be kept from any animal or human that might ingest it. No part of it is safe to eat.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Severe
Every tissue is toxic to a cat, and the diterpenoid alkaloids concentrate most in the seeds and young growth. Because these compounds block neuromuscular transmission, a poisoned cat shows drooling, colic, tremors, stiffness, and progressive weakness, which in severe cases advances to convulsions, cardiac failure, and death once the respiratory muscles are paralyzed.
Dogs Severe
A dog ingesting any part meets the same neuromuscular alkaloids, and the clinical course follows their action on nerve and muscle. Expect heavy salivation, constipation, colic, muscle tremors, and an ataxic, weakening gait that can proceed to recumbency, convulsions, and fatal respiratory paralysis. Veterinary intervention should not wait for the syndrome to deepen.
Horses Severe
Horses rank among the more sensitive species, and onset is rapid once the alkaloids are absorbed. The early phase brings colic, constipation, increased salivation, and stiffness, progressing to recumbency and convulsions, with cardiac failure and death from respiratory paralysis where the dose is high.
Rabbits Severe
The diterpenoid alkaloids that kill animals many times a rabbit's size act on the same neuromuscular targets in a rabbit. No portion of the plant is safe to browse, and even a small intake risks tremors, weakness, paralysis, and death. Site larkspur well away from any hutch or run.
Livestock Severe
Larkspur ranks among the deadliest range plants for cattle, which graze it readily and die in numbers annually. As the alkaloids block acetylcholine receptors at the neuromuscular junction, affected animals show muscle weakness and tremors, lose coordination, grow reluctant to move, and go down, often with bloating, a rapid irregular heartbeat, and labored breathing that ends in collapse and death.
Birds Severe
The alkaloids that poison mammals give no exemption to birds, and the seeds pose the greatest hazard to any bird that pecks at them. The entire plant should be treated as off limits, since even a small quantity can produce the tremors, weakness, and paralysis characteristic of the toxin.
People Caution
No part of larkspur is edible, and humans are susceptible to the same poison. Ingestion can cause vomiting, muscular weakness, a weak pulse, and respiratory difficulty, so keep seeds and foliage away from children and wear gloves whenever 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
Larkspur grown for gardens and cut flowers is selected chiefly for spike color and length. The following named types are among those in general circulation.
Stiletta Indigo Blue
A cut-flower selection valued for tall, upright spikes densely set with indigo-blue florets.
Cannes Deep Blue
A florist type that bears deep blue, spurred flowers on long, rigid stems suited to the vase.
White-flowered forms
Scarce white variants of a species that flowers predominantly blue and only occasionally deviates to pink or white.
How to propagate
Larkspur is propagated from seed, effectively the only practical method, because the plant resents disturbance to its roots.
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
Because larkspur resents root disturbance, direct-sow it into its final position rather than transplanting; a taproot-forming seedling establishes best where it germinates. Sow outdoors in fall in mild climates or in early spring elsewhere, and supply the cool soil the seed requires to break dormancy, after which germination follows in about 2 to 3 weeks. Thin the resulting seedlings to 9 to 12 inches apart to give each developing spike adequate room.
Myths
Larkspur becomes harmless to animals after it finishes flowering.
Alkaloid concentration does decline as the plant matures, but it never falls to zero. The seeds remain highly poisonous, and cattle losses continue late in the season, so a post-bloom plant carries real risk despite the lower whole-plant potency.
Larkspur endangers only cattle, not pets or people.
The same diterpenoid alkaloids act on cats, dogs, horses, and humans through the same neuromuscular mechanism that kills cattle. In people, ingestion can produce vomiting, muscular weakness, a weak pulse, and respiratory paralysis, which makes this a household hazard well beyond the pasture.