Ornamental · Ranunculaceae
Larkspur
Consolida ajacis
Tall blue spires to lift a cool spring garden, lovely to grow but poisonous in every part.
Full sun
Every 3–7 days
30–75°F · -1–24°C
USDA 2–11
Overview
If you're new to larkspur, you'll find it a kind plant to grow. It carries tall, airy spikes of blue through the cool part of the gardening year, and it's an annual cousin in the buttercup family, closely tied to the perennial delphiniums it often gets mistaken for. From June to August it lifts spike-like racemes of spurred flowers over soft, ferny leaves, and while most plants come up blue, you'll now and then be treated to pink or white.
The nice thing is how forgiving its timing is once you know the trick. Larkspur is a cool-season hardy annual that loves the mild window between frost and full heat, so sow it right where it will bloom and it takes off fast, often reaching 2 to 4 feet before it eases off in the heat. In cold zones you simply start it before the last frost, and in mild regions you sow in fall and let it settle in for an early show.
One thing does call for real care: 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 you'll want to give it a spot where curious mouths simply can't reach.
Care
The short version: give larkspur full sun, loose well-drained soil, and steady moisture, sow it early so it flowers before the heat, and snip off the faded spikes to keep more coming.
Light
Give larkspur full sun for the strongest, straightest spikes, though it will happily take a touch of partial shade. Sun keeps the air around the leaves on the dry side too, which helps hold powdery mildew at bay.
Water
Aim to keep the soil evenly moist but never soggy, watering roughly every 3 to 7 days as the weather and your soil call for it. Because the roots rot in wet, waterlogged ground, good drainage matters just as much as that steady moisture.
Soil
Set larkspur into loose, moderately rich, well-drained soil and it will settle in nicely. It's easygoing about pH and copes well with alkaline ground, 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 a light frost with no trouble but calls it quits once summer heat settles in, which is why an early sowing pays off so well.
Humidity
Everyday outdoor humidity suits it fine, so there's no number you need to chase. Air movement is what really matters, since still, damp air around crowded plants is the thing that invites powdery mildew.
Feeding
Work a balanced fertilizer into rich, organic soil at planting and you've done nearly all the feeding this plant needs. Larkspur isn't a hungry grower, and good ground at the start easily carries it through its short, fast life.
Pruning
Snip off spent spikes to coax more bloom over a longer stretch. As the heat builds and the plant begins to fade, go ahead and cut it back rather than leaving it to sit and sulk.
Size & spacing
Plants stand 2 to 4 feet (61 to 122 cm) tall and stay slim at 12 to 18 inches (30 to 46 cm) wide. Thin or space your seedlings about 9 to 12 inches apart so air moves freely and mildew stays away.
When to plant
Larkspur thrives in the cool half of the season and has no love for summer heat, which makes timing your easiest win. In cold climates you 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. Either way you're aiming for bloom from June to August, before the heat settles in and calls time on 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 a gentle plant with only a short list of troubles, and most of them trace back to heat or crowding. Learn to read the early signs on the leaves and get your sowing timed right, and you'll head off nearly all of it. Find what matches below, then step in while it still makes a difference.
Your plants begin to wilt and fade as the days warm up
This one isn't your fault at all. Larkspur is a cool-season hardy annual, so it naturally winds down once real heat arrives, and that happens soonest in the deep South. Once a plant starts leaving with the heat, it's kinder to let it go. The good news is that timing solves everything: sow in fall or very early spring so the blooms finish before high summer, and offer a little afternoon shade if your region runs hot.
Fix:
- Provide afternoon shade in hot-summer climates
- Sow in fall or very early spring so bloom finishes before peak heat
A pale, dusty film has settled on the leaves and stems
Don't worry, this is just powdery mildew, a common fungus that shows up when plants sit close together in still, muggy air. You can turn it around by opening up the spacing so a breeze can reach the foliage, watering low at the base instead of over the leaves, and gently removing any leaf that's already coated. A sunny spot keeps it from settling in again.
Fix:
- Improve air circulation by thinning and spacing plants
- Avoid overhead watering
- Remove and discard affected foliage
Leaves look nibbled, holed, or a bit misshapen
A few common pests enjoy larkspur foliage, and you may find aphids, slugs, snails, leaf miners, and mites among them. Take a peek under the leaves for little clusters, look for shiny slime trails after dark, and notice any pale winding tunnels the miners leave. You've got this: pick off slugs and snails by hand, knock aphids and mites away with a firm spray of water, and remove any leaf that's truly overrun.
Fix:
- Hand-pick slugs and snails
- Wash off aphids or mites with a strong water spray
- Remove heavily infested leaves
Toxicity
Please treat every part of this plant as poison. The flowers, leaves, stems, and above all the seeds hold diterpenoid alkaloids that block the nerves and muscles, and that's exactly why larkspur is a leading killer of range cattle. It's every bit 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
Please keep this one out of reach of your cat, because every part of it is dangerous and the seeds and young growth carry the most. The diterpenoid alkaloids work on the nerves and muscles, so a cat may drool, cramp, tremble, stiffen, and weaken, and in a bad case that can worsen into convulsions, heart failure, and death when breathing stops.
Dogs Severe
A dog that chews any part of larkspur meets the very same neuromuscular poison. Watch closely for heavy drooling, constipation, cramping, muscle tremors, and an unsteady, fading walk that can slide into collapse, convulsions, and fatal breathing failure. Head to the vet right away rather than waiting to see whether it passes.
Horses Severe
Horses feel this quickly, as they're very sensitive to the alkaloids. Early on you might see colic, constipation, more drooling than usual, and stiffness, and from there it can move to going down, convulsions, and in serious cases heart failure and death from breathing trouble. Call your vet the moment you suspect a bite.
Rabbits Severe
The same alkaloids that fell far bigger animals are just as unsafe for a rabbit. There's no part a rabbit can safely nibble, and even a little can lead to tremors, weakness, paralysis, and death. Keep larkspur well away from any hutch or run, and you'll spare yourself the scare.
Livestock Severe
Larkspur is one of the harshest range plants for cattle, which eat it readily and are lost in numbers each year. Affected animals grow weak and trembly, lose coordination, hang back from walking, and go down, often with bloating, a fast uneven heartbeat, and labored breathing. Grazing management is the real safeguard here.
Birds Severe
Birds aren't spared the alkaloids that harm mammals, and the seeds are the sharpest danger to anything that pecks. It's best to treat the whole plant as off limits, since even a small taste can bring the tremors, weakness, and paralysis this toxin is known for. A little caution keeps feathered visitors safe.
People Caution
No part of larkspur is food, and it can poison people too, so please handle it with care. Swallowing any of it may cause vomiting, muscle weakness, a weak pulse, and breathing trouble, so keep seeds and foliage away from children and slip on gloves before you work with 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 gardens and bouquets is chosen for the color and length of its spikes. Here are a few of the named types you're likely to meet.
Stiletta Indigo Blue
A favorite for cutting, this one sends up tall, straight spikes crowded with rich indigo-blue flowers.
Cannes Deep Blue
Grown for the vase, it holds deep blue, spurred blooms on long, sturdy stems that stand up beautifully.
White-flowered forms
These are the less common white variants of a plant that usually comes up blue and only occasionally shifts toward pink or white.
How to propagate
Larkspur comes up easily from seed, and seed is really the only sensible route, since it doesn't like 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 easiest path is to sow larkspur right where you want it to bloom, since it dislikes having its roots disturbed and grows best from a direct sowing. Put seeds in the ground in fall if your climate is mild or in early spring if it isn't, let them have the cool spell they need to sprout, and look for germination in about 2 to 3 weeks. Once seedlings are up, thin them to 9 to 12 inches apart so each spike has plenty of room.
Myths
Larkspur becomes harmless to animals once it has finished flowering.
It's true that the plant grows a little less potent as it matures, but it never becomes safe. The seeds stay highly poisonous, and livestock are still lost to larkspur late in the year, so a plant past bloom deserves the same caution as one in full flower.
Larkspur is only a danger to cattle, not to pets or people.
The diterpenoid alkaloids reach cats, dogs, horses, and humans just as readily as they reach cattle. In people, swallowing it can bring vomiting, muscle weakness, a weak pulse, and respiratory paralysis, so this really is a whole-household hazard, not just a concern out on the range.