Ornamental · Ranunculaceae
Monkshood
Aconitum napellus
Tall blue hooded spikes that earn the name queen of poisons, beautiful in the border and deadly in the hand.
Full sun
Every 3–7 days
USDA 3–7
3–4 ft · 0.9–1.2 m
Overview
Monkshood draws the eye late in summer, when its tall stems carry hooded flowers in deep purplish-blue. The shape of each bloom, like a cowl pulled over a face, gives the plant its name. It belongs to the buttercup family and thrives in cool, moist gardens across the colder USDA zones.
Behind the good looks sits one of the most poisonous plants you can grow. Every part holds aconitine, a fast-acting toxin that has earned old names like wolfsbane and queen of poisons. The danger is real enough that you should think hard before planting it where children, pets, or grazing animals roam.
It is a slow, long-lived perennial that resents being moved and asks for little once settled. Give it rich, damp, well-drained soil and some shelter for its tall stems, and wear gloves any time you touch it. Treated with respect, a clump returns reliably for years.
Care
The short version: full sun to part shade in cool climates, soil kept evenly moist but never waterlogged, rich ground worked with compost, and gloves on every single time you handle the plant.
Light
Monkshood does best in full sun in cool climates, but it welcomes some afternoon shade where summers run hot. Too much shade leaves the stems weak and floppy, so give it light and shelter both.
Water
Keep the soil evenly moist and never let it dry out, watering roughly every 3 to 7 days in a dry spell. It hates sitting in water just as much, so good drainage matters. In a hot summer, steady moisture and a little afternoon shade keep it happy.
Soil
Plant in rich, moist, well-drained ground packed with organic matter. It tolerates a wide pH band from 5.5 to 7.5, so most garden soil works once you dig in plenty of compost.
Temperature
This is a plant for cold places, hardy down to around -35°F (-37°C) and topping out near 84°F (29°C). It struggles in real heat, which is why a bit of afternoon shade pays off where summers bake.
Humidity
Ordinary outdoor air suits it, so there is no number to hit. Air movement counts for more here, since still, humid conditions invite powdery mildew onto the leaves.
Feeding
Feed in spring as growth begins, working in a balanced fertilizer or a layer of compost. One seasonal feed is plenty for a plant this unhurried.
Pruning
Cut the spent flower stalks once they fade to tidy the clump and sometimes coax a second flush, then cut the foliage back after it dies down in fall. Always wear gloves and keep the cut stems and sap off your skin.
Repotting
A settled clump resents disturbance and is best left alone, divided only every 4 to 5 years if it grows overcrowded. Reduced flowering, smaller crowded leaves, or a dead patch in the center tell you it is finally time.
Size & spacing
Plants reach 3 to 4 feet (91 to 122 cm) tall on narrow clumps spreading just 12 to 18 inches (30 to 46 cm) wide. Give each one room so air moves freely and mildew stays away.
When to plant
Monkshood runs on a cool-season clock. Sow the cold-treated seed in late winter or split established clumps in early spring or fall, then look for the tall blue hoods to open in the heat of July and August.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: Late winter; seed needs cold stratification and germination is slow and erratic
- Plant: Spring after frost, or divide established clumps in early spring or fall
- Harvest: July to August
Hardiness
USDA hardiness zones: 3–7
Temperature: down to -40°C
Troubleshooting
Most monkshood trouble starts at the roots in wet soil or among stems crowded too close. Catch the early signs and you can step in before a clump is lost. Match what you see below, and reach for gloves before you touch a sick plant.
Mushy brown crown collapsing at the soil line
This is crown rot, and it sets in when the soil stays wet for too long or the clump grows too crowded to breathe. Lift and bin the affected plants while wearing gloves, then fix the drainage before you replant. Healthy divisions go back into better-drained, amended ground.
Fix:
- Remove and discard affected plants (wear gloves)
- Improve drainage and avoid overhead watering
- Replant healthy divisions in better-drained, amended soil
Wilting leaves that never perk back up after watering
A soilborne fungus called verticillium wilt clogs the stem from the inside, so the plant fades no matter how much you water. There is no cure once it takes hold. Dig out and destroy the sick plants, then keep susceptible plants out of that ground for good.
Fix:
- Remove and destroy affected plants
- Avoid replanting susceptible plants in the same soil
- Maintain plant vigor with proper moisture and nutrition
Dusty white film coating the leaves
This is powdery mildew, a fungus that takes hold late in the season when humidity climbs and the air sits still. Thin or space the plants to get air moving, pull off the worst leaves, and keep water off the foliage. It looks rough but rarely does lasting harm.
Fix:
- Improve air circulation by thinning or spacing plants
- Remove and discard badly affected foliage
- Avoid wetting foliage when watering
Tall flower spikes flopping over after rain or wind
Those tall stems grow top-heavy and lean once a storm or a gust catches them, and a rich or shady spot only makes them softer. Stake the stems early, before they sprawl. Tucking the plant somewhere sheltered from the worst wind keeps it upright on its own.
Fix:
- Stake or support taller stems
- Site in a spot with some shelter from strong wind
Toxicity
Treat this plant as dangerous, because it is. All parts hold aconitine, a poison that can kill cats, dogs, horses, livestock, and people, sometimes within hours of swallowing only a little. The sap even passes through skin, so wear gloves whenever you handle it and keep it well away from animals and children.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Severe
A few mouthfuls can be deadly. Watch for heavy drooling, vomiting, a stumbling weakness, and laboured breathing, with the heart racing or stuttering toward sudden collapse. Treat any contact as an emergency.
Dogs Severe
Dogs that chew any part show drooling, bloating, and vomiting fast, then go weak and struggle to breathe. The real danger is the heart, where the toxin throws the rhythm into chaos and can stop it cold.
Horses Severe
Grazing on this plant brings salivation, colic, and trembling muscles within a short window. Breathing turns hard and the heartbeat falls out of rhythm, and a fatal cardiopulmonary failure can follow.
Rabbits Severe
The same aconitine that fells larger animals is just as deadly in a small body. Even a nibble can trigger drooling, weakness, and a failing heartbeat, so keep rabbits well clear of every part.
Livestock Severe
Cattle, sheep, and goats are poisoned by the foliage and roots, and losses in grazing stock are well documented. Expect staggering, slow heartbeat, and breathing trouble, often ending in death before help arrives.
Birds Severe
Aconitine is a systemic poison with no safe dose, and birds are not spared. Any pecking at leaves, seeds, or roots risks rapid weakness and a fatal disruption of the heart.
People Caution
No part of this plant is food. Swallowing even a small amount causes tingling, numbness, vomiting, and dangerous heart rhythms, and the sap can pass through the skin, so wear gloves and never handle it bare-handed.
Wear gloves whenever handling this plant, avoid skin or oral contact with plant juices and cut stems, cover any open cuts or abrasions, and wash hands thoroughly afterward. Keep away from pets, livestock, and children.
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 monkshood sold is the wild blue species, but a few named forms shift the color. These are the ones you are likely to find.
Album
A white-flowered form that trades the usual deep purplish-blue for clean white spikes.
Carneum
This one carries pale pink to pinkish flowers, a softer shade than the wild species.
Bicolor
Its blooms come marked in blue and white together, and it is often sold under Aconitum x cammarum.
How to propagate
Monkshood spreads either by splitting a clump or by sowing seed, and both ask for patience and a pair of gloves from start to finish.
DivisionModerate
Early spring or fall · Often slow to establish; may take a year or more to settle and bloom well
Split an established clump in early spring or fall, but wear gloves the whole time, since the sap is dangerous on bare skin. These plants resent being disturbed and often sulk, taking a year or more to settle in and bloom well again.
SeedDemanding
Sow in late winter; seed benefits from cold stratification · Slow and erratic germination over many weeks
Sow seed in late winter after a spell of cold to break its dormancy, and even then it comes up slowly and unevenly over many weeks. Handle the seed and the young seedlings with gloves, because every part already carries the toxin.
Myths
Monkshood is only dangerous if you eat it, so handling it bare-handed is fine.
The aconitine in the sap can pass straight through your skin, so touching cut stems or juices is a real risk on its own. Garden and poison authorities say to wear gloves and avoid all contact, and to take extra care over any open cut.
Only the roots and seeds are poisonous, so the leaves and flowers are harmless.
Every part of the plant is toxic, from the bark and leaves to the flowers, fruit, seeds, roots, and stems. The roots, seeds, and young shoots simply hold the most poison, which makes the rest no safer to touch or taste.