Ornamental · Ranunculaceae
Monkshood
Aconitum napellus
Tall blue hooded spikes that earned the name queen of poisons, a showstopper for the border and a plant that commands respect.
Full sun
Every 3–7 days
USDA 3–7
3–4 ft · 0.9–1.2 m
Overview
Monkshood grabs the spotlight late in summer, hoisting hooded flowers of deep purplish-blue on stems that climb well above their neighbours. Each bloom folds over like a cowl drawn across a face, and there is the name in one glance. It sits in the buttercup family and truly thrives in cool, moist gardens across the colder USDA zones.
All that late-season punch comes wrapped around one of the most poisonous plants you can grow. Every part carries aconitine, a fast-acting toxin that pinned old names like wolfsbane and queen of poisons onto it. The danger is the real deal, so weigh it hard before planting anywhere children, pets, or grazing animals roam.
The payoff is a slow, long-lived perennial that hates being moved and demands next to nothing once it takes hold. Hand it rich, damp, well-drained soil and a little shelter for those tall stems, and wear gloves any time you touch it. Give it that respect and a clump powers back, reliably, for years on end.
Care
The short version: full sun to part shade in cool climates, soil kept evenly moist but never waterlogged, rich ground charged with compost, and gloves on every single time you handle the plant.
Light
Monkshood powers along in full sun in cool climates, and it happily takes some afternoon shade where summers turn hot. Pile on too much shade and the stems go weak and floppy, so pair strong light with a touch of shelter.
Water
Keep the soil evenly moist and never let it dry out, watering roughly every 3 to 7 days through a dry spell. It hates sitting in water just as much, so drainage has to deliver. In a scorching summer, steady moisture and a little afternoon shade keep it charging.
Soil
Plant it in rich, moist, well-drained ground loaded with organic matter. It shrugs off a wide pH band from 5.5 to 7.5, so most garden soil works a treat once you dig in plenty of compost.
Temperature
This one is built for the cold, hardy down to around -35°F (-37°C) and topping out near 84°F (29°C). Real heat trips it up, which is exactly why a little afternoon shade earns its keep where summers bake.
Humidity
Ordinary outdoor air does the job, so skip chasing any number. What really counts is brisk air movement, since still, muggy conditions roll powdery mildew straight onto the leaves.
Feeding
Feed in spring as growth fires up, working in a balanced fertilizer or a layer of compost. Just one seasonal feed fuels a plant this unhurried.
Pruning
Cut the spent flower stalks the moment they fade to tidy the clump and sometimes spark a second flush, then cut the foliage back after it dies down in fall. Always wear gloves and keep those cut stems and sap off your skin.
Repotting
A settled clump would rather stay put, so divide it only every 4 to 5 years, and only once it crowds itself. Fewer flowers, smaller crowded leaves, or a dead patch in the center all shout that the moment has come.
Size & spacing
Plants rocket to 3 to 4 feet (91 to 122 cm) tall on trim clumps spreading just 12 to 18 inches (30 to 46 cm) wide. Hand each one some breathing room so air flows freely and mildew never gets a foothold.
When to plant
Monkshood runs on a cool-season clock, and it is a rewarding one to work. Sow the cold-treated seed in late winter, or split settled clumps in early spring or fall, then count down to July and August, when those tall blue hoods finally burst open.
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 kicks off at the roots in soggy soil, or wherever stems crowd in too tight. Spot the early signs and you can leap in before a clump is lost. Match what you see to the notes below, and grab gloves before you lay a hand on a sick plant.
Crown turning to brown mush and buckling right at the base
A soft, browning base points straight to crown rot, which moves in when the soil stays wet too long or the clump packs too tight to breathe. Jump on it: glove up, lift the affected plants, and clear them out. Sort the drainage first, then drop healthy divisions back into looser, richer soil and watch them bounce back.
Fix:
- Remove and discard affected plants (wear gloves)
- Improve drainage and avoid overhead watering
- Replant healthy divisions in better-drained, amended soil
Foliage flagging and refusing to firm up again after a soak
When leaves keep wilting and watering does nothing, verticillium wilt is the likely culprit, a soil fungus that jams the stem shut from the inside. This one pulls no punches, since nothing cures it once it lands. Dig out the sick plants, destroy them, and steer anything vulnerable away from 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
That chalky white dusting is powdery mildew, a fungus that pounces late in the season once humidity climbs and the air goes flat. Open the planting up so a breeze can rush through, strip the worst leaves, and aim your water at the base. It looks scruffy for a spell, but it rarely leaves any lasting mark.
Fix:
- Improve air circulation by thinning or spacing plants
- Remove and discard badly affected foliage
- Avoid wetting foliage when watering
Soaring flower stems pitching over the moment wind or rain sweeps in
Those soaring stems turn top-heavy, so one downpour or a stiff gust sends them sprawling, and a shady or overfed spot makes them floppier yet. Get ahead of it and stake early, before they tip. Slot the plant into a sheltered corner and it holds that tall line all 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 truly 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 drives 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
For a cat, a few mouthfuls can turn deadly. Look for heavy drooling, vomiting, a stumbling weakness, and hard breathing, and watch the heart race or stutter toward sudden collapse. Any contact means a dash to the vet, no waiting.
Dogs Severe
A dog that chews any part drools, bloats, and vomits fast, then weakens and fights for breath. The heart is the true danger zone, where the toxin scrambles the rhythm and can stop it outright, so move quickly.
Horses Severe
A horse grazing on it salivates, colics, and shakes with trembling muscles within a short window. Breathing turns laboured and the heartbeat falls out of step, and fatal cardiopulmonary failure can follow, so act fast.
Rabbits Severe
The aconitine that drops big animals hits just as hard in a body as small as a rabbit's. Even a single nibble can spark drooling, weakness, and a failing heartbeat, so keep rabbits well clear of every part.
Livestock Severe
Cattle, sheep, and goats all fall to the foliage and roots, and losses in grazing stock are well documented. Expect staggering, a slowed heartbeat, and breathing trouble that too often ends in death before help arrives, so fence them out.
Birds Severe
Aconitine strikes the whole body with no safe dose, and birds get no free pass. A little pecking at leaves, seeds, or roots can spark rapid weakness and a fatal upset of the heart, so keep it out of reach.
People Caution
No part of this plant is food, ever. Swallow even a small amount and expect tingling, numbness, vomiting, and dangerous heart rhythms, and the sap slips through skin too, so glove up and never grab 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 on offer is the wild blue species, but a handful of named forms shake up the color. These are the ones you are most likely to hunt down.
Album
This white-flowered form throws up clean white spikes instead of the usual deep purplish-blue.
Carneum
This one delivers pale pink to pinkish flowers, a softer shade than the wild species.
Bicolor
Its blooms flaunt blue and white together, and it frequently turns up for sale as Aconitum x cammarum.
How to propagate
Monkshood multiplies two ways, by splitting a clump or by sowing seed, and both reward a little patience and a pair of gloves worn from start to finish.
DivisionModerate
Early spring or fall · Often slow to establish; may take a year or more to settle and bloom well
Dig up an established clump and split it in early spring or fall, but keep those gloves on the whole time, since the sap bites bare skin. These plants hate being moved and tend to sulk afterward, so give them 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 cold spell wakes it from dormancy, and even then it pushes up slowly and unevenly over many weeks. Handle the seed and the young seedlings in gloves, since every part already packs the toxin from day one.
Myths
The plant is a danger strictly when eaten, so grabbing it barehanded poses no trouble at all.
The aconitine in the sap drives straight through your skin, so touching cut stems or juices is a real risk all by itself. Garden and poison authorities say it plainly, so wear gloves and avoid all contact, and guard any open cut with extra care.
Roots and seeds hold all the poison, leaving the leaves and blossoms completely harmless.
Every part of the plant is toxic, from the bark and leaves right through to the flowers, fruit, seeds, roots, and stems. The roots, seeds, and young shoots just pack the most poison, which does nothing to make the rest safe to touch or taste.