Monkshood
Aconitum napellus ·
Tall spires of hooded, deep blue flowers in late summer, above glossy, deeply cut leaves. One of the most poisonous garden plants.
- Size
- 4 ft tall, 18 in wide
- Zones
- 3 to 7
- Light
- Full sun to part shade
- Flowers
- Mid July to mid August
- Pets
- Toxic (severe)
- Job in a bed
- Accent
A bed built around monkshood
The planner designed this bed around monkshood when this page was made: a 12 × 6 ft (3.7 × 1.8 m) cottage garden in zone 6, part sun. It chose the other plants from the 708 that suit those conditions, and says below why monkshood stands where it does.
The plants
- 1 Monkshood × 3, blue
- 2 Lingonberry × 5, pink
- 3 Green-and-gold × 13, yellow
- 4 Bee balm 'Purple Rooster' × 3, purple
- 5 Globe arborvitae 'Danica' × 1, green leaves
- 6 Northern sea oats 'River Mist' × 3, green
- 7 Meadow saffron × 12, white
- 8 Snowdrop × 5, white
Why this plant
- The only spires. The others are bells, daisies, globes and others. Shape reads at a distance, where colours blur.
- The only blue.
Why here
- Anchors a back corner. At 4 ft it is the tallest plant here, and the tallest close the ends of the bed, where they stand in front of nothing.
- A step in the back row. Next to northern sea oats 'River Mist' (2½ ft): no two neighbours there are the same height, so the back steps up and down like a skyline.
- 3 plants in one clump. A single spire is too slight to hold a place alone, so here the accent is a clump.
Why now
- In flower for five weeks. From mid July to mid August.
- Part of the bed's peak. With bee balm 'Purple Rooster' and northern sea oats 'River Mist' in late July: 3 kinds at once. No week has more.
Planned as a cottage garden: 28 of the 28 plants (100%), bulbs aside, stand alone or in groups of no more than five, small enough for the kinds to mingle. The flowers run from early March to mid October, but stop for 3 weeks from early April. Neither end outweighs the other: the visual weight is split 52 to 48, left to right.
The planner's own design check: 12 Excellent, 4 Good, 1 Satisfactory.
Open this bed in the planner Change its size, the sun or the style, move plants about, or swap one for another.
Where monkshood goes in a border
At 4 ft it belongs at the back of a bed. In the planner's ten height classes it is moderately tall (3 to 4 ft).
Its job: accent. An upright plant used singly, like a punctuation mark, and repeated at intervals to give the bed rhythm.
It gives the true blue of a delphinium in shade and without staking, which is why gardeners put up with its toxicity. Keep it at the back, away from paths, and wear gloves to cut it down.
- Blue spires for part shade, where delphiniums fail
- Late-summer height
- The back of a cool, moist border
What to plant with monkshood
Plants recorded as good partners for it, and what each pairing does:
- Astilbe: low enough (24 in) to stand in front of it; happy in the same moist soil.
- Japanese anemone: happy in the same moist soil.
- Hosta: its green leaves are a setting for the flowers, before and after; bold leaves against its medium ones; a low mound at the foot of its spires.
Questions about monkshood
Does monkshood need sun or shade?
Monkshood grows in full sun (6 or more hours of direct sun a day), part sun (4 to 6 hours of sun, including some afternoon sun) and part shade (2 to 4 hours of sun, mostly in the morning).
It wants soil that stays evenly moist, rich in organic matter, and a good soak in dry spells.
What zones does monkshood grow in?
USDA zones 3 to 7. It is hardy to zone 3, which means it survives winter nights down to about −30 °F (−34 °C).
Above zone 7 it struggles: the zones measure winter cold, but a plant's top zone usually means the summers are too hot or the winters too mild for it to rest.
Gardening in Canada? Use the USDA zone for your town: Canada's own zone map uses different numbers.
When does monkshood flower in my zone?
In zone 5 it flowers mid July to mid August. As a summer flower it moves about half a week for every zone: earlier where it is warmer.
| Zone 3 | Late July to late August |
|---|---|
| Zone 4 | Late July to late August |
| Zone 5 | Mid July to mid August |
| Zone 6 | Mid July to mid August |
| Zone 7 | Mid July to early August |
Worked out from Hopkins' bioclimatic law: spring arrives about four days later for each degree of latitude north, and one zone is roughly two degrees. A warm spring or a cold one moves any garden two weeks either way.
How far apart should monkshood be planted?
18 in (45 cm) apart, from the middle of one plant to the middle of the next. That is about its full width, so each plant keeps its own outline.
It grows 4 ft (120 cm) tall. Give it the full distance from anything taller, or it will lean out towards the light.
How many monkshood plants do I need?
At 18 in apart, set out in staggered rows:
- 5 to fill 10 square feet (about 1 m²)
- 7 for a single row 10 ft (3 m) long
- 17 to fill a 4 × 8 ft bed on its own
In a mixed border, plant it as one at a time, repeated along the bed at intervals; a slender one reads better as a clump of three.
In the bed above, the planner used three in one group.
Is monkshood poisonous to dogs and cats?
Yes: severe toxicity to people, dogs, cats and horses. All parts are extremely poisonous if eaten, and the poison can be absorbed through the skin. Wear gloves to handle it, and never plant it near vegetables or where children play.
What flowers after monkshood?
Monkshood is over by mid August. These start as it finishes and carry the same spot on, so it never goes flat:
- Japanese anemone: from mid August to early October, 3 ft tall, to stand in front of it and cover its fading stems (a recorded partner).
- Garden chrysanthemum: from mid September to late October, 24 in tall, to stand in front of it and cover its fading stems.
- Lilyturf: from early August to mid September, 16 in tall, to stand in front of it and cover its fading stems.
What does monkshood look like in winter?
Monkshood dies down to the ground and there is nothing to see until spring. The old stems can be cut in autumn, or left until spring as shelter for overwintering insects.
More Aconitum
- Autumn monkshood Aconitum carmichaelii 'Arendsii'
- Bicolor monkshood Aconitum x cammarum 'Bicolor'
In these guides
- Garden plants toxic to cats
- Garden plants toxic to dogs
- Tall perennials and grasses for the back of a border
- Plants with spires: vertical accents
- Plants with blue flowers
Every fact on record
- Type
- Perennial
- Size
- 4 ft (120 cm) tall, 18 in (45 cm) wide
- Plant
- 18 in (45 cm) apart
- Height class
- Moderately tall (3 to 4 ft)
- Place in the bed
- Back
- Flowers
- Mid July to mid August; upright spires; blue
- Leaves
- Medium texture, herbaceous; dark green
- Shape
- Upright, clumping
- Hardy in
- USDA zones 3 to 7
- Light
- Full sun, part sun, part shade
- Soil
- Rich, evenly moist
- Water
- Moderate
- Feeding
- Light, in spring
Checked against VASCAN (plants of Canada). Flowering times are for zone 5: later in colder zones, earlier in warmer ones. Toxicity and spread information is a general gardening guide, not veterinary or medical advice, and invasive status varies by region. If a person or pet eats a plant, contact poison control or a vet. Check your local invasive species list before planting anything with a spread warning.