Autumn monkshood
Aconitum carmichaelii 'Arendsii' ·
The last blue of the year: sturdy spires of rich blue hooded flowers in September and October, on stems that need no staking.
- Size
- 4 ft tall, 18 in wide
- Zones
- 3 to 7
- Light
- Part sun to part shade
- Flowers
- Mid September to mid October
- Pets
- Toxic (severe)
- Job in a bed
- Accent
A bed built around autumn monkshood
The planner designed this bed around autumn 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 autumn monkshood stands where it does.
The plants
- 1 Autumn monkshood × 3, blue
- 2 Bloody cranesbill 'Max Frei' × 8, pink
- 3 Creeping mahonia × 5, yellow
- 4 Purple milkweed × 3, purple
- 5 Inkberry 'Gem Box' × 1, dark green leaves
- 6 Northern sea oats × 5, tan
- 7 Double snowdrop × 12, white
Why this plant
- The only spires. The others are open bowls, dense clusters, flat plates 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 inkberry 'Gem Box' (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 six weeks. From mid September to mid October.
- Closes the season. For its last four weeks nothing else on the plan is in flower.
Planned as a cottage garden: 25 of the 25 plants (100%), bulbs aside, stand alone or in groups of no more than five, small enough for the kinds to mingle. Something is in flower for 33 weeks between early February and mid October, with a pause of 3 weeks from late March. The scheme leans cool (blue, pink and purple) and uses yellow for warmth, with tan and white to calm things between them.
The planner's own design check: 13 Excellent, 2 Good, 2 Satisfactory. It flags one thing to watch: Autumn monkshood is highly poisonous to people, dogs, cats and horses. Northern sea oats spreads aggressively.
Open this bed in the planner Change its size, the sun or the style, move plants about, or swap one for another.
Where autumn 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.
Blue is rare in autumn, when most late flowers are yellow, russet or pink, so it lifts everything near it. It shares the extreme toxicity of all monkshoods: plant it at the back and handle it with gloves.
- Autumn colour for part shade
- A late partner for Japanese anemones
- The back of a moist border
What to plant with autumn monkshood
Plants recorded as good partners for it, and what each pairing does:
- Japanese anemone: in flower together for four weeks, from mid September; happy in the same moist soil.
- Toad lily: in flower together for six weeks, from mid September; low enough (2½ ft) to stand in front of it; 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 autumn monkshood
Does autumn monkshood need sun or shade?
Autumn monkshood grows in 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 autumn 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 autumn monkshood flower in my zone?
In zone 5 it flowers mid September to mid October. As an autumn flower it keeps much the same dates in every zone: it answers to shortening days and cool nights more than to the warmth of spring.
| Zone 3 | Mid September to mid October |
|---|---|
| Zone 4 | Mid September to mid October |
| Zone 5 | Mid September to mid October |
| Zone 6 | Mid September to mid October |
| Zone 7 | Mid September to mid October |
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 autumn 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 autumn 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 autumn 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 autumn monkshood?
Autumn monkshood is over by mid October. These start as it finishes and carry the same spot on, so it never goes flat:
- Bodnant viburnum 'Dawn': from early November to late December, 10 ft tall.
- Common witch hazel: from mid October to late November, 15 ft tall.
What does autumn monkshood look like in winter?
Autumn 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
- Bicolor monkshood Aconitum x cammarum 'Bicolor'
- Monkshood Aconitum napellus
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 that flower in fall
- 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 September to mid October; upright spires; blue
- Leaves
- Medium texture, herbaceous; dark green
- Shape
- Upright, clumping
- Hardy in
- USDA zones 3 to 7
- Light
- Part sun, part shade
- Soil
- Rich, evenly moist
- Water
- Moderate
- Feeding
- Light, in spring
Not yet checked against an outside reference: its figures rest on general horticultural knowledge. 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.