How to grow aconitums, or monkshood

Aconitums shouldn’t be shunned due to their toxicity because their flowering spikes, which come mostly in shades of blue, are extremely handsome.

There are whites, which are poor things in my opinion, and a rose-pink or two. However, it’s the blues or silver-blues that steal the limelight.

Where to plant

Aconitums, like many members of the buttercup family, prefer deep fertile soil and some shade due to their woodland-edge provenance, and their roots like to be cool and moist in summer. They can also flower in deep shade. Those who garden on lighter soils should add copious amounts of garden compost for moisture retention. You can also mulch around crowns in spring.

They can rot in cold, wet soil and in the wild they are usually found on slopes where water can drain away.

Growing from seed

Seeds can be sown when ripe and they will normally germinate in the following spring.

When they flower

Some aconitums flower in May or June, others repeat-flower, and some are very late. Growing a range is an excellent idea.

Space should be found for taller, September-flowering varieties which can reach 6ft. They come in a range of blues, perform in shade and complement tall, late-season grasses that include Miscanthus sinensis. The late Alan Bloom pioneered this combination at The Dell Garden in Bressingham, Norfolk, years before grasses were fashionable. He admired The taller September-flowering aconitums and let them rub shoulders with substantial plumed grasses that included pampas grass (Cortaderia selloana 'Sunningdale Silver') and tall Miscanthus sinensis varieties such as 'Silberfeder'.

Caring for aconitums

Some aconitums develop ragged lower leaves in dry summers and this is the nature of the beast. Hide them behind things so that it doesn’t show.

Most aconitums do not need staking. Cut faded stems back in late autumn to prevent wind rock from loosening roots. This will also prevent unwanted seedlings.

After flowering

Cut back the stem after flowering to prevent unwanted seedlings, but always handle with care.

They are all bee-friendly and will often set seeds. Dead heading will prevent inferior seedlings popping up.

When to divide

Divide plants carefully just as they start into growth in late February or early spring. Handle any with tuberous roots carefully, replanting vigorous pieces, and divide those with fibrous roots using two forks back to back. Then wash your hands.

Plant with

The deep blue varieties, including 'Spark's Variety', look vibrant emerging behind the crimson torches of Persicaria amplexicaulis Taurus (syn 'Blotau').

These deep blues also flatter pastel roses, which could include the apricot 'Buff Beauty' or the shorter floribunda 'Champagne Moment'.

Varieties

There are 300 species of aconitum found in the temperate regions of the Northern hemisphere, mostly with lobed leaves and roots that are mostly tuberous or bulbous, though not always. We have a questionable British native, Aconitum napellus, which is found in damp woodlands in the western half of England and Wales.

Crûg Farm Plants have introduced several wild species into cultivation, mainly from Korea, and these include A. jaluense which Bleddyn Wynne-Jones describes it as “bearing the largest flowers we have yet seen.” It was found growing on the island of Kõjedõ, South Korea where it carpeted the ground as far as the eye could see, on wooded hill sides.

Early aconitum varieties

These varieties flower in May, June or early July

'Stainless Steel'
A silver-grey that shines in shade. This easy perennial began to be grown here in the 1990s and was grown on the Continent having been found by Aad and Elly Geerlings from Holland.

A. x cammarum 'Bicolor'
This bears a loose spire of fresh white flowers edged and flushed in violet-blue. Very reliable and very crisp in summer, although the flower heads are gappy. Needs regular division and moving about.

Mid-season aconitum varieties

These varieties flower in July and August.

A. hemsleyanum
This normally blue twiner, collected near Mount Omei in China, is hard to grow well unless your garden is wet and warm in summer. There is also a wine-red form, ‘Red Wine’. Easy to raise from seed.

‘Bressingham Spire’
Raised by Alan Bloom in 1957, with very dark flowers held on a narrow spike. It is thought to be a cross between A. napellus and ‘Newry Blue’. The true form is willowy and slender.

‘Spark's Variety'
An old plant (although still brilliant) launched in 1898 by the Maurice Prichard’s Riverslea Nursery, once based at Christchurch in Hampshire. This famous nursery also launched the chalk-pink Geranium x riversleaianum 'Russell Prichard' (circa 1915) and the violet Campanula lactiflora 'Prichard's Variety', both garden classics. I am unsure why this plant was named Spark's, but it’s unique with a main stem that supports well-spaced side branches that branch out at an angle of 45 degrees. It will give you glints of deep-blue over many weeks and should be in every garden.

Late aconitum varieties

These flower in September to October.

A. carmichaelii 'Kelmscott'
This rich lavender-violet monkshood, reaching 5 ft (150 cm), raised by J. Barker of Kelmscott near Ipswich.

A. carmichaelii ‘Barker’s Variety’
Also from J. Barker, but a stable seed-raised aconitum with lavender flowers.

A. carmichaelii ‘Wilsonii’
A late, true-blue tall aconitum often flowering in October.

A. carmichaelii 'Spätlese'
A paler, grey-blue aconitum that can flower as early as August - before the others.

A. carmichaelii Arendsii Group
This is a name for seedlings rather than the true plant. All form upright, man-high spires with enormous soft-blue flowers. The stiff stems are self-supporting and attractively downy with handsome leathery green foliage.

Did you know...?

The hooded, helmet-shaped part of the flower, which gives the aconitum the common name of monkshood, is a weather-resistant sepal rather than a petal.

Aconitums are are also known as wolf’s bane, indicating that this plant contains enough poison to kill a wolf.

The Anglo-Saxon name thung also means poisonous. Akon translates as dart, from the Greek. The potent alkaloid toxin may well have been daubed on arrow and spearheads and, although the roots are the most toxic part of the plant, great care must be taken when handling the flowers and stems too.

There is an upside. Deer and rabbits leave this plant well alone.