Acanthus are mainstays of the herbaceous border, providing statuesque clumps of handsome, lobed basal leaves and spikes of flowers hooded by papery bracts. Most gardeners are aware of the more common species A. mollis and A. spinosus, but there are also some rarely seen colourful dwarf species that can be used in smaller spaces such as a rock garden.
There are 30 species of acanthus, which are generally perennial herbs. But there are a few shrubby types, some of which are even found in mangroves. They tend to have basal, arching, dark-green leaves that can be either glossy, glaucous or covered in downy/bristly hair.
The leaves are large with a pinnatifid margin, often deeply lobed and sometimes spiny. They produce flowers that are commonly white with purple papery bracts that create a hood over the flowers.
There are a few with different coloured flowers, such as the red A. sennii, as well as varieties such as ‘Whitewater’ that has purely white flower heads.
The most commonly grown acanthus in UK gardens are mollis and spinosus. Both varieties are quite large plants, creating spreads of more than 1m, so need to be allowed significant space, ideally towards the back of the border.
A. mollis has larger leaves that are deeply lobed, soft and floppy, while those of A. spinosus are spiny, as its name suggests. A. mollis is semi-evergreen, sometimes holding onto its leaves through a mild winter but should it be knocked back it will reshoot in the spring.
You need to be aware that they can become invasive in a border so you should be certain that you really want them. If you try and dig plants out, little sections of the fleshy, brittle roots that get left behind will produce new plants.
A. spinosus is better behaved and therefore perhaps a better choice for the mixed border. It also produces a greater quantity of flower spikes. Opposite to A. mollis, it dies down in the autumn and will send up new growth in the spring.
Aside from the two well-known species there are a few interesting lesser-known types. One of these is A. sennii, a species from Ethiopia that has red flowers. Two small species that would make a nice feature in a rock garden are A. dioscoridis var. perringii, with deeply toothed grey to green leaves and intricate pink and grey to green flower spikes, and the hairy-stemmed A. hirsutus, which bears soft yellow flowers hooded with hairy green bracts.
Despite the lush look of the leaves such as those of A. mollis, which might make you think they are from wet climes, most acanthus, including that species, are found growing wild in dry, rocky locations, like a well-drained soil and do not appreciate being wet for a prolonged period of time in winter.
They can tolerate sun or shade but they will flower more profusely if they are given a sunny position, especially if they have been well baked in the previous summer. They are more likely to produce masses of large foliage in shady positions. The plants will produce long tap roots, which once established mean they are tough plants. Give new plants a thick protective mulch for the first two years.
The main problem affecting acanthus is powdery mildew. This is more prevalent if the plants are in very dry soil and positioned in the shade. It is best either to move them to a better spot or water them well during the growing season. Remove any leaves that become covered in the white mould.
What the specialists say
Sarah Millington, director, Hillview Hardy Plants, Shropshire
"Acanthus is a brilliant garden plant, but make sure it’s planted where it’s going to stay. There are enough different ones to fit into almost any garden. Some only grow to about 30cm wide and tall, and there are others that will easily fill a space of 150cm squared.
"There are a couple of species or varieties that particularly stand out for me. A. sennii, a native of Ethiopia but hardy in the UK, is very different to the herbaceous varieties, with scarlet flowers. A. spinosus ‘Lady Moore’ has beautiful pale, almost white, foliage in spring.
"They have better flowers in a sunny position and bigger leaves in the shade. Snails like to overwinter near the base of the leaves and will often nibble the flower spikes as they emerge. Watch out for powdery mildew. It can occur if they don’t have enough air movement around them in the summer."
Simon Rice, head gardener, Culham Court, near Henley-on-Thames
"I think A. mollis is a particularly handsome plant with its broad-lobed foliage. It always did well in my native Vancouver, but unfortunately on the dry, chalky soil we have here in Berkshire it suffers heavily with mildew, particularly A. spinosus, and I just wouldn’t plant it — and it can be a pernicious once it gets established. Because it will never look at its best here I tend to treat it as a weed and spray it.
"That said, in the right place and optimum conditions it will thrive, and when you see it growing well it’s quite a sight. Give acanthus the room it needs and once you’ve got it let it go, because you’re never getting rid of it. Monty Don recommends two golden-leaved varieties called ‘Fielding’s Gold’ and ‘Hollard’s Gold’, which are said to be highly attractive and less vigorous so ideal for the mixed border."
Species and varieties
A. dioscoridis var. perringii has deeply toothed grey to green leaves along with intricate pink and grey to green flower spikes between June and August. Height: 45cm.
A. hirsutus is a choice dwarf species of acanthus from Turkey that is quite different from the others, having hairy stems and bracts. It has long, narrow and deeply divided leaves, forming dense clumps. The flowering spikes carry soft yellow flowers hooded with hairy green bracts. Height: 40cm.
A. hirsutus subsp. syriacus (syn. A. syriacus) has short broad spikes of dark-maroon to purple flowers between July and September. Height: 45cm.
A. ‘Hollande du Nort’ is an easy-to-grow showy variety that does not get huge like some other acanthus. The leaves are deeper cut than most but not spiny. Height: 45cm.
A. hungaricus is a highly shade-tolerant species that produces upright, stiff flower spikes of white or pale-pink flowers, each with a green or purplish hood in summer, above mounds of deeply divided foliage. Height: 1.5m. Spread: 90cm.
A. mollis forms a large clump of shiny, pinnatifid-lobed, green leaves and produces spires of white flowers emerging from dusky purple bracts in late summer. Height and spread: 1.5m.
A. mollis Latifolius is a group with broad, shallowly lobed, shiny green leaves that are conspicuously veined and very long — up to 120cm.
A. ‘Morning’s Candle’ is a cross between A. mollis and A. spinosus, producing upright flower spikes of rich pink and white flowers on tall stalks in midsummer. Its shallowly scalloped, narrow grey-green leaves are slightly spiny. Height: 1-1.2m.
A. sennii is a lovely, relatively rare red-flowering acanthus. It has attractive silvery/green, deeply lobed, spiky foliage and dark-purplish to black stems. Its young growth is greyish pink with purple edges. Bright-red flowers are borne on a spike in later summer or early autumn. Height: 1.6m.
A. spinosus has large, deeply lobed and spine-tipped green leaves. Erect spikes of white flowers with spiny purplish bracts are produced in late spring and summer. Height: 1.2m.
A. ‘Whitewater’ is also known as variegated bear’s breeches because it has variegated green and white spiky leaves. It produces showy pink and white flowers on red stems.
Thank you to Floramedia, which supplied the images for this article from its photo library