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Acanthus - Care Guide
Acanthus are perennial herbaceous plants from the Mediterranean region which are striking architectural plants but, make no mistake, they are actually also fairly invasive garden ‘thugs’ with quite nasty prickles even on their flower spikes. As such, we tend to position them in dark corners, rubbish dumps or to shield something unpleasant from view. This may be a bit unfair, but it is certainly justified. Just the thing in fact to deter an intruder from your garden edge.
With us Acanthus mollis and Acanthus spinosus only die down completely in an extremely cold winter. Cutting them back hard to ground level in the autumn does not prevent immediate regrowth from their fleshy roots. If you get fed up with this spreading thug and try to dig it out do not think that the roots will not reshoot and invade your compost heap just as they have done on ours in several locations. Every fleshy root needs to go onto the bonfire to ensure containment or eradication. Yes, as you will have guessed, I am not a fan of this plant, whatever its undoubted merits, particularly as a dried flower when the 3-4ft tall flower spikes with their purple shaded bracts and stems are cut and dried just before the flowers emerge.
A. mollis is a vigorous clump forming species achieving heights of up to 5ft with its flowering spikes. Its leaves are deeply lobed and dark green. A. spinosus grows to a similar height and its flowers are similar but the leaves are more finely cut with more nastily spiny margins.
This plant propagates itself and it is hard to imagine anyone wanting to let it spread more than it does but the roots will reshoot when dug and chopped up into almost any size. It will grow absolutely anywhere in poor or rich soils, in shade or in full sun. The reference books recommend a ‘spacious’ border to grow it in. We would recommend starting it on the rubbish dump where its invasiveness can do no real harm. Strim the hell out of it once a year and take immense satisfaction at that! If you enjoy using a ‘Poldark’ scythe instead make sure you are wearing protective clothing!
Under drought stress they will sometimes get powdery mildew but this, sadly, has few lasting or terminal effects.