Acanthus mollis is the old-fashioned oyster plant. From a wide clump of large, glossy, jagged-edged leaves, arise spires to a height of a metre or more, of white flowers delicately veined with purple and held within purplish-brown shell-like bracts. It is in bloom from late spring to early summer. Its foliage forms an attractive groundcover below trees or between shrubs, tolerating root competition quite well. It is best not to let them be overgrown by nearby plants - give them enough room to show the architectural features of the foliage and flowers. On the other hand, the huge leaves can at times swamp nearby plants - one idea is to remove some of the leaves at the base. it doesn't seem to affect the vigour of the plant! The leaves sometimes disappear for a time after flowering but soon return and they are particularly luxuriant in winter. A member of the broader Acanthaceae plant family, most of which grow very well in Sydney, it shares with its relatives an ease of cultivation; an ability to grow in shady places; lush and attractive leaves; and decorative two-lipped flowers, often surrounded by distinctive bracts.
The oyster plant looks its best in shade or part shade. It may flower less well in heavy shade. It needs reasonable moisture, and protection from snails. Otherwise, it is an undemanding plant. However, it is very hard to eradicate if you ever decide to get rid of it, as the tiniest piece of root left behind will become a whole new plant. To propagate it, put a spade into the ground near its base in autumn to cut a few roots: baby plants will appear in spring and can be dug up. It is a keen self-seeder, so its flower spikes should be removed and destroyed (not composted) before they set seed. The flower spikes can be used as cut flowers in a vase.
The distinctive dark/light tonal colour scheme of its flower spires suggests some interesting possibilities for companion plants. A nearby substantial shrub with purplish-brown leaves - such as Euphorbia cotinifolia or smaller-growing Alternanthera 'Little Ruby' could mirror the hue of the acanthus's flower bracts. The contrasting whiteness of the flowers of the oyster plant can be echoed by other white flowers in bloom at the same time in shady spots, such as frothy renga-renga lilies (Arthropodium cirratum, ht 60cm). Some of the oyster plant's own relatives also provide suitable companions for it: one that flowers at the same time is the forest bell bush (Mackaya bella, ht 1.5-2.5m) with pale lilac-white, bell-like flowers, finely etched with purple, the same shape as those of the oyster plant. Shrubby winter-flowering goldfussia (Strobilanthes anisophyllus, ht 1.5m) has slim, dark purple-suffused leaves which echo the dark bracts of the oyster plant's flowers.
Various cultivars of the oyster plant exist: a lovely white-variegated leaf form (which I have seen growing most effectively combined with a lush plain green leaved rhizomatous Begonia, though it doesn't seem as vigorous as the species; robust 'Hollard's Gold', which has beautiful lime-gold leaves in winter and early spring (they turn more green with age); and 'Bendigo Spires', said to grow to 3m tall! The cultivar known as 'Beth Chatto's Form' is said to be a more compact plant; I have not yet tried it.
The leaves of Acanthus mollis inspired the design of the tops of Corinthian columns in ancient times. The story goes that a few plants were growing next to some marble pillars that lay on the ground in Ancient Corinthia, awaiting a few finishing touches, and that the leaves' form was carved into the top of these columns. The name Acantha comes from a Greek legend: Acantha was a nymph who scratched the face of amorous Apollo in an effort to fend him off, and was turned into a plant with spiky leaves!