Acanthus mollis (bear's breeches): how to grow it, and how hard it is to remove
Quick answer: Acanthus mollis is a hardy shade perennial, 1 to 1.2m tall and about a metre across, happiest in partial shade in ground that holds moisture but drains. Ours is $16.50 plus GST. The part most listings leave out: it regrows from root fragments, so once a clump is established, removing it takes two or three seasons of digging rather than one afternoon.
Most older properties have a corner nothing wants to grow in. Shaded for half the day, dry under the eave, soil that is more builder's fill than topsoil. Whatever gets planted there sulks for two seasons and then dies, and you plant something else, and that dies too.
Acanthus mollis will fill it. Leaves up to 50cm long and 30cm across, deeply cut, dark and glossy enough to look wet an hour after rain. In early summer a flower spike stands up out of the middle of the clump, white flowers under purple hoods stacked the whole way up, and it holds them for weeks.
It is a good plant, and we grow it.
It is also one of the few things on our list where the downside matters more than the growing notes, so that is where this starts.
What is bear's breeches, actually?
Not a native, and not related to anything you would guess from the leaf.
Acanthus mollis comes from south-west Europe and North Africa. It reached New Zealand as an ornamental and was recorded as naturalised in 1958. The leaf is the reason it has been planted for two thousand years: it is the leaf carved into the top of a Corinthian column, which is a long innings for any piece of foliage.
The plant is a clump-forming perennial herb growing to about 1.2m. Leaf stalks are long, 10 to 50cm on the leaves coming straight out of the crown, much shorter on the leaves up the flowering stem. Flowering is October through to April, with seed capsules 3 to 4cm long forming from November into May. Underneath it all sits a set of thick, fleshy roots.
Where does it grow well, and where does it struggle?
Partial shade, in ground that stays moist without staying wet.
Our own growing notes put it at frost hardy, medium wind tolerance, well drained to moist, preferring partial shade but tolerating sun. The RHS rates the genus H6, which is hardy through anything a New Zealand winter produces. Cold is not the limiting factor here, anywhere in the country.
Shade and sun change the plant rather than deciding whether it lives:
In shade you get bigger leaves and fewer flower spikes. NZPCN records it as tolerant of moderate shade, flowering best with at least partial light.
In full sun you get more spikes and smaller leaves, and in a hot dry position the leaf edges can scorch.
So a bed on the south side of a house in Balclutha, cool and cloudy and reliably damp, gives you the foliage plant. A bright spot with a bit of afternoon shelter gives you the flowers.
Two places where I would think twice. Arrowtown and the harder inland sites give you a plant that dies right back to nothing over winter and stays gone until well into spring, which is a long empty patch if it is the only thing in the bed. And a dry, root-filled position under established trees in Rangiora country, where the nor'wester takes what moisture there is, tends to bring on powdery mildew rather than growth. Dry soil is what triggers it.
Deciduous or evergreen depends on where you are. It generally drops its leaves in colder districts and can hold them through winter in mild ones.
Why is it so hard to get rid of?
Because every piece of root you leave behind is a new plant.
That is not a figure of speech. Washington State University's extension guidance classifies it with the groundcovers for exactly this reason: any piece of root cut from the original plant can contribute to further spread. The New Zealand Plant Conservation Network says the same thing, that root fragments grow easily and an established plant can be hard to eliminate from a site.
Add deep roots to that and you get the problem. You dig, you get most of it, you miss a finger-thick piece at 400mm down, and next spring there are two plants where there was one.
The practical consequences, if you ever have to take one out:
Do not rotary hoe the patch. A rotary hoe turns one plant into fifty pieces of root and plants all of them for you.
Sieve the soil as you go and lift every fragment you can find, not just the crown.
Expect to come back. Removing every bit of root over two or three growing seasons is what does it, not one big dig.
Do not compost the material. Weedbusters' control advice is refuse, burnt, or buried deeply.
If you want to go the herbicide route, that is a conversation with your supplier rather than something I will prescribe here.
It is still a good plant. Put it where you want it in ten years and none of the above ever comes up.
Is bear's breeches a weed in New Zealand?
It is on DOC's 2024 list of 386 environmental weeds in New Zealand.
That list is not a ban and it is not the Pest Plant Accord. Acanthus mollis is legal to sell and plenty of New Zealand nurseries carry it, ourselves included. What sits behind the listing is this: the plant forms large clumps, spreads fast in disturbed ground, and shades out small native plants in shady sites. It turns up on riverbanks, roadsides in partial shade, and in pasture. Seed moves by gravity, water and birds, and root fragments move in soil and green waste.
What that means for where you put it:
Fine in a bounded garden bed, a courtyard, a townhouse strip, a bed between a building and a concrete path.
Not against a bush margin, a gully, a reserve boundary or a waterway.
Cut the spent flower spikes off before the capsules ripen if you are anywhere near any of the above. Flowering goes October to April, seed November to May, so a spike cut in late summer never gets there.
Anything you dig out goes to refuse, not over the fence and not into the green waste bin.
How do you plant it so you don't regret it?
Pick the position on the basis that it is staying.
The three positions that work:
1. A bed with hard edges on at least three sides. Building on one side, concrete or paving on the others. The roots go down but they cannot travel sideways into lawn or a neighbouring bed.
2. Inside a root barrier. The same product used for running bamboo. Dig the hole, set the barrier, plant inside it.
3. A large container. Contained absolutely, and it will take the shade a pot on a courtyard usually sits in. It will fill the pot and want splitting every few years.
The rest of the planting is ordinary. Get some organic matter into the ground, plant at the same depth it sat in the pot, water it through its first summer. Autumn is the best window for this sort of thing down here, and mid-winter is fine too. Lift and divide clumps in autumn or spring when the middle gets congested.
Two things to watch. Slugs and snails go for the new spring leaves, and a shaded damp bed is where they live. And it takes a season or two to settle after being moved or divided before it flowers properly, so a plant that produced nothing in its first summer is not a failure yet.
Is there a native that does the same job?
Not really. The bold, deeply cut, glossy leaf in shade is not something the New Zealand flora offers in a herbaceous perennial. Two things get part of the way:
Myosotidium hortensia (Chatham Island forget-me-not). Big, ribbed, glossy leaves and blue flower heads in spring, and it wants exactly the cool dappled shade and steady moisture that suits Acanthus. The limit is frost. It is half hardy, takes light frost, and needs protection when young and shelter from cold drying wind, so it is a coastal Otago or Southland-coast plant rather than an inland Central Otago one. It also needs more care than Acanthus ever will.
Pseudopanax arboreus (five finger). Glossy fans of five to seven leaflets, tolerant of moderate frost and happy from full sun through to partial shade. But it is a small tree of 3 to 6m, shallow rooted, and it needs shelter from strong wind. It will not fill a 600mm gap in a border.
If the brief is bold foliage in a difficult shaded corner and it has to be a native, change the plant type. There is no like-for-like swap to find.
Common questions
Can I put the leaves and roots in the compost?
No. Weedbusters' advice is that all plant material goes to refuse, gets burnt, or is buried deeply, and that is about the root fragments rather than the leaves. A home compost heap will not reliably kill a piece of Acanthus root.
Will it grow in full sun?
Yes, and it will flower better there. You get smaller leaves for it, and in a hot dry spot with no afternoon relief the leaf margins can scorch. Sun with some moisture in the ground is the sweet spot.
Why has mine never flowered?
Two usual reasons. Deep shade will give you a beautiful leaf and very few spikes, because the plant needs at least some direct light to flower well. And a clump that has been moved or divided recently often sits out a season before it flowers again.
Does it die down completely over winter?
In colder districts, generally yes. It behaves as a deciduous perennial where winters are cold and can stay semi-evergreen in mild areas. Mark the spot, or you will put a spade through the crown in August and then discover you cannot kill it anyway.
Can I grow it in a pot to stop it spreading?
That is the simplest containment there is, and it suits the plant well enough given it tolerates shade. Give it a decent volume of mix, keep the water up in summer, and plan to split it every few years when it fills the pot.
Before you order
Bear's breeches is a straightforward plant to grow and a slow one to remove. If you know that going in, and you put it somewhere with edges, it will hold that difficult corner for as long as you own the place.
If you are ordering for a job near a reserve boundary, a gully or a waterway, pick something else.
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