Invasive Alien Species in Northern Ireland

Acaena ovalifolia, Two-spined acaena

Acaena ovalifolia Ruiz & Pav.
Acaena is a genus of around 100 species of southern hemisphere plants in the Rosaceae, the rose family; they are perennial, low, creeping herbs and subshrubs with prickly seeds. The most commonly encountered species in Northern Ireland is the South American Acaena ovalifolia. This species was first described by Ruiz & Pavon in 1798 and its introduction to the British Isles may well have been by two means – as a deliberate import for garden ornament, primarily as a rockery plant, and also as an accidental introduction in raw wool bales from the southern hemisphere. Whatever the means of introduction to Britain, through subsequent careless disposal of garden waste, and also by being transported on the clothing of textile workers, the plant is now present in the wild in Britain, where it has proved to be a very successful coloniser of light woodland conditions and, consequently, a nuisance in those conditions.
Several other similar Acaena species and cultivars are grown in gardens in Northern Ireland, and have also become nuisances in the wild. The commonest is the New Zealand species A. novae-zelandiae, pirri-pirri bur, bidi-bidi.
Plant with:
stems — of both woody and herbaceous creeping and rooting habit (and occasionally rhizomes), prostrate or sometimes ascending, green, occasionally flushed red, with long fine white hairs
leaves — more or less oval, pinnate, 5-12cm long, divided into 7-11 toothed leaflets
stipules (leaflets at base of petiole) — with a section 3-5mm long fused to the petiole
leafletsupper surface bright green, finely wrinkled and sometimes glossy when young, hairless, – lower surface covered with soft silky hairs, especially on the midrib and main veins, the two outermost pairs of leaflets elliptic/oblong, 15-30 x 9-16mm, with 17-23 acute teeth, clefts between the teeth extending up to one-third of the way to the midrib of the leaflet
scape (flower stem) — 6-12mm long in fruit, green and densely appressed-hairy
flowers — tiny, without petals, with purple anthers, crowded on to a spherical inflorescence c.8-10mm diameter in flower, 18-30mm diameter in fruit (including spines); each flower with two spines, 8-10mm long, red, barbed, at the top of the receptacle
fruit — the head readily breaks up when the fruit is mature; each individual fruit has two barbed spines.
Country of origin
South America, from the Magellan region northwards through the Andes to Colombia, including the Falkland Islands.
Current distribution
Acaena spp. are endemic to the southern hemisphere, but have now become serious weeds in some northern hemisphere countries including the USA, UK and Ireland.
Location in Ireland
Scattered across Ireland in suitable habitats — open woodland, forestry plantations and tracks, sand dunes, railway cuttings and embankments etc. often with well-drained, gravelly surfaces.
Life cycle
Reproduction is from seed, and sometimes from pieces of rooted stolon. It freely produces viable fruit which can attach to clothes, shoes etc by means of the long spines. It can also be propagated vegetatively as it has a creeping and rooting rhizomatous stem.
Wildlife and habitat impacts
In its own native continent, it is proving a highly invasive alien on the IUCN-designated World Biosphere Reserve of Juan Fernandez Island off the coast of Chile, competing with native grasses and endangering elements of an extremely vulnerable endemic island flora.
Because it is an aggressive coloniser, it can form large pure colonies which swamp other low-growing native species. This loss of native species will lead to the impoverishment of habitats and the concomitant loss of hostplants for native animal life.
It is illegal to introduce this plant into the wild in Northern Ireland.
Human impacts
Key vectors
Dispersed by means of barbed receptacles containing the achenes (seeds), which are easily detached from the seedhead; these catch on to the coats and feet of animals or the clothes and shoes of humans, or mud in vehicle tyres, etc. The plant can also spread vegetatively, as displaced stems and rhizomes will root readily in suitable environments this may be the original source of some colonies.
What you can do as an individual
Do not dispose of garden plants at waste disposal sites — burn them instead.
Try to discourage garden centres from selling this species.
Exercise good environmental hygiene by cleaning walking boots, socks, etc after visiting sites where the plant grows.
Advice to specific groups relevant to this species
For land managers and staff involved in environmental projects, good hygiene as described above is important when working in areas where this plant is a problem.
Management measures
It may be possible to control this locally by application of suitable weedkillers such as glyphosate.
Management groups
Managers of large estates, forestry plantations, forest parks and woodland or sand dune nature reserves should be aware of the problems associated with Acaena species.
Further information
BSBI Atlas maps scheme gives up to date distribution maps for the whole British Isles
Manaaki Whenua - Flora of New Zealand web site with identification keys for NZ species at
Other Habitas web sites:
Garden Flora of Northern Ireland
Flora of Northern Ireland
Preston, C.D., Pearman, D.A. and Dines, T.D. (2002). New Atlas of the British & Irish Flora. Oxford University Press.
Yeo. P.F. (1973). 'The species of Acaena with spherical heads cultivated and naturalized in the British Isles' in Plants, Wild and Cultivated, Ed. P.S. Green. BSBI (Classey).
Rich, T.C.G. and Jermy, A.C. (1998). Plant Crib. BSBI. pp. 154-5 (also accessible on line at
Text written by:
Catherine Tyrie, Curator, Botany, Ulster Museum