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{{Taxobox
| color = lightgreen
| name = ''Acaena''
| image = Acaena novae-zelandiae1.jpg
| image_width = 250px
| image_caption = ''[[Acaena novae-zelandiae]]'' foliage and various fruiting stages
| regnum = [[Plant]]ae
| divisio = [[Flowering plant|Magnoliophyta]]
| classis = [[Magnoliopsida]]
| ordo = [[Rosales]]
| familia = [[Rosaceae]]
| subfamilia = [[Rosoideae]]
| tribus = [[Sanguisorbeae]]
| subtribus= [[Sanguisorbinae]]
| genus = '''''Acaena'''''
| genus_authority = [[Carolus Linnaeus|L.]]
| subdivision_ranks = [[Species]]
| subdivision =
See text.
}}

'''''Acaena''''' is a genus of about 100 species of [[perennial plant|perennial]] [[herb]]s and [[subshrub]]s in the [[Rosaceae]], native mainly to the [[Southern Hemisphere]], notably [[New Zealand]], [[Australia]] and [[South America]], but with a few species extending into the [[Northern Hemisphere]], north to [[Hawaii]] (''A. exigua'') and [[California]] (''A. pinnatifida'').

The [[leaf|leaves]] are alternate, 4-15 cm long, and pinnate or nearly so, with 7-21 leaflets. The [[flower]]s are produced in a tight globose inflorescence 1-2 cm diameter, with no petals. The [[fruit]] is also a dense ball of many [[seed]]s; in many (but not all) species the seeds bear a barbed arrowhead point, the seedhead forming a [[burr (fruit)|burr]] which attaches itself to [[animal]] [[fur]] or [[feather]]s for dispersal.

;Selected species
{|
|- valign=top
|
*''Acaena adscendens''
*''Acaena anserinifolia''
*''Acaena argentea''
*''Acaena buchananii''
*''Acaena caesiiglauca''
*''Acaena exigua'' - Liliwai
*''Acaena fissistipula''
*''Acaena glabra''
*''Acaena inermis''
*''Acaena laevigata''
*''Acaena lucida''
*''Acaena magellanica''
*''Acaena microphylla''
|
*''Acaena myriophylla''
*''[[Acaena novae-zelandiae]]'' - [[bidibid]]
*''Acaena ovalifolia''
*''Acaena ovina''
*''Acaena pallida'' - Sand [[bidibid]]
*''Acaena pinnatifida'' - Sheepburr
*''Acaena platyacantha''
*''Acaena pumila''
*''Acaena saccaticupula''
*''Acaena sanguisorbae''
*''Acaena sericea''
*''Acaena splendens''
*''Acaena trifida''
|}

===Invasive species===
Some species have been introduced accidentally to other areas, attached to [[sheep]]'s [[wool]], and have become [[invasive species]]. ''A. novae-zelandiae'', the [[bidibid]] from New Zealand, is the most commonly encountered species in [[Great Britain]], where it is often abundant on coastal sand dunes, crowding out native vegetation and creating an often painful nuisance with the barbed burrs. In California, ''A. pallida'', from New Zealand and southeast Australia, is similarly a problem species.

[[Category:Rosaceae]]
[[Category:Pantropical flora]]