Acaena species - Pirri-pirri-burs

There are four species of these aliens from New Zealand and Australia. They escape from gardens and can easily be established and spread due to the spines attached to the seeds. So they can be found where garden waste is discarded but also alongside forestry tracks well away from houses.

The crucial distinguishing features are the shape of the last pair of leaflets, the number of teeth (count round the leaf including both sides) and the colour and shininess of the leaves. The commonest are Acaena novae-zelandiae and especially Acaena ovalifolia.

Acaena ovalifolia Two-spined Acaena is the commonest species in our area. The leaves are glossy green on upper side, with (7-)9(-11) leaflets, apical pair 10-30mm long and 1.7-2x as long as wide, with (11)17-23 teeth. Has two barbed spines on the hypanthium.

Acaena anserinifolia Bronze Pirri-pirri-bur has distinctive bronzy tinged leaflets. Apical pair of leaflets 3-10mm 1.2-2x as long as wide, spines mostly 3-10mm, spines mostly 3.5-6mm often 1 or 2 much shorter.

Acaena novae-zelandia Pirri-pirri-bur procumbent woody stems. mat forming and ascending herbaceous shoot. Leaflets glossy green 9-13(15) 1, the apical pair 5-20mm, 1.8-2.5x as long as wide, with 5-12(15) teeth. Spines up to 4,  mostly 6-10mm with 1 or 2 much shorter.

Acaena inermis  Spineless Acaena Has leaflets (7-)11-13, bluish grey-green-tinged brown or orange, the apical pair 2-8mm, as long as wide, with 5-10 teeth. Spines are usually 0 and ±not barbed. (photo awaited)