Piri-piri-burs
What are they?
These low-growing, mat-forming plants are mostly native to the southern hemisphere and are members of the rose family (Rosaceae). Although they mostly hug the ground, they are considered to be sub-shrubs because they form perennial, woody stems that put out short, herbaceous growths of leaves and flowers. The plants are famous - or infamous, perhaps - for their spiny seedheads with backward-pointing barbs. These seedheads readily attach themselves to the fur of wild mammals or domestic livestock and will attach to socks in the same way! The spines became stiff and rigid with age and can be quite painful if they get into the skin.
Where are they found?
At least a dozen species are grown in the UK as garden plants but in the East Anglian region, only one species has so far been recorded in the wider countryside. These plants can be very aggressive in the garden, causing gardeners to remove them when they get out of hand. Unfortunately this sometimes results in the unwanted plants being dumped under cover of darkness, so grassy or open areas close to country car parks and lay-bys are the most likely places for these plants to appear.
Identification
These are all low-growing, creeping plants with pinnate leaves and distinctive, drumstick flowerheads that are followed by spiny seed clusters. Identification of the various species can sometimes be difficult and is based largely on details of the leaves and the seedheads. Make a note of leaf colour and take measurements (width and length) of the endmost pair of leaflets and note the number of teeth present on these leaflets. Note also the colour of the seedhead spines and the presence or absence of barbs at their tips.
Common Pirri-pirri-bur Acaena novae-zelandiae
Introduced from Australia or New Zealand as a garden plant and recorded from a handful of sites, most notably at Kelling Heath where it is well established and spreading from dumped garden waste. Flowers June to July. Leaves bright, glossy green, the distal leaflets 2-2.5 times longer than broad with 8-15 teeth. Spines on the seed capsules are tipped with backward-pointing barbs.
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