Not to be semantically confused with :
Piri Piri [an African word for
Chili Peppers from the Capsicum genus and also a Portuguese hot chili sauce. Note the differing spelling from Pirri-pirri-bur, although note also that the American spelling for this plant is indeed Piri-piri-bur (!).]
Pirri-pirri-bur (Acaena novae-zelandiae) shown here has: 9-13 (up to 15) leaflets - those at the tip of a leaf being 1.8x to 2.5x longer than wide, 5-20mm long with 5-12 (up to 15) teeth. 3 to 4 spines per flower. Spines 6-10mm long, often with 1 or 2 much shorter. Compare this with the Pirri-pirri-burs listed below.
Easily mis-identified as :
All are introduced and naturalised and most are grown in gardens. All are invasive, and hard to eradicate once they get into the landscape. There is still a chance of eradicating the outbreak just north of Stanage Pole since at the moment your Author only saw it over a 100 yard section of paved pathway, but then, it is hard to see amongst any taller vegetation which is plentiful around here, such as Heather, Bilberry, grass, etc. Your Author could hardly see it even amongst short grass, so it is possible it has already spread deeper into the acidic moorland common around those parts. Get digging...
Bronze Pirri-pirri-bur (Acaena anserinifolia) but that invariably has bronzed (not glaucous) leaves. The pair of leaflets at the tip of a leaf are only 3-10mm long. Spines only 3.5-6mm long but often one or 2 much shorter.
Glaucous Pirri-pirri-bur (Acaena caesiiglauca) with blue-greem glaucous leaves often tinged purple. The pair of leaflets at the tip of a leaf are only 6-14mm long. Spines only 4.5-7mm long.
Two-spined Pirri-pirri-bur (Acaena ovalifolia). With (7) 9-(11) leaflets in a leaf; the pair of leaflets at the tip of a leaf are longer at 10-30mm, are 1.7 - 2 x longer than wide and have more teeth ((11) 17-23. Only 2 spines per flower.
Spineless Pirri-pirri-bur (Acaena inermis) without spines, or with imperfect spines, or the spines are not barbed. There are usually no spines each a flower, but sometimes between 1-4 spines per flower. Leaves bluish-grey-green, the pair at the tip being very small, just 2-8mm long, and are as long as they are wide, with 5-10 teeth.
A hybrid between
Bronze Pirri-pirri-bur (Acaena anserinifolia) and
Spineless Pirri-pirri-bur (Acaena inermis) is also grown in gardens and can escape. These have 2 stigmas and develop spines.
Some similarities to : Salad Burnet (Sanguisorba minor) which also has similar pinnate leaves and a similar flower-head, but the flower-head is smaller, red and lacks the spines. Fodder Burnet (Poterium sanguisorba ssp. balearicum) has more similar (more oblong) pinnate leaflets but a longer flower-head, again lacking spines. Both plants are in the same Rosaceae family as the Pirri-pirri-burs.
Unusual for a plant belonging to the Rose family Rosaceae in that it has flowers with zero petals (rather than the usual complement of 5), but it does have 4 (rather than the usual 5) sepals, which are green. The anthers are white and prominent whereas the style, although also white, is much smaller with a hairy stigma. Like most plants it is bisexual.
The stems themselves are spine-less, with all the spines appearing in the flower-head when it is fruiting. There are 4 spines per flower (on
Two-spined Acaena (Acaena ovalifolia) there are only 2 spines per flower). The spines are barbed and adhere to clothing or the fur of passing animals, distributing the seeds (which are attached singly to each barb).