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
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 spines are not barbed, usually zero per flower, but sometimes 1-4 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.
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...
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 burred and adhere to clothing or the fur of passing animals, which spreads the seeds (which are attached singly to each bur).