This species is named after the Isle of Purbeck in south-east Dorset where it is locally abundant on scrubby heathland. Its historical range also extends eastwards to the Isle of Wight and Hampshire, as far as the southern edge of the New Forest. In recent decades it has also been found in scattered locations in Devon and parts of central and eastern Ireland as well as Normandy in France. It can be recognised by its white flowers with large white or pale pink concave petals, ovate terminal leaflet, dark-margined sharply serrate leaflets and hairy stem.
Rubus purbeckensis is a low-arching species which forms low bushes or grows up through gorse scrub. The panicle is narrow in shape with a congested head of flowers at the apex. There are a few trifoliate leaves lower down and often several small single leaflets just below the main head of flowers. The rachis is flexuose below, green or reddish, thickly pubescent, with long slender declining prickles. Very short-stalked glands and patent prickles occur on the pedicels.
The upper panicle leaflets are a matt yellow-green to dark green colour and shortly pubescent above, ovate or somewhat rhomboid in shape, plicate, with a short or sometimes long apex. The leaflets are sharply and narrowly serrate, giving a finely jagged and crisped appearance, made more distinctive by hairs showing between the teeth and a dark reddish-brown tinge around the margin.
Flowers are relatively large, 2.5-3cm diameter, with white or very pale pink elliptical petals 12-15 x 8-9mm, which are sometimes toothed or apiculate at the apex and often distinctly convcave. Stamens are dense and much longer than the styles, with white filaments and glabrous to hairy anthers (one specimen examined had glabrous and hairy anthers in the same inflorescence). Styles are pale green; young carpels pilose (may also be glabrous); receptacle hairy.
Sepals are densely pubescent with a few short-stalked glands and acicles, narrowly pale-bordered, with long finely-pointed tips; they become loosely reflexed (downward pointing at about 45°) soon after the flowers open (so usually visible between the petals). Note the long stamens in the photo below.
Leaves are usually 5-foliate (sometimes 3-foliate), mid to dark green in colour, with the leaflets contiguous to slightly imbricate. The terminal leaflet is fairly small, c.7 x 4.5cm (published measurement), or perhaps a little wider, usually ovate, with an emarginate to subcordate base and an acuminate apex. Leaflets are often moderately plicate like the panicle leaflets, with sides angled inwards, and often also have a dark-brown tinged, deeply and irregularly serrate margin.
Leaflets are greenish or whitish-green felted below, both on the stem and in the panicle.
The stem is bluntly angled with flat or slightly rounded sides, becoming dull purple; usually shortly pubescent with no stalked glands. The prickles are shorter to about the same length as the width of the stem, moderately stout, either wholly yellow or with a red base and yellow tip.