This is a garden species of North American origin which has only rarely been recorded naturalised in the wild in the UK. These photos are taken from a patch I found in a wood in Southampton, Hampshire in 2011 and from a cultivated bush in a garden. The species is only briefly mentioned in Edees & Newton's Brambles of the British Isles, so the description below is just a brief summary of what is visible in these photos. Its main features are the suckering habit typical of Section Rubus, the long racemes of white flowers and bright red stems which are almost completely thornless (giving rise to two of its vernacular names, Thornless Blackberry and Smooth Blackberry).
The stems are thin, cane-like and deeply grooved with rounded ridges. The inflorescence develops along them in the second year and causes them to bend over and turn a bright purplish red in the sun. The inflorescence consists of clusters of 2-3 branches arising from the stem, which are also branched again, each producing a head of 10-15 or more flowers. Each main branch and some of the sub-branches has a 3-foliate leaf with broadly elliptical leaflets. The canes are completely glabrous, though not particularly shiny; the branches and pedicels are quite thickly pubescent and sometimes have one or two small acicles.
Flowers are about 2-2.5cm across with broadly elliptical white petals. The white stamens are about as long or a liitle longer than the styles, but are strongly deflexed in a manner characteristic of several other species in Section Rubus. The styles are pale green, becoming yellow and progressively turning pinkish from the tip downwards.
Sepals are ovate with a short, stout tip (often leafy), green or turning yellowish or reddish above, thinly pubescent and slightly shiny, not white-edged. They are patent at flowering, becoming loosely reflexed.
Leaves are 3-foliate with sessile basal leaflets, all somewhat concave with finely serrate margins; mid green to yellowish green in colour, rather thin in texture and slightly glossy above. The terminal leaflet is broadly ovate to elliptical with an erose to subcordate base and a relatively short, poorly differentiated cuspidate apex.
Leaflets are green below, sparsely pubescent.