Flower:

Raceme up to 6 inches (6 to 15 cm) long at the tips of short lateral shoots along 1-year-old stems, each cluster with 8 to 20 flowers. Flowers are white, about 1 to 1½ inches (2.2 to 4+ cm) across with 5 rounded petals. In the center is a green cluster of many styles surrounded by a ring of numerous, creamy white-tipped stamens.

Cupping the flower are 5 green sepals, triangular to broadly oblong, mostly with an elongated tip, the outer surface densely covered in non-glandular hairs. Flower stalks are similarly hairy, sometimes with a few slender prickles.

Leaves and stems:

Leaves are alternate and palmately compound, the non-flowering first-year stems (primocanes) with 5 leaflets or sometimes 3 on the lower stem, the flowering second-year stems (floricanes) with mostly 3 leaflets. Leaflets are generally elliptic, sharply toothed around the edges, sparsely hairy on the upper surface, velvety hairy on the lower. The terminal leaflet on primocanes is 2¾ to 5 inches (7 to 13 cm) long, up to about 3 inches (4.5 to 8.5 cm) wide, widest near the middle, rounded to heart-shaped at the base, abruptly tapered to an extended or tail-like tip.

Leaflet stalks are covered in non-glandular hairs and may have a few broad-based, curved or declined prickles. At the base of the compound leaf stalk is a pair of appendages (stipules), lance-linear to thread-like, up to about ½ inch (10 to 15 mm) long.

Prickles are up to about ¼ inch (4 to 7 mm) long, broad-based, mostly straight, very sharp, and moderately abundant but unevenly spaced along the stem. Stems are up to 6 feet (to 2 m) long, green to dark red or purple, hairless, initially erect but usually lean or arch over, the tip not reaching the ground so not rooting at the tips, and die the second year after fruit matures. Colonies may form from spreading rhizomes.

Fruit:

Fruit is a round to cylindric cluster of fleshy drupelets, up to about ½ inch (8 to 15 mm) diameter, maturing from green to red to black, and are quite tasty.

Notes:

Plains Blackberry is an occasional species in Minnesota, where it reaches the northwestern tip of its range. It sometimes forms large, dense colonies, but tends to be in the minority when growing with other Rubus. In Minnesota it's most often found in sandy soil or sandy loam, in the part-shade of forest edges and grassy clearings with pine, aspen, birch or oak.

Rubus is a large and difficult genus; both first year (non-flowering primocane) and second year (flowering/fruiting floricane) stems from the same plant may be necessary for a positive ID. Multiple species frequently grow together so stems from the same plant is recommended. Primocanes should be used for stem and leaf characteristics, floricanes mostly for just flowers and fruit. Characteristics to look for are the size and shape of the flower cluster as well as the flower, whether there are glandular and/or non-glandular hairs (on sepals, leaves, stalks and/or stems), whether there are any broad-based prickles or needle-like bristles, number of leaflets on the primocane and whether they are palmately or pinnately compound, whether canes are low-growing or trailing along the ground and/or root at the tip. In some species, the leaflet shape may also be relevant. Floricane leaves are frequently different from primocane leaves in shape and/or number of leaflets so are not a good substitute, and keep in mind that primocanes mature and tip-rooting occurs later in the season than peak flowering time.

Plains Blackberry is identified by the combination of: lacking any glandular hairs; prickles are broad-based, mostly straight, up to 7 mm long; leaf lower surface is velvety hairy; the flower cluster is an elongated raceme with up to 16 flowers, and sepals are densely covered in non-glandular hairs. Canes typically lean or arch over but do not root at the tips. Primocane leaves mostly have 5 leaflets, sometimes 3 on the lower stem, the terminal leaflet usually with an extended tip (long-acuminate).

It most closely resembles Allegheny Blackberry (Rubus allegheniensis), with which it may grow, and which most obviously differs in having sparse to abundant glandular hairs on sepals, flower stalks, leaf stalks and the upper portion of primocanes, and its raceme is notably long and narrow. While the raceme of R. ablatus may be similarly long and narrow, it may also be decidedly wider than R. allegheniensis.

Rubus ablatus is lumped in with Rubus pensylvanicus in many references, along with R. frondosus, R. heterophyllus, R. missouricus, and R. recurvans, but we follow the treatments by Mark Widrlechner and documented by Welby Smith in his book “Trees and Shrubs of Minnesota”, which keeps them split as separate species. These species all have broad-based prickles, velvety leaves, and lack glandular hairs. R. ablatus flowers are in an elongated raceme where the others are in a shorter raceme or a flattish/convex cluster (corymb).