Flower:

Raceme up to 7 inches (6 to 18 cm) long, up to 4 inches (6 to 10 cm) wide, at the tips of short, leafy, lateral shoots along 1-year-old stems, each cluster with 5 to 19 flowers. Flowers are white, 1 to 2 inches (2.5 to 5 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, generally triangular, the tip abruptly narrowed with a tail-like extension, the outer surface moderately to densely covered in a mix of glandular and non-glandular hairs. Flower stalks are similarly hairy, and may also have a few 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 to oblong-elliptic, sharply toothed around the edges, sparsely hairy on the upper surface, velvety hairy on the lower. The terminal leaflet on primocanes is 3 1/3 to 6 inches (8.5 to 15.5 cm) long, up to 4 inches (5 to 10 cm) wide, half to 2/3 as wide as long, widest near the middle, slightly to distinctly heart-shaped at the base, abruptly tapered to an extended tip.

Leaflet stalks are variably covered in non-glandular hairs, sometimes mixed with a few glandular hairs, and scattered broad-based, curved prickles. At the base of the compound leaf stalk is a pair of appendages (stipules), lance-linear, up to about ½ inch (7 to 15 mm) long.

Prickles are up to about 1/3 inch (4 to 8 mm) long, broad-based, straight to slightly curved, very sharp, and moderately abundant but unevenly spaced along the stem. The upper stem may also have scattered glandular hairs, though these can wear away as the season progresses. Stems are up to 9 feet (to 2.7 m) long, green to dark red, initially erect, becoming arching, but the tip not usually reaching the ground so not rooting at the tips, and die the second year after fruit matures. Colonies may form from root suckers.

Fruit:

Fruit is a round to cylindric cluster of fleshy drupelets, up to about ¾ inch (9 to 18 mm) diameter, maturing from green to red to black, and are quite tasty.

Notes:

Old Field Blackberry is an occasional species in Minnesota, where it reaches the northwestern edge of its range. It's most often found in the part shade of woodland edges, less often in old fields, wetland edges, or roadsides, usually in sandy soil.

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.

Old Field Blackberry is identified by the combination of: abundant glandular hairs on sepals, flower stalks, sometimes on the upper portion of stems and/or a few on leaf stalks; prickles are broad-based, straight to slightly curved, 4 to 8 mm long; lower leaf surface is velvety hairy; the flower cluster is typically a wide, leafy raceme with up to 19 flowers. Canes can reach 9 feet in length and typically lean or arch over but are not known to root at the tips. Primocane leaves mostly have 5 leaflets, sometimes 3 on the lower stem, the terminal leaflet is widest near the middle, half to 2/3 as wide as long, has an extended tip (acuminate) and is somewhat to distinctly heart-shaped at the base.

In many references Rubus alumnus is lumped in with R. allegheniensis, along with R. rosa, 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. All have velvety leaves, broad-based prickles and a mix of glandular and non-glandular hairs. R. ithacanus also has these traits but is low-growing, the canes trailing along the ground and rooting at the tips.

R. alumnus closely resembles both R. allegheniensis, which has a more elongated and narrow raceme, and R. rosa, which has a terminal leaflet broadly egg-shaped to nearly round, at least ¾ as wide as long, and sometimes has a more branching flower cluster (compound corymb).