Flower:

Flower clusters at the tips of short lateral shoots along 1-year-old stems, each cluster with 5 to 24 flowers. The cluster can be in one of two forms: a relatively wide raceme with flower stalks more or less perpendicular to the stem, or a more branching, flattish/convex cluster at the tip with smaller clusters on long stalks arising from leaf axils, known as a compound corymb. Flowers are white, 1 3/8 to 2 inches (3.5 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 long tapering or 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 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 sharply toothed around the edges, softly hairy on the upper surface, velvety hairy on the lower. The terminal leaflet on primocanes is 3¾ to ~6¾ inches (9.5 to 17 cm) long, to 5+ inches (8.5 to 13 cm) wide, broadly egg-shaped to nearly round, at least ¾ as wide as long, rounded to heart-shaped at the base, tapering to a pointed tip.

Leaflet stalks are densely covered in a mix of glandular and non-glandular hairs, and scattered broad-based, straight or curved prickles. At the base of the compound leaf stalk is a pair of appendages (stipules), lance-linear, ½ to ~¾ inch (12 to 20 mm) long.

Prickles are up to about 1/3 inch (6 to 8 mm) long, broad-based, straight to slightly curved, very sharp, and moderately abundant but unevenly spaced along the stem. The upper stem is also sparsely to densely covered in glandular hairs, though these can wear away as the season progresses. Stems are up to 8 feet (to 2.5 m) long, green to dark red or purple, initially erect, becoming leaning or 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 spreading rhizomes.

Fruit:

Fruit is a round to cylindric cluster of fleshy drupelets, up to about 2/3 inch (8 to 16 mm) diameter, maturing from green to red to black, and are quite tasty.

Notes:

Rose Blackberry is an occasional species primarily in the north-central counties of Minnesota, where it reaches the northwestern tip of its range, with scattered populations around the Twin Cities and into our southeastern counties. Like many other blackberries, it has a preference for the part shade of woodland edges but has also been found on open, grassy slopes and creek banks. The preference is for dry, sandy soil but it tolerates loam and moister conditions. It is rumored this is the species from which horticultural varieties were developed for their fruit.

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.

Rose Blackberry is identified by the combination of: abundant glandular hairs on sepals, flower stalks, leaf stalks and the upper portion of stems; prickles are broad-based, straight to slightly curved, 6 to 8 mm long; lower leaf surface is velvety hairy; the flower cluster is either a raceme or compound corymb with up to 25 flowers. Canes can reach 8 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 notably broad, has a short extended tip (acuminate) and is rounded to heart-shaped at the base.

In many references Rubus rosa is lumped in with R. allegheniensis, along with R. alumnus, 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. rosa is most easily recognized by the large, broad terminal leaflet, at least ¾ as wide as long, but its flowers and fruits tend to be larger than others as well. The compound corymb flower cluster is another distinction, though, per Welby Smith's book, this has only been seen in the north-central part of the state and not elsewhere (our photos of that came from Pine County). The terminal leaflet on both R. alumnus and R. allegheniensis is more narrowly egg-shaped or elliptic, about half to 2/3 as wide as long, and the raceme on R. allegheniensis is a narrow, elongated raceme.