Flower:

Leafy raceme or flattish/convex cluster (corymb) of at the tip of lateral shoots along 1-year-old stems, each cluster with 2 to 8 flowers. Flowers are white, 1 to 1½ inches (2.5 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, oblong to egg-shaped, abruptly tapered to a tail-like tip, the outer surface hairless or thinly covered in non-glandular hairs. Flower stalks are hairless to thinly hairy, sometimes with a few needle-like prickles.

Leaves and stems:

Leaves are alternate and palmately compound, the non-flowering first-year stems (primocanes) with 5 leaflets, sometimes 3 on the lower stem, the flowering second-year stems (floricanes) with 3 leaflets. Leaflets are thin, generally elliptic, sharply toothed or double-toothed around the edges, hairless or sparsely hairy along major veins on the lower surface. The terminal leaflet on primocanes is 4 to 5+ inches (10 to 13 cm) long, up to about 3½ inches (6 to 9 cm) wide, widest at or below the middle, rounded to heart-shaped at the base, the tip abruptly tapered to a long, extended or tail-like tip.

Leaf stalks are hairless, sometimes with a few small, weak prickles. At the base of the compound leaf stalk is a pair of linear appendages (stipules) about ½ inch (11 to 15 mm) long.

Prickles are small, weak, needle-like, usually widely spaced or lacking altogether. Stems are up to 6 feet (to 2 m) long, green to red or purple, hairless, initially erect but usually arch over at knee to waist height, not rooting at the tips, and die the second year after fruit matures.

Fruit:

Fruit is a round to cylindric cluster of fleshy drupelets, average ½ inch (8 to 15 mm) diameter, maturing from green to red to black.

Notes:

Sharp-tooth Blackberry is only known to be in Minnesota, though based on records from near the Canadian border, it is possibly also somewhere in southwest Ontario. The first official record is from 1944, collected in Itasca State Park, but several herbarium records going back to 1912 from elsewhere in northern MN have been tentatively IDed as Rubus acridens as well (most generally lack sufficient plant material for a definitive ID). In any case, it is not common. Habitats are similar to other High-bush Blackberries, in sandy or sandy loam soil, upland forest, and part shade.

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.

Rubus acridens is identified by the combination of: hairless or nearly so, lacking any glandular hairs; prickles are needle-like, small and weak, usually widely spaced or absent altogether; leaves are thin, hairless or sparsely hairy only on major veins; flower cluster a short, leafy raceme or corymb with up to 8 flowers, and sepals hairless to thinly hairy. Canes mostly arch over knee to waist high but do not root at the tips. Primocane leaves all have mostly 5 leaflets, the terminal leaflet usually with a narrowed, extended tip (long-acuminate). My first impression of the leaves was suprise at how thin they are compared to other Rubus.

Most similar is Rubus canadensis, which has broad-based prickles and the flower cluster is a well-defined raceme with up to 16 flowers, where R. acridens prickles are more needle-like and the cluster is a short, ill-defined raceme or corymb with up to 8 flowers. Some references lump R. acridens in R. canadensis and others lump it with R. kennedyanus. Per the original L.H, Bailey text published in 1945 describing North American Rubus, R. kennedyanus is a smaller, much pricklier plant and restricted to Newfoundland in Canada. Some say the differences are too minor to warrant splitting them into separate species, but we split them, following the work of Mark Widrlechner and documented by Welby Smith in his book “Trees and Shrubs of Minnesota”.