Flower:

Clusters of 5 to 15+ flowers on lateral shoots along 1-year-old stems, the cluster shape variable, sometimes a single raceme at the tip of the shoot, sometimes ascendate with single flowers or small clusters from leafy nodes below the terminal cluster, rarely in a flattish/convex cluster (corymb). 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, mostly triangular, the tip typically 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 sometimes have a few needle-like 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 elliptic to egg-shaped, sharply toothed or double-toothed around the edges, sparsely hairy on the upper surface, moderately to velvety hairy on the lower. The terminal leaflet on primocanes is 3 to 5½ inches (8 to 14 cm) long, up to 3½ inches (6 to 9 cm) wide, widest at the middle or towards the base, rounded to slightly heart-shaped at the base, abruptly tapered to an extended or tail-like tip.

Leaflet stalks are variably covered in a mix of glandular and non-glandular hairs, and scattered broad-based, curved prickles. At the base of the compound leaf stalk is a pair of linear to lance-linear appendages (stipules).

Prickles are up to about ¼ inch (2 to 5 mm) long, broad-based, straight to slightly curved, very strong and sharp, and sparsely to moderately abundant but unevenly spaced along the stem. Primocanes usually have at least a few glandular hairs scattered along the stem, sometimes just near the tip. Stems are up to 9 feet (to 3 m) long, green to dark red or purple, initially erect, soon arching over and the tips trailing along the ground. Arching stems tend to fall over each other and form a dense, tangled mound, and some trailing stems will take root at the tip. Stems die the second year after fruit matures. Colonies may form from root suckering and tip-rooting.

Fruit:

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

Notes:

Ithaca Blackberry is uncommon in Minnesota, where it reaches the northwestern tip of its range. Not a lot is known about its history here, except it appears to have a preference for sandy soil and habitats where it can get at least some sun, rather than constant shade. In Minnesota, it's mostly been found in or near woodlands dominated by oaks.

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 ithacanus is identified by the combination of: glandular hairs on sepals and flower stalks, usually some also on primocane stems and leaf stalks; prickles are broad-based, straight to slightly curved, 2 to 5 mm long, very strong and painful(!); lower leaf surface is usually velvety hairy; the flower cluster is variable in shape (raceme or ascendate, rarely a corymb) with 5 to 15+ flowers. Primocane leaves mostly have 5 leaflets, the terminal leaflet elliptic to egg-shaped, its base rounded to somewhat heart-shaped. Canes can reach 9 feet in length but arch over anywhere between ankle and waist height, resulting in a tangled, mounding colony, the tips usually trailing along the ground, some canes rooting at the tip.

The tip-rooting technically makes this species a Dewberry rather than a Blackberry. When trailing canes and tip-rooting are not obvious (as is the case earlier in the season), the glandular hairs and broad-based prickles could easily confuse this with one of the High-bush Blackberries, R. allegheniensis in particular (someone once described R. ithacanus as looking like R. allegheniensis that's been knocked to the ground), but the latter typically grows taller before arching over, its flower/fruit cluster is consistantly a narrow raceme, and of course its canes do not trail along the ground or tip-root.

In many references R. ithacanus is lumped with Rubus flagellaris, along with other MN species R. ferrofluvius, R. heterophyllus, R. multifer, R. satis, and R. steelei, 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. R. flagellaris is apparently a dumping ground for Dewberries that have broad-based prickles and stems that trail along the ground, rooting at the tips, but other characteristics can be rather variable.

Most similar from this group is R. satis. Both have velvety leaves and can grow low like a ground cover, but R. satis has smaller, less numerous (and less painful) prickles that are all declined to curved, lacks any glandular hairs, and its flower/fruit clusters are smaller and more compact, usually a flattish/convex cluster (corymb). I've also observed the R. satis leaflet stalks are typically quite short and leaflets seem to be more irregularly toothed compared to R. ithacanus, though that isn't noted in any references and may not be consistant characteristics.