Rubus odoratus and Rubus parviflorus identification

Identifying Thimbleberry (R. parviflorus) and Purple-flowered Raspberry (R. odoratus)

Thimbleberry and Purple-flowered Raspberry are two very closely related species that can look quite similar when they are not flowering. Purple-flowered Raspberry (Rubus odoratus) is an eastern species, while Thimbleberry (R. parviflorus) is a western species that can also be found around Lake Superior and Northern Michigan. Where their ranges overlap, the two species might hybridize. Hybrids can also be found in cultivation as Rubus x fraseri.

Purple-flowered Raspberry (R. odoratus)

Key ID mark: sepals

The sepals densely covered in stalked glands that are slightly sticky to touch. These glands are dark purplish, which distinguishes it from its close relative, Thimbleberry, which has glands that are yellowish to reddish (1). The stalks are also longer: in Thimbleberry, these stalks are sessile or no more than 5 times longer than the gland (2). The pedicels and peduncles are also densely covered by glands.

Flowers and fruit

The flowers have 5 petals that are usually deep pink. Occasionally, flowers can have white petals like Thimbleberry - these might be the result of hybrid introgression between the two species (2). Hybrids between the two species are referred to as Rubus x fraseri.

After the petals have fallen, the fruit starts out very pale greenish. Once ripe, it will turn red.

Leaves

The leaves are palmate and large. The tips of the lobes are acuminate, more so than Thimbleberry (2).

The adaxial (upper) surface of the leaves is soft, with short fine hairs.

The abaxial (lower) surface of the leaves is soft and fuzzy, with short fine hairs growing along the veins.

The petiole and the largest veins on the underside of the leaves also have sticky stipitate glands.

At the based of the petioles, there are two thin stipules.

Stems

The young branches are green and with stipitate glands. The older branches are woody with pale brown bark that peels.

Thimbleberry (R. parviflorus)

R. parviflorus can be differentiated from R. odoratus using three key field marks:
1) flowers are white (instead of pink) and are larger than R. odoratus
2) stipitate glands are yellowish/greenish instead of purple
3) Leaves have blunter tips than R. odoratus (less acuminate) but this difference can be subtle

In the above comparison photo, note the difference in flower colour/size and colour of the glands between R. odoratus (left) and R. parviflorus (right).

Flowers

Note the R. odoratus in the background on the left, with noticeable purple stipitate glands.

Stipitate glands

Like R. odoratus, the sepals, young stems, and undersides of the leaves are covered in sticky stipitate glands. These glands are paler than they are in R. odoratus, appearing yellowish or greenish instead of dark purple. Note the dark purple R. odoratus buds in the background of the first and fourth photos here.

Leaves

The leaves are very similar to R. odoratus, but the tips tend to be more blunt, less acuminate than R. odoratus. In these photos, note that there are some purple flowers in the background: those are from an R. odoratus that is growing side-by-side with the R. parviflorus.

Note that young leaves that have not fully expanded can appear more pointed, similar to R. odoratus

Stem

The older stems are woody, like R. odoratus, with peeling bark.

Comparisons

Here are some comparison photos. In this location, a Thimbleberry is growing intertwined with a Rubus odoratus. Even at a distance the difference in colour of the stipitate glands is visible.