25. Rubus L. (raspberry, blackberry, bramble)

Contributed by Mark P. Widrlechner

Plants shrubs (perennial herbs elsewhere), sometimes suckering or reproducing vegetatively from rooted stem tips. Stems (referred to as canes) biennial or (in sect. Rubus) occasionally persisting longer, prostrate, climbing, arched, or erect, all those found in Missouri armed with prickles, sometimes also with long, stiff bristles (except for horticultural selections); first year’s stems generally vegetative (called primocanes), unbranched or few-branched (moderately branched in a few species); second year’s stems fertile (called floricanes), usually dying back to the rootstock at the end of the second growing season. Leaves rarely evergreen. Stipules small to conspicuous, persistent, leaflike, fused to the petiole laterally or attached at the junction of the stem and petiole. Leaf blades palmately or pinnately compound, the margins toothed and sometimes lobed, the surfaces often hairy, sometimes with gland-tipped hairs or bristles, the main veins sometimes also with prickles. Inflorescences typically axillary on floricanes, of branched or simple clusters, racemes, panicles, or occasionally solitary flowers. Flowers perfect (imperfect elsewhere), with a noticeable hypanthium. Sepals 5, fused basally, persistent at fruiting, erect, horizontally spreading, or reflexed. Petals usually 5 (rarely more in horticultural selections), white or less commonly pale pink to rose pink, glabrous or hairy, the margins usually entire, rarely lobed (in R. laciniatus). Stamens numerous, the filaments attached at the mouth of the hypanthium. Pistils many, attached to an enlarged receptacle, this hemispheric, or becoming conic or cylindric as the fruits mature, each with 1 carpel and 2 ovules, 1 of these aborting as the fruit develops. Style 1, threadlike or occasionally narrowly club-shaped, glabrous or hairy. Fruits drupelets positioned on the enlarged receptacle to form an aggregate, which either separates freely from the receptacle when ripe (raspberries) or is shed with the receptacle attached (blackberries). About 800 species (although species concepts and the resulting estimates differ widely), worldwide, particularly abundant in temperate regions of the Northern Hemisphere.

Rubus is a large genus that is diverse in the midwestern United States. Like Crataegus, its reputation for hybridization, polyploidy, and apomixis has given it a well-deserved reputation as a taxonomically difficult genus. Additionally the prickly canes that change appearance from the first to second growing seasons have caused many collectors to avoid the plants. Rubus is thus underrepresented in herbaria and many of the specimens that are present are relatively incomplete. Plants often thrive in disturbed, successional habitats, sometimes in mixed-species thickets. A number of mixed collections in various herbaria involve the primocanes of one species and the floricanes of a second species growing at the same site.

The genus produces a number of important fruit crops, including raspberries, blackberries, and loganberries. In addition to being consumed as fresh fruits, the fruits are used extensively in baked goods, jams, jellies, preserves, and as juices and flavorants. The fruits also are an important food source for wildlife. A few species are cultivated as ornamentals.

Whenever possible, users of this treatment should examine primocane leaves for identification characters involving leaves. References to primocane leaves and leaflets in the descriptions that follow are based on fully expanded leaves. It should be noted that in leaves with five leaflets, the apical or middle leaflet is called the central leaflet, the adjoining pair of lateral leaflets are termed the middle leaflets, and the pair positioned closest to the petiole are known as basal leaflets. Floricane foliage is primarily found on inflorescence branches, but can also be found on sterile branches. Large, sterile branches that sometimes form near the base of floricanes are called parcifronds and can easily be confused with primocanes. Floricane leaves on sterile branches generally resemble those of primocanes, but often are smaller and have only 3 leaflets. Floricane foliage on inflorescence branches is denoted in the species descriptions below as bracts or leafy bracts.

In the present treatment, inflorescences are described and measured from well-developed examples on floricanes. Inflorescences can also emerge directly from the crown, generally when a primocane has been damaged or winter-killed. These atypical inflorescences are called novirames, can flower out of season, and are extremely difficult to identify to species.

North American herbaria hold thousands of fragmentary Rubus specimens that cannot be determined to species. When vouchering Rubus, it is important to collect pieces of both the primocane and floricane, with the primocane displaying both sides of fully-expanded leaves and the floricane displaying well-developed inflorescences, collected at any time between the start of flowering and the formation of ripe fruits. These pieces should be kept together, either by mounting on the same sheet or through appropriate labeling. On labels, it is also useful to note the overall plant habit and the presence or absence of rooting at the tips of the canes.

The genus Rubus generally is divided into five subgenera and numerous sections. The relationships among the sections are poorly understood, and it is not clear if the traditional classification circumscribes natural subgroups. The nomenclature of the sections is also still somewhat controversial, as Liberty Hyde Bailey (1858–1954; the leading specialist on the genus in the first half of the twentieth century) initially treated these as unranked groups and by the time that he validated the sectional names, some might already have been published by other authors. Thus the names applied to the sections should be treated as provisional, pending further review of the literature on Rubus. Grouping the species into morphologically based subgenera and sections, as is done in the present treatment, has the advantage of saving space by requiring that characters shared among species within a section be listed only once in the section description. Users are cautioned to read the subgeneric and sectional descriptions carefully in addition to the species descriptions for help in confirming the identity of plants being determined. The following key to the subgenera and sections of Rubus uses a combination of characters from primocanes and floricanes, but can be navigated with only one type of cane, with the exception of couplet 4, which requires inflorescences.