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There are over 100 species of Rhamnus worldwide, most of which are native in northern temperate regions.  We are primarily interested in R. cathartica and R. frangula which have been identified as overwinter hosts for the soybean aphid and which are themselves problematic invasive species.

Quick ID: Both species occur as large shrubs or small trees (up to 6 m) that produce abundant small rounded dark fruit (drupes) containing 3-4 seeds per drupe each autumn.  The bark is very similar to plum species (Prunus) and both species have a yellow sapwood and a pinkish to orange heartwood.  R. cathartica commonly has spine-tipped twigs; R. frangula does not.

Click here for a downloadable PDF image file (522 K).

Key for Rhamnus species (modified from Swink, F and G. Wilhelm.  1994.  Plants of the Chicago region.  4^th ed.  Indianapolis: Indiana Academy of Sciences and Mohlenbrock, R.  Guide to the vascular flora of Illinois.  1986.  Southern Illinois Press: Carbondale, IL.).

1. Low shrub less than 1 m tall; petals absent	R.alnifolia
1. Shrubs or trees well over 1 m tall at maturity, petals absent	2
2. Some branchlets end in a spine; nutlets with deep narrow groove	3
2. Branchlets do not end in spine; nutlets without groove or groove is broad and open	4
3. Leaves less than 7.5 cm long, elliptic-ovate to rotund, rounded at base	R. cathartica
3. Larger leaves more than 7.5 cm long, lance-oblong to elliptic or elliptic-ovate, mostly tapered at the base	R. davurica
4. Leaves with 6-9 pairs of lateral veins, winter buds scaly, flowers opening with leaves, nutlets with a broad open groove, leaves finely serrulate, short-acuminate to acuminate	R. lanceolata
4. Leaves with 8-10 pairs of lateral veins, winter buds without scales, flowers appear after	5
5. Leaves obscurely serrulate, about 3 times as long as wide	R. caroliniana
5. Leaves nearly or quite entire, usually less than twice as long as wide	R. frangula