Common buckthorn (Rhamnus cathartica)
Appearance: Tall understory shrub or small tree up to 20' high with a spreading loosely branched crown, often multiple stems at the base. Brown bark with elongate silvery corky projections (Caution: native plums or cherries have a similar bark). Female and male plants.
Branches: Cut branch exposes yellow sapwood and orange heartwood. Twigs often end in stout thorns.
Leaves: Alternate, sometimes opposite; broadly elliptic pointed at the tip, smooth, dark glossy and small-toothed. Leaves stay green late into fall.
Flowers: Inconspicuous, appear in May or June, clustered in the axils of leaves.
Fruit: Clusters of black 1/4 inch fruit ripen on female plants in August and September. Seeds are viable for 2 - 3 years in the soil.
Roots: Extensive fibrous root system.
Minnesota Conservation Volunteer magazine article The Trouble With Backyard Buckthorn