Much-branched shrub or small tree, usually with thorny branches; lvs oblanceolate to obovate, 4-7 cm, obtuse to broadly rounded above, narrowed to rounded at base, hairy beneath, sometimes eventually glabrous; fls only 1 or 2(3) in a cluster, 2 cm wide; fr dark blue to black, 2-3 cm, the stone subglobose, scarcely keeled, adherent to the flesh; 2n=48. A European cultigen, thought to be derived by hybridization of P. cerasifera with P. spinosa, occasionally escaped from cult. along roadsides and fence- rows in our range. Most of our plants are ssp. insititia (L.) C.K. Schneid., the bullace or Damson-plum, as described above. (P. insititia) Less often we have ssp. domestica, the common plum, more arborescent, generally unarmed, with larger fr 4-7 cm, and a compressed, keeled stone that often separates from the flesh.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.