A shrub or small tree growing to 6 m. in height.
Some branches end in short thorns.
Leaves mostly opposite or subopposite, with some alternate. Flowers appear with the leaves and have parts in 4s. Birds eat the fruit and spread the seeds such that even scientific and natural area quality fens, prairies and woodlands now have infestations of buckthorn. Common buckthorn can form dense thickets that shade out species of sedge meadows, calcareous fens and the spring ephemerals of woodlands. It is especially prolific in drained peatlands and upland oak forests.