Buckthorn is a deciduous shrub with clusters of green flowers followed by black berries. The small flowers have 4 petals arranged to form a square base, with the stamens at the corners of each square.
The leaves are very similar to Dogwood, being oval in shape with pronounced side veins curving up towards the tip, but the edges of the leaves have small teeth. Another difference is that the Buckthorn flower/fruitclusters are located below the current season's leaves, whereas in the Dogwood they are at the tips of the stems.

Toothed leaves and grey stems		Flower close-up, showing arrangement of stamens relative to petals