Japanese Barberry (Berberis thunbergii)
A foreign invader; it makes a fine hedge, but it is beginning to show up where it is not wanted. The red-leaved form is rarely found in the wild; this one was growing deep in the woods in Fox Chapel, where it was blooming in early May.
The National Park Service (in a “least wanted” posting) gives us this description:
Japanese barberry is a dense, deciduous, spiny shrub that grows 2 to 8 ft. high. The branches are brown, deeply grooved, somewhat zig-zag in form and bear a single very sharp spine at each node. The leaves are small (½ to 1 ½ inches long), oval to spatula-shaped, green, bluish-green, or dark reddish purple. Flowering occurs from mid-April to May in the northeastern U.S. Pale yellow flowers about ¼ in (0.6 cm) across hang in umbrella-shaped clusters of 2-4 flowers each along the length of the stem. The fruits are bright red berries about 1/3 in (1 cm) long that are borne on narrow stalks. They mature during late summer and fall and persist through the winter.