Japanese barberry (Berberis thunbergii) is an exotic, invasive, thorny shrub. It is native to Japan and has been cultivated around the world as an ornamental. It was introduced into North America in the 1800s where it occasionally escapes cultivation. It is now common from Maine to Minnesota south to North Carolina and Missouri, with scattered populations in the west. In Minnesota it is common in the eastern half of the state with scattered populations in the western half. It is found in open, bottomland and upland woodlands; woodland edges and openings; pastures, meadows, and old fields; and roadsides and other disturbed places. It grows in well-drained, moist to dry soil under full sun to medium shade. It is a restricted noxious weed in Minnesota. The state recognizes – and prohibits – 25 cultivars in addition to the parent species.
Japanese barberry is a dense, compact shrub that sometimes forms large, impenetrable thickets. The shrubs are usually no more than 3′ tall but can be much taller. The narrow, spatula-shaped leaves appear in tight clusters on short shoots along the stems and branches. There is a single, half-inch long spine at the base of each leafy shoot. In May and early June clusters of 1 to 5 small yellow flowers appear at at the ends of the shoots. These are replaced in late summer by bright red juicy berries. The berries remain on the plant throughout the winter.