Japanese Barberry
Berberis thunbergii [ USDA, Wikipedia, GoBotany, CISMA ]
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Japanese Barberry is a small deciduous shrub from 2-4 ft. tall (although a 10 ft. tall shrub was found growing in the edge of the Grassy Pond meadow). The thin, grooved branches have paddle-shapped leaves up to 1 in. long, and thin, straight spines on every node. The pale-yellow flowers occur in drooping clusters of 2-5 and develop in mid-spring to early summer. The berries ripen to a bright red color in late summer and are 0.25-0.3 in. long. The wood of the Japanese Barberry stem and roots is a brilliant yellow.
While this plant isn't as aggressive as other invasive plants, it is very shade-tolerant and can form dense stands which shade out and displace native species.
Removal
Ideally, it should be uprooted. The roots are numerous and relatively short and shallow, making this possible.
Alternatively, it may be cut, but will require multiple cuttings to kill.
Protective Gloves should be worn when removing this plant, due to the long thin straight spines.
Common Mis-identifications
It may be confused with Common Barberry (Berberis vulgaris) [ USDA, Wikipedia ]. Serrated leaves, juicy berries, and three pronged spikes differentiate this species from Japanese Barberry. It was largely eliminated in an extermination effort in the 20th century due to being a carrier of wheat rust. This species hybridizes with Japanese Barberry.
It is less likely to be confused with the following, as they don't have thorns: