This thorny large shrub or small tree is a rare escape from cultivation native to China and Korea. More commonly known as Poncirus trifoliata (L.) Rafinesque, but a 2003 study showed that Poncirus is nested within Citrus. The fruits look and taste like small sour oranges. Also called Hardy Orange because it's relatively frost tolerant.

Morrow Mountain State Park, Stanly Co., NC 4/17/2011.

Morrow Mountain State Park, Stanly Co., NC 4/17/2011.

Trifoliate Orange forms impenetrable thickets.

Morrow Mountain State Park, Stanly Co., NC 4/17/2011.

Haywood Co. NC 5/10/08.

The thorns are truly wicked.

Haywood Co. NC 5/10/08.

The bark is striped with green.

Haywood Co. NC 5/10/08.

The shrub at left was photographed in the woods along Towerview Drive on the Duke campus, where there is a large impenetrable thicket in the woods.

Durham, NC 3/28/07.

Bark detail.

Durham, NC 3/28/07.