Thuja occidentalis, also known as white cedar, is an evergreen coniferous tree, in the cypress family Cupressaceae, which is native to eastern Canada and much of the north, central and upper Northeastern United States, but widely cultivated as an ornamental plant.nlike the closely related species Thuja plicata, it is only a small tree, growing to a height of 10–20 metres (33–66 ft) tall with a 0.4 metres (1.3 ft) trunk diameter, exceptionally to 30 metres (98 ft) tall and 1.6 metres (5.2 ft) diameter. The tree is often stunted or prostrate.