Thuja occidentalis is commonly cultivated and plants may persist near old homesteads. Some of these may be unusual horticultural forms. In addition, the Asian Platycladus orientalis, Oriental arborvitae, is commonly cultivated in southern Michigan, and may also persist (and is known to escape south of Michigan). It differs from Thuja in the flattened sprays of foliage being held predominantly in a vertical alignment rather than more or less horizontal, and in the cone scales having striking 'horns.'
All species found in Thuja
MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. March 2, 2021. https://michiganflora.net/genus.aspx?id=Thuja.