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Thuja occidentalis is
easily recognized. The leaves are small, scale-like and
tightly appressed to the branches. The leaves are of two
distinct types, the first flat and broad and the second
appearing folded and narrower. They are arranged as shown
above to create a strongly flattened appearance to the twigs.
Male and female flowers are found in separate cones that
are so small as to frequently go unnoticed (see photos above).
After anthesis (the act of dispersing pollen) the male cones
quickly wither. The flowering female cones persist and eventually
develop a few woody scales oppositely arranged and containing
seeds in their axils (the acute angle formed by the scale
and the central axis of the cone). Flowering begins about
mid-April in upland sites in Brown County.
Thuja occidentalis ranges
from Ontario and northeastern Minnesota to Nova Scotia,
south through Wisconsin to northeastern Illinois, Michigan
and New York and at scattered locations as far south as
North Carolina in the Appalachians. In Wisconsin it is most
common in the northern half of the state and farther south
in the east along Lake Michigan. It is most often found
on calcareous sites, including the thin soils of Door County
and other eastern Wisconsin sites underlain by dolomite.
It is also prominent in swamps where it may be the dominant
species. Thuja occidentalis is not reproducing well
in Wisconsin, at least in part due to the large numbers
of deer which find
the leaves highly palatable, thereby reducing the survival
of seedlings to a very small number in some areas.
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