TeachingTuesday: American Arborvitae

This week, our plant is American arborvitae, or Thuja occidentalis. American arborvitae is a slender, evergreen tree native to eastern North America. It has a broad pyramidal shape, and rich green summer color. The most common cultivar is ‘Emerald’, which is a dwarf form that only reaches about 10-14’ high and 6’ wide, whereas the natural species grows 40-60’ tall. The bark is grayish or reddish brown and exfoliates on mature branches and trunks. Pictured above is Thuja occidentalis 'Hetz Wintergreen'.

American Arborvitae is low maintenance, and will tolerate clay soil, wet sites, black walnut and air pollution, but does not perform well in overly shady, dry, or windy sites. If the site is too shady, the branches will thin out and die back dramatically, leaving a very unattractive skeleton of a tree. The foliage may become winter burned (turn yellow or brown) in dry, exposed locations. These trees are also susceptible to damage and broken branches in winter if ice or snow accumulates in or on the branches.

For maintenance, American Arborvitae rarely requires pruning when planted in the right location. Make sure to plan for the mature size if planting these trees on each side of a door or a walkway to avoid having to prune them back. Bagworms can be an issue in the spring, so watch for them and remove them by hand. This will help keep them from becoming a larger problem needing chemical treatment. American arborvitae cannot tolerate drought, so make sure the trees receive adequate water, even in the winter, to prevent drying out and winter burn. If your soils are naturally very dry, you may want to go with a different evergreen tree, as American arborvitae may not survive long-term.

ID Tips

• Foliage is rich green in the summer, but may be yellow-green or even brownish at other times of year, and grows in flattened sprays of scale-like leaves.
• Leaves have an aromatic scent.
• Because it is a gymnosperm, arborvitae does not have flowers. It does have small cones that start yellowish green and turn brown at maturity.
• May be confused with Juniper, Chamaecyparis, or Leyland cypress.
  • How to tell from Juniper: Juniper has small, blue waxy berry-like cones, while arborvitae has little brown cones that open up like pine cones. Juniper foliage is not as flat as arborvitae foliage.
  • How to tell from Chamaecyparis: The most common species of Chamaecyparis are C. obtusa (hinoki falsecypress) and C. pisifera (threadleaf falsecypress). C. obstua has what looks like white x’s all over the back of the leaves, due to a waxy layer that builds up on the edges of the scales (this is absent in arborvitae). C. pisifera has very long thread-like leaves also composed of scales, whereas arborvitae would have much shorter ones.
  • How to tell from Leyland cypress: Leyland cypress also looks very similar to arborvitae. Arborvitae trees tend to be denser, fuller, and wider, whereas Leyland cypress tends to grow in a more open habit. If you have any good tricks for telling these two apart, please send me a message at caitlin@myattlandscaping.com!