| PSC 2620: Woody Trees and Shrub | Course Home | Week 12 |
Thuja occidentalis - American Arborvitae
Plant Viewer
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| Small scale-like leaves overlap and hug the stem. They are bright green, but may discolor during the winter. | The foliage usually has a horizontal alignment, creating noticeable planes in the foliage. Image: Albert Jankowski |
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| Egg-shaped cones are 1/2 inch long and held above the stem. Image: Dan McKay |
Plant Description
Thuja occidentalis or American Arborvitae, is a hardy, medium-sized evergreen tree reaching 20-30 feet high and 10-15 feet wide. It grows best in a temperate and humid climate and in rich, moist soils. Once established, it will tolerate a drier condition and our alkaline soils, but they always need regular water to keep them healthy. Plant in full sun.
The bark is narrowly furrowed and two-toned. The ridges are gray while the furrows are a reddish-brown color. It may exhibit a slight exfoliating nature, with the bark peeling in long strips. It is a somewhat ornamental feature, but not outstanding and usually hidden by the foliage.
The foliage is composed of small scale-like needles that overlap each other and hug the stem tightly. They are true green to dark green in color. In the winter the foliage may turn a yellow or even an unattractive brown color. Small cones up to 1/2 inch long and composed of overlapping scales form on the limbs. They are egg-shaped and begin greenish color before ripening to a brown. They are normally born in small amounts and not noticeable.
It can be difficult to tell the difference between Thuja occidentalis and Thuja orientalis. The easiest method to tell the two species apart is to look at the orientation of the foliage. If the foliage is held in horizontal planes, it is Thuja occidentalis, if it is held in vertical planes than it is Thuja orientalis. While this is a good general rule, be aware that it does not hold true for all of the cultivars.
Landscape Use
It is most widely used as a screen or hedge, thanks to its dense evergreen foliage and upright habit. Smaller cultivars can be used as foundation plantings. Similar to Junipers, Arborvitae has been overused and has fallen out of favor, especially with private consumers.
Points of Interest
Thuja occidentalis is like cotton candy for deer and will is a favorite for deer foraging in yards over the winter. They can literally be stripped clean of foliage in a matter of days if not protected. Snow damage can lead to branch breakage, so it is best if the plants are wrapped in burlap (or at least tied up) over the winter.
Needle burn is also a major problem in our climate, and it is especially troublesome during the winter with the reflective light. Spider mites can be problematic.
Notable Cultivars
Degroot's Spire
A tight and narrow form and spiraled branchlets that create a wavy texture. It grows 6-12 feet tall and 3-5 feet wide.
Hetz Midget
A small, slow-growing and globe-shaped cultivar with dark green foliage. It does not require pruning to maintain its good globe shape. It grows 3-4 feet high.
Holmstrup
This smaller cultivar has a strong pyramidal form and good bright green foliage that it retains through the winter. Grows 5-6 feet tall and 2-3 feet wide.
Linesville
Also known as Mr. Bowling Ball. A small, globe-shaped cultivar with light green foliage and a more airy branching habit. I prefer the texture and color of Linesville to Hetz Midget. Grows 2-4 feet high and wide.
