Thuja Genus (arborvitae)

6 Species with 212 Trinomials

Thuja_occidentalis-350x522.jpg

Thuja is a genus of coniferous trees in the Cupressaceae (cypress) family. There are five species in the genus, two native to North America and three native to eastern Asia. The genus is monophyletic and sister to Thujopsis. They are commonly known as arborvitaes (from Latin for tree of life) or Thujas; several species are widely known as cedar but, because they are not true cedars (Cedrus), it has been recommended to call them red-cedars or white-cedars.

Description. Thujas are evergreen trees growing from 10 to 200 feet (3 - 60 m) tall, with stringy-textured reddish-brown bark.

  • The shoots are flat, with side shoots only in a single plane.
  • The leaves are scale-like, measuring 0.04 to 0.4 inch (1 – 10 mm) long, except in young seedlings during their first year, which have needle-like leaves. The scale leaves are arranged in alternating decussate pairs in four rows along the twigs.
  • The pollen cones are small, inconspicuous, and are located at the tips of the twigs.
  • The seed cones start out similarly inconspicuous, but grow to about 0.4 to 0.8 inch (1 – 2 cm) long and are mature at about 6 to 8 months old. They have 6 to 12 overlapping, thin, leathery scales, each scale bearing 1 to 2 small seeds with a pair of narrow lateral wings.
foliage and cones of <em>Thuja sutchuenensis </em> (left), <em>Thuja plicata </em> (center), and <em>Thuja occidentalis </em> (right)
foliage and cones of Thuja sutchuenensis (left), Thuja plicata (center), and Thuja occidentalis (right)

The five species in the genus Thuja are small to large evergreen trees with flattened branchlets. The leaves are arranged in flattened fan shaped groupings with resin-glands, and oppositely grouped in 4 ranks. The mature leaves are different from younger leaves, with those on larger branchlets having sharp, erect, free apices. The leaves on flattened lateral branchlets are crowded into appressed groups and scale-like and the lateral pairs are keeled. With the exception of western red-cedar (Thuja plicata), the lateral leaves are shorter than the facial leaves (Li et al. 2005). The solitary flowers are produced terminally. Pollen cones have 2 to 6 pairs of 2 to 4 pollen sacked sporophylls. Seed cones are ellipsoid, typically 0.36 to 0.55 inch (9 - 14mm) long, they mature and open in their first year. The thin woody cone scales number from 4 to 6 pairs and are persistent and overlapping, with an oblong shape, they are also basifixed. The central 2 to 3 pairs of cone scales are fertile. The seed cones produce 1 to 3 seeds per scale, the seeds are lenticular in shape and equally 2 winged. Seedlings produce 2 cotyledons.

Another very distinct and only distantly related species, formerly treated as Thuja orientalis, is now treated in a genus of its own, as Platycladus orientalis. The closest relatives of Thuja are Thujopsis dolabrata, distinct in its thicker foliage and stouter cones, and Tetraclinis articulata is distinct in its quadrangular foliage (not flattened) and cones with four thick, woody scales.

The genus Thuja, like many other forms of conifers, is represented by ancestral forms in Cretaceous rocks of northern Europe, and with the advance of time is found to migrate from northerly to more southerly regions, until during Pliocene time it disappeared from Europe. Thuja is also known in the Miocene beds of the Dakotas.

The foliage is also readily eaten by deer, and where deer population density is high, can adversely affect the growth of young trees and the establishment of seedlings.

They are widely grown as ornamental trees, and extensively used for hedges. A number of cultivars are grown and used in landscapes. Homeowners will sometimes plant them as privacy trees. The cultivar 'Green Giant' is popular as a very vigorous hedging plant, growing up to 80 cm/year when young.

The wood is light, soft and aromatic. It can be easily split and resists decay. The wood has been used for many applications from making chests that repel moths to shingles. Thuja poles are also often used to make fence posts and rails. The wood of Thuja plicata is commonly used for guitar sound boards.[14] The combination of light weight and resistance to decay has led to Thuja plicata (Western red cedar) being widely used for the construction of bee hives.[citation needed]

Oil of Thuja contains the terpene thujone which has been studied for its GABA receptor antagonistic, with potentially lethal properties.

The natives of Canada used the needles of Thuja occidentalis (Eastern white cedar) to make a tea that has been shown to contain 50 mg of vitamin C per 100 grams; this helped prevent and treat scurvy.

In the 19th century Thuja was commonly used as an externally applied tincture or ointment for the treatment of warts, ringworm and thrush, and a local injection of the tincture was used for treating venereal warts.

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Attribution from: Wikipedia

Comments

Peter Pliszka

do Arborvitae loose leaves during fall? It seems like my are loosing some leaves? Should they be sprayed from preventing to get sick?

David Olszyk

yes, very normal. An arborvitae will lose up to 1-third of its interior foliage every fall. No disease to treat.

Nikki Isakson

I have four 3 yr old emerald green arborvitae that were shipped to me and planted two weeks ago. The very tops were all bent in shipment. They are not broken, just bent. I was hoping they would straighten on their own, but so far, nothing. Do I need to place a support at the top and wrap it in place? I don't want to wait too long.

David Olszyk

I have seen arbs bent by snow, ice, and other mechanical disruptions. For the most part they will straighten themselves out (unless they get bent to the ground). I've seen it take an entire season. For exceptional ugliness, I've wrapped the entire plant with old bicycle tires to hasten the process.

Carol Holstead

I have a conifer I can't identify. Its leaves and cones look like an aborvitae but it has a very unusual shape. The trunk branches and spreads near the base, somewhat like the lone cypress. I cannot find any pictures of aborvitae that look like this. I think the tree is around 60 years old. Would love to send a photo of the tree and leaves. Thank you.

Brandy Reece

How much do the Emerald Green arborvitaes grow a year.

David Olszyk

... of course, by Emerald Green™, you mean 'Smaragd', the proper cultivar name for this conifer. Please see this link for the answer
https://conifersociety.org/conifers/thuja-occidentalis-smaragd/

Emily Paine

I have recently, w/in past month, planted a arbavatae @ 6 ft. Tall purchased @ Home Depot. The tree was not the healthiest, but in reasonably good shape @ a competitive price. It is turning brown in an area getting direct morning sun. An entire vertical section is turning brown. What should I do, I would like to save this tree and have it in my landscape. I live in Chicago.

David Olszyk

Hi Emily, unfortunately, Summer is absolutely the worst time to plant a large tree. I fear that it's already gone and that HD gave you a "reasonable price" because they knew it was distressed (very common this time of year). I recommend seeking out a refund.

Lisa Becvar

We are considering planting arborvitae a long a 6ft fence to give additional privacy. They would be on a property that is waterfront on long island, but not at the edge of the water. So are they salt/wind tolerant and if so which type would you reccommend?
Thanks!

Mitzi Weil

Are any arborvitae deer resistant?

David Olszyk

they don't seem to mess much with plicata, but occidentalis is definitely one of their preferred food sources.

Kay S

I am waterfront in Santa Cruz CA Can the American Pillar Arborvitae handle wind and salt spray?
Thanks :)

Patricia Vealey

Due to the heavy snow our arborvitae are bent over. Should we top them when spring arrives some are very tall.
Thank you

David Olszyk

Hi Patricia ... this happens all the time with these plants. They are not very good choices for parts of the country where it snows or gets icy in the winter. Nonetheless, people really like them.

As for topping them ... they're your trees. You're free to do as you please with them. Just be warned that they're going to look pretty weird and chopped-off for several years as they sort themselves out. Also, when you "top" a tree, it responds by creating lots of upright, weak growth, which will make subsequent snow / ice events even more tragic.

Doris Zajec

During this past Tuesdays snowstorm, several of my arborvitae had branches bent to the ground. One appears to be split, so that one is ruined. What should I do with the rest of them. Once the snow melts, I’m hoping the spring back up. They’re huge - very tall - about the height of 2 of the 3 stories of my house. Should I have someone come and tie them to stand up? If the show doesn’t melt I’m thinking of leaving them and dealing with removal in the spring and Judy cutting my losses. They are already getting sparse due to the large numbers of deer that seem to think my backyard is their home. Any advice?

David Olszyk

this sort of thing happens every winter when it snows and arborvitae are present. These particular trees are poorly suited for planting where wintry weather is the norm. They are also one of deer's favorite things to eat.

Without pictures, it's nearly impossible to suggest a path forward. I would probably have not planted them in the first place.

Catherine Berou

Hi, Can I plant the Thuja occidentalis 'Smaragd' in zone 9B? I have been looking for an evergreen shrub to plant in my front yard as a privacy screen. Please advise.

Dennis

I bought five 2-foot-tall 'Emerald Green' arborvitae in December, Zone 5. The ground isn't frozen, so it's okay to plant them? What about how often to water them, other than the initial planting? Thanks!

David Olszyk

hi Dennis. 100% of all trees that I've planted in December were dead by the next summer. In my world (USDA Zone 8a), the second week of October has proven to be the "sweet spot" for tree planting.

Andy Pink

Do Emerald Green Arborvitae get thinner and lose needles during the winter months (N Georgia - zone 7)

David Olszyk

conifers never shrink unless somebody takes shears or pruners to them. They will, however, shed up to 30% of their old, internal foliage every year.

I have observed that deer really like to eat arborvitae. They'll definitely thin them to the trunk if they're hungry enough.

Isabel Mack

Our neighbor recently reached out to us concerned about our new privacy trees being toxic to their horse. We have planted 40 Thuja Green Giants a couple feet off of our property line. About half of those trees are parallel to our neighbors’ horses’s pasture fence. Internet searches have shown information saying they ARE and ARE NOT toxic to horses. Are these trees truly toxic to horses? We absolutely do not want to knowingly cause harm to an animal, but also want to have privacy for our family.

David Olszyk

they are in no way toxic. In fact, Thuja is a well known favorite food of deer.

Isabel

Hmm. I was asking about horses? Also deer don’t like these trees.

David Olszyk

deer and horses are closely related enough that what would be toxic to a deer would also be toxic to a horse, just as if something is toxic to a chimpanzee, it would be toxic to a human.

Deer browsing is a very different problem in all areas depending on how much natural deer food is available. In my area, every arborvitae is deformed.