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I’ve had several calls over the past three weeks concerning Arborvitae (Thuja sp.) and/or Leyland cypress (x Hesperotropsis leylandii). Do you know if it is an arborvitae or Leyland cypress? How can you tell?

To be honest, I am the horticulture agent and I cannot rattle off a list of differences for you. But, I can tell you that if you will pick a leaf, crush it and smell it, you can tell the difference very quickly. An arborvitae will smell just like a green apple Jolly Rancher. A Leyland cypress will smell more like a Christmas tree. Incidentally, did you know that Leyland cypress is sold as a Christmas tree across the South?

All the calls that I’ve received recently have been concerning the browning of the trees. They have nearly all been arborvitaes as well. Not to say that we have a multitude of issues with arborvitae, but I am much more accustomed to answering questions about Leyland cypress. I would say Leylands are probably in the top three of all tree questions that I answer on a yearly basis. They are not well suited to our area. As such, they have a life expectancy (typically) in our landscapes of between 10 and 15 years.

Arborvitaes are much like our crepe myrtles, they come in many different shapes and sizes. There is a species or cultivar for your space from a potted plant to a tall tree. One of the most notable cultivars is the Green Giant (Thuja standishii x plicata ‘Green Giant’). These are larger trees that are typically planted for privacy borders. The Green Giant is a hybrid of two species of Thuja. Emerald Green (Thuja occidentalis ‘Emerald Green’) is another cultivar that has gained popularity in landscapes as a border plant.

Arborvitaes have relatively few problems other than they don’t like wet feet. Bagworms can be an issue but not like we would see with Leylands. They are susceptible to root rots such as armillaria or phytophthora that will kill them. Armillaria is a wood decay fungus that attacks the roots. Normally there isn’t too much of an issue with healthy trees but I do see it take out healthy trees from time to time. Armillaria is most prevalent in areas where trees have been cut and subsequently replanted. The fungus establishes on the roots and stump of the dead tree and spreads to the new tree. Phytophthora root rot typically becomes an issue when there is poor drainage. The pathogen is nearly always present in the soil but doesn’t become an issue unless there is poor drainage or soil saturation for longer periods. Leylands are highly susceptible to both of these issues but the arborvitaes tend to fair a little better.

The trees both perform the best when there is plenty of sunshine. Shading tends to cause the trees to be very thin and will also cause browning of the leaves. One of the main issues I see on Leylands, that causes browning, is Passalora needle blight. With this disease, the leaves will turn brown and then eventually drop from the inside out and the bottom up. Often by the time I get a call, there is only a little green left at the top of the tree.

Let’s return to the calls from the past few weeks. These were mostly due to internal browning. Earlier, I mentioned that the trees will begin to thin in the shade; this happens to the inside of the tree as well. This time of year, the sun is much lower in the sky and the light period is shorter allowing less light to penetrate. This often causes a natural internal browning of the leaves. If you are seeing excessive browning, however, there may be another issue, it may pay for you to call the office to talk about it.

Some of the issues that I see are circling or girdling roots and root rots. Often, when we get containerized trees, we don’t shave the roots or prune any circling roots before they are planted. Once a root begins to grow in a circular pattern, it will continue to grow in that same pattern eventually girdling the rest of the roots or main stem. When this happens, it cuts off the tree’s vascular system; essentially strangle the rest of the tree cutting it off from vital water and nutrients.

If you are having trouble with growing in your home landscape, call the Extension office at (252)946-0111 or email Gene Fox at gene_fox@ncsu.edu. Topics begin to get slim this time of year, if you have a question, send it in and I may use it for my next article. Until then, happy gardening.