Plant Health Alert – Arborvitae Problems

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Arborvitae trees are quickly becoming the most popular tree in the nursery industry. Displacing the ‘Leyland Cypress’ as a favorite screening plant, the arborvitae blocks unwanted views. The plant is mostly pest free but as we have seen with so many other plants that we thought were ‘pest free’, once we plant too many of these or plant them improperly, diseases and insects can move in and cause problems.

arborvitae canker disease

‘Emerald’ arborvitae that are in shade are susceptible to stem diseases during wet years. In this case mowers had blown grass clippings under these plants holding moisture on the bark even longer which led to canker disease.

The two most common cultivars are ‘Green Giant’ and ‘Emerald’. ‘Green Giant’ is a large, needle bearing evergreen coniferous tree which is a hybrid cultivar of Thuja plicata and Thuja standishii  the Western and Japanese arborvitae. It is a densely branched pyramidal habit reaching a mature height of  60 feet and width 20 feet.

T. plicata cultivars

‘Emerald’ is an American Arborvitae (Thuja occidentalis). ‘Emerald’ is an upright narrow plant that typically reaches a height of 20′ and width of 6′. There are many other species of arborvitae including some from Asia. T. occidentalis cultivars

arborvitae diseased large

These ‘Green Giants’ are in too much shade and have developed canker diseases.

In the main, here in western NC, we grow and use ‘Green Giant’ and ‘Emerald’. We have been seeing more problems with these trees. Spider mites can be a problem especially for trees located in areas of high heat such as the southwest facing sides of structures. Other arthropods that we can see are scale insects.

Diseases such as bot canker have started to infect trees that stay wet for too long, especially those in shade. Trees in shade will take longer to dry after a rain. Both ‘Green Giant’ and ‘Emerald’ are dense plants that stay wet after rains. Wet bark on branches eventually cracks and allows fungal infections to start.

A natural phenomena in all conifers is dropping older leaves. Each year conifers drop older leaves from the interior of the canopy. This will manifest as a general browning of the interior of the plant. This situation is not a disease. What can happen however is that the dropping leaves can build up in the interior of the plant holding moisture against the stems and trunk leading to disease. It is recommended to clear the interior of the trees of these fallen needles at least annually.

natural leaf drop on the interior of arborvitae

Browning of the interior needles of a conifer is natural leaf drop.

natural leaf drop on the interior of arborvitae

Natural leaf drop on the interior of arborvitae can lead to dead needles piling up on the interior branches and trunk leading to disease. Clean out the dead needles inside your arborvitaes annually.