The beauty of arborvitae trees is that they are evergreen and will not shrivel or turn brown like most other trees. However, if you notice that your thuja is losing its luster or turning brown, it can be quite disheartening. No one wants to see something they’ve planted around their home, looking less than vibrant and healthy.
It is not difficult to prevent your thuja from turning brown, but the first step is to understand why it is changing colors. Keep reading to find out how you can save your evergreen tree.
Why They Turn Brown
Arborvitaes turn brown for a handful of reasons, both natural and induced by disease, winter burn, and pests. It is essential to determine if the browning is natural or a symptom of a larger problem to prevent reoccurrence.
Seasonal Needle Drop
A natural cause of browning is seasonal needle drop.
Thuja trees are conifers, meaning they don’t drop their foliage every year like deciduous trees, and they don’t lose needles every year like evergreens. Instead, they shed their “leaves” every few years as a natural part of their life cycle. An entire branchlet on the tree turns brown as it ages and may remain on the tree for some time before dropping.
How can seasonal needle drop be prevented?
Seasonal needle drop from a tree cannot be prevented since it is a normal and natural process. Follow the best cultural practices to keep your arborvitae trees healthy, and the needle drop will be reasonable and restricted to old-growth instead of new growth.
Fungal Diseases
While arborvitaes are known and revered for being easy-to-care-for trees, they are susceptible to several fungal problems that can result in your arborvitae turning brown.
- Kabatina twig blight kills the tips of one-year-old branches. Where the dead wood meets living tissue, you will see black, pimple-like fungal structures. It can coincide with Phomopsis blight.
- Pestalotiopsis tip blight affects the ends of the branches, causing dead spots or blotches. As the infection progresses, it moves towards the base of the needles. The area may be dotted with black, pimple-like structures. It is opportunistic and usually occurs when there is already an insect or pathogen problem.
- Phomopsis twig blight starts in the tips of immature branches. Yellow spots morph into faded light green, that ultimately brown. It can co-occur with Kabatina blight.
- Cercospora leaf blight affects Oriental arborvitae but not American (Thuja occidentalis). Browning begins on low branches close to the trunk and spreads upward, leaving only the tree’s top unaffected.
How can diseases be prevented?
The best way to prevent fungal diseases is by keeping your thuja tree in good health through proper care. Unhealthy, stressed trees are more susceptible to disease problems.
- Ensure you are watering and fertilizing the arborvitae correctly.
- After planting, spread a layer of mulch on the ground around the plant. Mulching prevents soil-borne fungal spores from splashing up onto the branches when watering.
- Be careful when mowing around trees to prevent damage. Wounds in the bark are a quick entry point for some pathogens.
- Quickly treat disease problems on other plants in your garden to minimize spread.
Winter Burn
If you observe your arborvitae turning brown during the winter or early spring, the cause is likely winter burn. A combination of freezing temperature, dry winter winds, sun, and a lack of water in the soil causes arborvitae foliage to turn brown because it is drying out. Symptoms typically appear at the tips of the branches and progress inward towards the center trunk.
How can winter burn be prevented?
Winter burn is preventable by protecting trees from the wind, minimizing water loss, improving the root system, and fertilizing appropriately.
- Use burlap or canvas to create barriers protecting the plant from dry winter wind and sun. Drive stakes slightly outside the drip line around the perimeter of the tree, wrapping the protective material around them to create
- Apply two to four inches of mulch around the tree’s base, out to the edge of the drip line. Mulching insulates the roots from severe temperature fluctuations and keeps water in the soil.
- Encourage a deeper root system during the growing season by providing more water less frequently. Deeper roots can access soil moisture deeper in the ground.
- Use your home as a wind block. Plant on the northeast or east side of your property to help shield them from the winter wind.
- Plant trees in the spring or late summer to give the roots time to establish before the tree goes dormant for the winter, and the ground freezes.
- Avoid fertilizing plants in the late summer or fall. Late fertilization triggers the growth of new foliage, potentially inhibiting the onset of dormancy.
Pests and Solutions
The last cause of browning is problematic pests. Although thuja trees have fewer pest problems than other landscape trees and garden plants, a couple may cause damage.
- Bagworms produce spindle-shaped cocoons that are very difficult to distinguish from evergreen foliage. The dense bags incorporate twigs and green “leaves” from the tree, protecting the bagworms from insecticides.
- Spider mites are incredibly destructive of arborvitae, sucking sap from leaves’ underside, causing the needles to lose their color and brown. Mites are about the size of pepper grains and may be black, brown, or red.
- Leafminers attack all Thuja varieties but prefer pyramidal, globe, and golden varieties. After hatching, green larvae bore into the leaves and feed, hollowing the needles from the inside.
How can pest damage be prevented?
Regularly inspect your plants and garden for pests, treating the insect problems quickly. You can also incorporate the following tips to help prevent pest damage.
- Grow plants in the garden that repel arborvitae-specific pests or attract their natural predators.
- Keep mulch about three inches from the trunk.
- Do not water trees excessively. High moisture will attract insects.
- Plant deer-resistant varieties: “Holmstrup”, a cultivar of the American arborvitae (Thuja occidentalis) or the Giant arborvitae (Thuja plicata) cultivars “Green Giant”, “Spring Grove”, and “Zebrina”.
- Create barriers around plantings to prevent deer from eating them in the winter, when food is scarce.
