First: is it normal?
Every fall, arborvitae shed their oldest needles from the inside of the plant. The interior turns brown and drops while the outer foliage stays green. If the browning is mostly inside and showed up in September or October, it's normal and nothing to worry about.
Browning on the outside of the plant, at the tips, or across whole sections is different. That's a sign of stress. Here are the usual causes.
1. Drought stress
The most common cause in Oregon, especially for trees planted in the last two or three years. Our dry summers and heat waves dry out the root ball, and browning often shows up weeks later. Water deeply once or twice a week through summer and mulch the roots.
2. Winter burn
Cold, dry wind and winter sun pull moisture out of the foliage when roots can't replace it. Plants turn bronze or brown on the exposed side, usually noticed in late winter. Water well in fall before the ground gets cold, and screen exposed plants from wind.
3. Root rot from wet soil
Arborvitae can't sit in waterlogged soil. In heavy clay with poor drainage, roots rot and the whole plant browns and thins, often starting low. Improve drainage, avoid overwatering, and replant in a raised area if needed.
4. Planted too deep or mulched too high
Burying the trunk under soil or a mulch volcano suffocates the bark and roots. The root flare, where the trunk widens at the base, should sit at or slightly above soil level.
5. Dog urine or de-icing salt
Brown patches low on one side, near a walkway, driveway or a favorite dog stop, usually point to urine or salt. Rinse the area with water and add a small barrier.
6. Spider mites
In hot, dry late summers, spider mites can bleach and brown foliage. Tap a branch over white paper. Tiny moving specks mean mites. A strong spray of water every few days often controls them.
Can brown arborvitae turn green again?
No. Brown arborvitae foliage won't regrow, and arborvitae doesn't sprout new growth from bare wood. The good news is that if the cause is fixed, the green growth keeps growing and fills in over time. Gently remove dead foliage, and don't cut back into bare wood.
Replacing gaps in a hedge
If a tree is too far gone, replace it with a similar size so the hedge stays even. See our Emerald Green arborvitae guide for spacing and care, and check current stock of Emerald Green arborvitae in our catalog.