Hello and thanks for contacting Ask Extension. It’s difficult to determine exactly what is wrong with your arborvitae from your pictures but here are some possibilities.

My first thought is winter injury, caused by the winter sun and wind causing excessive water loss from the needles while the roots are in frozen soil and unable to replace the water so they dry out and turn brown. This results in desiccation of the plant tissue. Bright sunny days during the winter warms plant tissue, which in turn initiates cellular activity. Then when the sun sets or goes behind clouds, foliage temperature can drop, injuring or killing the foliage. Bright, cold winter days destroy chlorophyll in the foliage. The chlorophyll does not re-synthesize when temperatures are below 28 degrees. This results in a bleaching of the foliage. Cold temperatures occurring early in the fall before plants have hardened off completely or in late spring after new growth starts can result in injury or death of this tissue. Arborvitae are very susceptible to this type of winter browning. Needle damage normally occurs on the south, southwest, and windward sides of the plant, but in severe cases it may affect the whole plant. With this kind of injury the needles turn brown from the tip of the branch first which is what I see in your pictures.

Arborvitae are also very sensitive to de-icing salts. Needles turn brown from the tips and then progress back towards the trunk. I cannot tell from your pictures if they are near a walkway or road that is salted in the winter.

Another possibility is the plants aren’t receiving adequate water during dry spells. Arborvitae are also sensitive to root rot from excess moisture so either extreme can cause this type of damage. Avoid over-fertilization, which can lead to nutrient burn. Monitor for environmental stressor like extreme temperatures or wind exposure.

Check for signs of pests such as spider mites or bagworms.

You can reduce winter injury by planting arborvitae on north and northeast sides of building or in areas protected from wind and winter sun. Prop pine boughs or Christmas tree greens against or over evergreens to protect them from winter wind and sun. Construct a barrier of burlap or similar material on the south, southwest, and windward sides of arborvitae. If your plants exhibit injury on all sides, surround it with a barrier, but leave the top open to allow for some air and light penetration. Keep arborvitae properly water throughout the growing season and into the fall. Never stress plants by under- or overwatering. Decrease watering slightly in September to encourage hardening off, then water thoroughly in October until ground freezes. Do not prune after August as pruning can induce the growth of new foliage that doesn’t have time to harden off before winter.

The brown needles you see on your arborvitae are most likely dead and will not green up. But the buds, which are more cold hardy than needles, will often grow and fill in areas. Wait until mid-spring before pruning out injured needles. If the buds have not survived, prune dead branches back to living tissue. Provide appropriate protection next winter.

Here is a link to an article about some of the insect and disease problems that affect arborvitae.

https://apps.extension.umn.edu/garden/diagnose/plant/evergreen/arborvitae/foliageturnsbrown.html

If you want to know for sure what is happening to your arborvitae I recommend sending an email to <personal data hidden>. Attach your pictures of the affected arborvitae. Include some close ups and some from further away. This is the email address for the Plant and Pest Diagnostic Lab at Michigan State University. They may ask you to send in some samples. Here is a link to the Plant and Pest Diagnostic Lab at Michigan State University. Plant & Pest Diagnostics

Here is another link to an article about keeping arborvitae healthy.

https://extension.umn.edu/planting-and-growing-guides/protecting-trees-and-shrubs-winter#how-to-reduce-evergreen-winter-injury-1264311

1. The brown needles you see on your arborvitae are most likely dead and will not green up. You can prune off the brown parts of the branch back to a viable bud (green growth). It is possible that new growth will start to fill in the open spots. However, if there are no viable buds and you have to prune that branch all the way back to the trunk, new growth will not happen in that area. If you are able to correct the problem the arborvitae will survive and continue to grow but depending on how much you have to prune out it may always have some open spots.

2. If you can figure out the problem and correct it your arborvitae will continue to grow but depending on how much you have to prune out, if may not have the ideal shape.

3. Yes, pruning out the dead, brown growth will help the arborvitae recover. However, if there is no life (green) in some branches, those will not recover.

That’s great that you provide irrigation water during the growing season but what about late fall, in October? Most people drain and blow out their irrigation system at that time and if we have a dry October, arborvitae continue to need water because their needles are transpiring and they need a source of water. Arborvitae also need water in late winter and early spring before most people have their irrigation system turned on. If the ground is not frozen and we have sunny weather they may need water.

You are correct, if the root systems of the arborvitae were under water last fall that could be the cause of the browning. Arborvitae prefer a well-drained soil and don’t tolerate a soil that is water logged for very long. Roots need air and if the soil is water logged parts of the plant may die.

My advice would be to prune back to a point where you still see green needles. Make sure you provide adequate water, but do not over water. Provide some winter protection from the drying sun and winter winds. If the arborvitae fills back in that would be great. If it doesn’t and you don’t like how it looks, I would either replant with another arborvitae or a different evergreen.