Pests and Problems

• Thuja shrubs can suffer from a number of insect pests and problems. They will not grow well in full shade and can suffer from spider mite infestations. Only two problems cause brown foliage, however.
  
  Bagworms are voracious pests that can quickly devour and kill an entire plant in mere days. The shrub will turn brown and may even lose all of its foliage. Unfortunately, thujas are very susceptible to bagworm infestations. Foliar burn can also occur if the shrub is planted in an area that is exposed to harsh, cold winter winds.
  
  

Inner Branches

• Although these shrubs are evergreen, the inner branches often do not receive enough sunlight to remain healthy and green. The same dense growth that makes them so attractive also means that the inner foliage often dies and drops when autumn arrives. If you gently pull the branches away from the plant, you may notice that the innermost branches toward the center are brown and shedding leaves. You will probably also notice a dusting of fine brown leaves around the base of your plant. This is normal and nothing to be worried about.
  
  

Winter Color

• If you notice your plant is turning brown as the months turn to winter, it might not be a problem with your shrub at all. Some types of Thjua occidentalis normally turn brown in cold weather. Although some cultivars, such as 'Nigra', keep their rich green color through the winter months, many other cultivars turn a yellow-brown or bronze. Luckily, once spring weather arrives, the foliage will turn green again.
  
  

New Growth

• If it's just the tips of the branches that are turning brown, take a closer look. Many types of Thuja have new growth that emerges grown. As spring turns to summer, the foliage will turn greener. This is not true for cultivars that are brown year-round, however, such as 'Rheingold'.