My emerald cedars are 10 years old and have been doing very well up to this year. There are several that are almost entirely brown and look like they are dying. My soil has a high ph and I started using an acidifying fertilizer last year to adjust the ph (ammonium sulphate). Could this be the cause of the cedars dying? Fertilizer is high in nitrogen as well.
Thuja occidentalis ‘smaragd’ is a cultivar commonly known as Emerald Cedar or Emerald Green Cedar. It is a popular choice for gardeners, particularly as a hedge or privacy screen because of its dense deep green foliage.
You indicated that you applied an acidifying fertilizer because of the high pH of your soil. Thuja occidentalis prefers slightly alkaline to neutral soils, although it can tolerate a fairly wide range of soil pH conditions from about pH 5.0 to 8.0. It is possible that you’ve created soil conditions that are too acidic for these trees. A soil test will give you a better sense of what is going on with your soil pH following the application of ammonium sulfate last year. While home soil testing kits are available, you will find that a professional soil test will be more accurate. A list of accredited soil testing services can be found here.
Depending on the timing, rate and method of application, it might also be possible that you burned the roots of some of the trees. As you noted, ammonia sulfate is a soluble fertilizer with a relatively high nitrogen content (21%). If applied incorrectly, soluble nitrogen fertilizers will draw moisture away from the roots, causing “burn” that will eventually turn foliage brown. This previous response provides some helpful information on fertilizing Emerald Cedars.
The Toronto Master Gardeners frequently gets questions regarding issues with Emerald Cedars. Here are some other responses that might also provide some information on the effective management of these trees, and potential causes for browning or yellowing of foliage.