Hi Don, Kimberly here from landscape-solutions-for-you. Thank you for asking the question on your Cedar trees. Could you give me some clarification on the size and depth of your trench? Depending on the size of the trench, the trees could be collecting water. Once Cedars are established, they don't need as much water as when you first planted them. If you could let me know about the size of the trench, that would be great.
One thing you can do is to scratch the surface of a couple of branches on each tree. If there is green, the trees are still alive. If they are brown, they are in shock or could be dying.
You said your soil is sandy and rocky, one thing if your trees are still alive is to loosen the soil in the trench area. Add some organic mushroom compost to it. This is an organic fertilizer that will last up to one year. The mushroom compost may give your Cedars a boost. You can also have your soil tested to see what nutrients may be lacking, or the soil may have too much of a certain nutrient. In your particular situation, this may be a good idea.
One other thing, sometimes Cedars and other closely related specimens will brown in the middle during cold winters.
I hope this has helped a little, and let us know about the size and depth of your trench. My crew and I have dug several different size trenches, and this may be one of the keys to why your Cedar trees are struggling.
Thanks again for asking your question here at landscape solutions for you. I look forward to hearing from you. Have a great day!
Kimberly