Dead cedars are a definite eye sore on your yard and keeping them around is of no benefit to you. The best solution for dead cedars is to replace them completely. If you plan to replant new cedars where you old one once stood, you will need to do the following:
- Remove full cedar tree including the root
- Clean up existing soil, if soil is old consider laying down fresh soil
- Mix soil with feed (bonemeal) for new cedars
Before doing that you may wish to consult the company that sold you the cedars to see if they may be covered under a warranty so you can avoid the costs of buying new ones.