So sorry about your root rot problem. This infection occurs in soil with poor drainage, whether due to compaction, clay soil, or drainage issues. Some plants are resistant to phytophthora root rot, including shrubs such as hypericum, lonicera, spiraea, weigela and tamarix.  If you are looking for taller trees than cryptomeria, Ginkgo and Picea (spruce) are considered resistant. Sadly, your arborvitae hedge is considered susceptible to this root disease, so try to divert any heavy rainfall or other drainage water away from your hedge. Another tip is not let a hose or sprinkler run near the root zone, even on hot days. You can also consider some garden art or structures in this area if you continue to have problems.   Think about a bird bath or a tall garden arch with a bench in this area, then add colour with large containers filled with potting soil so you can grow shrubs, perennials, and annuals here without dealing with root rot fungus. Keep growing, Marianne Binetti