Arborvitae and other needle evergreens are alive year round and can be stressed with general weather changes and more so with extremes in temperatures, rainfall and dry periods. Needles will be the first lost, then twigs, small branches. if you are not finding disease or insects, the cause is more likely environmental. Best practices for needle evergreens is to be sure they are well watered late into the fall, watering the last time just before you put the hose away sometime in November. Consider feeding the plants next spring as growth resumes. these two practices will give the plants help in recovery.
