Conifer trees go brown because of pests, diseases or unfavorable growing conditions, such as drought, frost and cold winds. Cypress aphid is a common pest that causes conifer browning. It could also sometimes be caused by mistakes in trimming routines like over-trimming and trimming at the wrong time of year.
Browning in conifer trees can be prevented by following proper procedure and timing when trimming hedges. It is best to trim only twice or thrice a year and never during hot weather or in the fall. When there's already some browning in the trees, it can be controlled with nonchemical solutions like tying healthy shoots to fill bare areas. Browning caused by pests can be remedied by using an insecticide, which should be applied as early as possible.