Cedar hedges discolor and die back for many reasons. The most common causes stem from misunderstanding the plants cultural requirements. According to North Dakota State University, cedars are low maintenance compared to other shrubs, but it is not a no-maintenance genus. Without proper care, the plants are stressed, become weak and attract opportunistic insects and diseases that do further damage. If the shrubs are still alive with some remaining green foliage, you can attempt to save the plants allowing a few seasons to let the hedge recover.
Water the soil around the cedar hedges immediately and deeply. Maintain evenly moist, but not soupy wet, soil at all times or cedars discolor and die back from persistent drought stress.
Don a pair of gloves and a long sleeve shirt and insert your hand and arm into the hedge's interior. Gently swish you hands around to dislodge all of the loose dead foliage and allow it to fall to the ground. This creates better airflow and sunlight penetration within the shrub and helps prevent disease and insects from breeding in the build up of decaying foliage.
Prune broken and clearly dead or diseased branches. Do not cut any branches with green foliage remaining at the tips.
Rake all the dead brown foliage and branch cuttings that drop out of the canopy. Discard them, leaving a clean soil surface.
Laying down a several inch blanket of coarse mulch helps keep the roots moist and cool and weeds away. Replenish the mulch once or twice a year, as it breaks down into the soil.
Test the soil for nutrients and soil pH. Maintain a pH between 6.0 and 6.5. Amend overly acidic soils less than a 6.0 pH with dolomitic lime.
Fertilizer the hedge plants in the early spring or late fall. Apply the fertilizer lightly over rehydrated and very moist soil. If the hedges were exposed to excessive fertilizer that may have caused the dieback, do not add more fertilizer. If in need of fertilizer, use a slow release product with a guaranteed analysis of 12-6-3 or 16-6-6. Apply lightly according to product label directions and always err on the side of less not more fertilizer when in doubt. Water in deeply.