The answer to 'why is my hedge yellowing' is that it is likely due to a nutrient deficiency, environmental stress, or disease. Hedges can start to turn yellow for a variety of reasons, including lack of nitrogen, too much or too little water, or pest or disease issues.
Common causes of yellowing hedges include overwatering, underwatering, nutrient deficiencies (especially nitrogen), pest infestations, or fungal diseases. The specific cause will depend on the type of hedge, the growing conditions, and any other symptoms you notice. If only a few leaves are turning yellow, it may be a normal part of the plant's life cycle. But if the entire hedge is yellowing, it indicates a more serious problem that needs to be addressed.