The answer to 'why is my hedge brown patches' is that the most common causes are drought stress, pests, disease, or improper pruning. Brown patches in hedges are often a sign of underlying issues that need to be addressed.
Drought stress from lack of water can cause leaves and foliage to brown and die off. Pests like aphids, spider mites, or scale insects can also damage the plant and lead to discoloration. Fungal diseases like leaf scorch or canker can also cause brown patches to develop. Improper or overzealous pruning can also stress the plant and lead to patchy browning.