The answer to 'why is my shrub yellowing' is that there are several common causes, including overwatering, nutrient deficiency, disease, or pest infestations. Yellowing leaves are a sign that the shrub is under stress and not getting the right balance of water, nutrients, and care.
If the leaves are turning yellow and wilting, it could be due to too much water, which can lead to root rot and prevent the plant from absorbing nutrients. Conversely, if the leaves are turning yellow and crispy, it may be a sign of underwatering or drought stress. Nutrient deficiencies, like a lack of nitrogen or iron, can also cause yellowing leaves. Diseases like fungal infections or pests like aphids or spider mites can also impact the shrub's health and lead to yellowing foliage.