The answer to 'why is my flower bed yellowing' is that there are several common causes, including nutrient deficiencies, too much or too little water, disease, or pest infestations. Yellowing leaves can be a sign that your flowers are not getting the right balance of nutrients, or that there is an underlying problem affecting their health.
Some of the most common causes of yellowing flower beds include lack of nitrogen, overwatering, underwatering, fungal diseases, or pest infestations like aphids or spider mites. The specific solution will depend on the underlying cause, so it's important to diagnose the issue correctly.