The answer to 'why is my flower bed brown patches' is that there are a few common causes, including drought stress, disease, pests, or lack of nutrients. Brown patches can indicate that the plants are not getting enough water, have a fungal or bacterial disease, are being damaged by insects or other pests, or are lacking the right nutrients in the soil.
To troubleshoot, check the soil moisture levels and water the flower bed if it's dry. Inspect the plants closely for signs of disease or pests. You can also have a soil test done to see if the pH or nutrient levels need to be adjusted. If the problem persists or you're unsure of the cause, it's best to call in a professional landscaper or horticulturist to diagnose and treat the issue.