At times, fertilizer can be harmful to Yellow bellflower instead of helpful, especially if the plant is stressed for another reason.
Avoid fertilizing if Yellow bellflower is struggling with diseases (downy mildew, powdery mildew, gray mold, lef spot) or insect problems like aphids, spider mites, or whiteflies. The addition of fertilizer will further weaken the plant's defenses, making it more vulnerable to other pest problems or disease infections.
Avoid fertilizing when the soil surface has hardened or become too dry. In this instance, plants are at a greater risk of fertilizer burn on their roots. Water the potting soil to get it slightly moist, and then fertilize.
During the hottest part of the summer, Yellow bellflower may go dormant until temperatures cool down. When growth slows, avoid fertilizing plants until growth resumes.