At times, fertilizer can be harmful to Tall Fescue instead of helpful, especially if the plant is stressed for another reason.
Avoid fertilizing if Tall Fescue is struggling with diseases such as leaf spot, pythium, large patch, brown patch, or fairy rings and insect problems like grubs, chinch bugs, or armyworms. The fertilizer will further weaken the plant's defenses, making it more vulnerable to other pest problems or disease infections.
Never fertilize Tall Fescue when it is dormant in the winter or growth is slow during the summer. Always wait until the grass is actively growing. When it’s hot, avoid fertilizing during the hottest part of the day to minimize fertilizer burn—try to fertilize in the morning or evening.
Avoid fertilizing just after a rainstorm too. Let the soil dry out for a day or two before applying fertilizer to minimize runoff and possible contamination of ground and surface waters.