Knockout rose leaves can yellow because of quite a few reasons. Solving the problem and keeping your roses of all breeds healthy, requires proper care and attention. Most roses stay healthy unless they are suffering from some kind of deficiency or over feeding of water.
Cold and Heat
Your knockout rose leaves could turn a yellowish color owing to extreme weather. They are not resistant to heat or cold and will certainly wilt in both weather conditions. Keep them at a constant temperature between sixty and seventy five degrees.
Watering and Feeding
Do not over water the knockout roses. The roots can drown and effectively make the rose shut down. Do not under water them, especially in hot weather, as they will wilt quickly.
Soil and Atmosphere
Keep the Ph balance in your soil. Some roses respond to different levels of Ph balance and clay soil can depreciate the quality. Do not overly spray your roses with chemicals. You will effectively over medicate them. Make sure there is enough nitrogen reaching the roses. If the knockout rose is planted amongst others of the like and it is the only one not doing well, it could be the rose bush itself that is ill. Keep the roots cool and use high nitrogen fertilizer.