Overwatering

• If you overwater your Knock Out roses, you may waterlog the shrub's roots, and the leaves will turn yellow as an indicator that the plant cannot receive the oxygen it needs. The plant will dehydrate and die if you don't improve your rose's drainage system.
  
  

Spider Mites

• One of the common pest problems for Knock Out roses is spider mites. A colony of spider mites will result in a decrease in chlorophyll in the leaves. Carefully pressure wash or apply mite controlling poison to your Knock Out roses.
  
  

Poor pH Balance

• If the pH balance of your plant is too low or high, yellowing of the leaves can occur. Unlike watering or pest issues, yellowing via poor pH balance appears slowly. Test the pH balance and add a soil acidifier or base as needed to prevent continued yellowing.