The Knock Out® Family of Roses will perform beautifully even without fertilizing. However, if you choose to give them an additional boost, it's very important not to fertilize until after the roses are established and go through one bloom cycle.
Some additional tips to keep in mind:
Use a balanced fertilizer or fertilizer formulated for roses from your local garden center and apply after the first wave of flowering (be sure to follow the specified rates and method of application provided on the product label).
Make sure the soil is moist before you fertilize to avoid burning the roots.
Do not fertilize late in the summer as this is the time the rose should be preparing themselves for dormancy and you do not want to create unnecessary new growth that will likely die back from the first hard frost.