The best thing to feed roses that I’ve found is a mixture of chicken manure or compost, rose and flower food, and alfalfa meal.
You want to start feeding when you see the new growth. You don’t want to fertilize too early because this will force out new growth that could be hit by a freeze.
Roses need a specific rose and flower food that is formulated with nitrogen, phosphorus, and potassium as well as other ingredients they need to thrive.
I fertilize roses by digging a shallow, circular trench around the base, working the food into that trench, and covering it back up with soil.
I plant bare root roses with a good amount of compost and make sure they were well watered in. About a month later, I will begin the fertilizing/feeding routine.
You keep roses healthy by giving them the 5-6+ hours of sun a day, providing regular water, pruning properly, and feeding as needed for your growing zone.