I have always had good luck with roses. Perhaps it’s because I’m half English, so it is ingrained in my particular genetic mixture. More likely though, it’s because I really like them, and because of that, I have made a point to learn what makes them happy. One thing that I do, no matter how busy I am, is to give them regular fertilizer once a month from April to September. I try to do it on the first of the month, in order to keep it consistent, and to help me remember to do it. I use Rose-tone, by Espoma, which has a nutritional breakdown of 4-3-2. (Nitrogen-Phosphorous-Potassium). I’m sure there are other good brands out there, but I like this one.
The application is simple: Just sprinkle it around the “drip line” of the plant, and either water it in, or let the rain do it. The “drip line” is the outermost circle of leaves of the plant. (See illustration.) By spreading the fertilizer that far out, as opposed to right at the base of the plant, you encourage healthy root growth and spread.
And that’s it! Really, the hardest part is just remembering to do it.