There are several things to look for in a fertilizer:
1) Choose one that is labled as being especially for roses. This will have the right balance of nitrogen, phosphorous, and potassium. Nitrogen stimulates top-growth. Phosphorous stimulates bloom, and potassium supports root development.
The labels state the amount of each in this order N-P-K (Nitrogen-Phosphorous-Potassium) . So a fertiilizer labled 5-8-6 would have 5% available nitrogen, 8% available phosphorous, and 6% available potassium. Such a formulation should work for roses.
A fertilizer too rich in nitrogen will cause a plant to grow fast, but may retard flowering. It may also invite pests such as thrips and aphids which thrive on soft new-growth tissues. Most rose formulations have somewhat more phosphorous than nitrogen or potassium.
2) Choose one that is derived principally from organic materials. Remember, you are not just feeding the rose, but are also feeding the soil that supports it. If you feed the soil and keep its flora and fauna flourishing, the soil will augment the health of the rose. Inorganic fertilizers can retard or kill beneficial flora and fauna in the soil, making the rose more dependant on inorganic sources of nutrition. Too much inorganic fertilizer can put the soil and the plants that depend upon it on a spiralling path to doom.
3) If using non-organic fertilizers, choose slow-release formulas. Fast-release formulations may provide the nutrients fast, but they tend to stress the beneficial organisms in the soil. These organisms do a lot to extract nutrients from the organic and inorganic parts of the soil. The food that the rose uses - the chemicals of the fast-release formulas - are waste products for these beneficial organisms. Use too much, and the soil loses fertility. Then the only way the rose can survive is if you continue to feed it fertilizer. It becomes a fast-release fertilizer junky.
4) Look for fertilizers that claim to build the soil. Kelp and humic acid are some ingredients that can work here. Fertilizers extracted from plants and animals will be better at this than inorganic fertilizers. Alfalfa, peanut hulls, bone meal,and fish meal are some common organic materials.
5) Test the soil and determine what soil ammendments are necessary to provide adequate levels of iron, calcium, potassium, copper, magnesium, and other trace minerals. Then ammend the soil accordingly. Also determine what you need to add to bring the soil to a pH of 6.5 to 7.5. Roses will grow in soils of 6.0 or 8.0, but they will be weakened and more prone to disease and attack by pests when soil is outside the optimum pH range.
Always read and follow the labling. Too much fertilizer can cause serious harm. Strong fertilizers applied directly to the roots could kill the plant. Too much fertilizer could cause other problems. It may make the plant more subject to attack by pests, or it may make it short lived, or it may kill it. Overfertilization ultimately kills the soil, making the soil incapable of supporting plant life until the toxic fertilizer level is drawn down by the slow process of percolation.
Fertilization is an important part of getting the most out of a rose, but it is much less important than most of the other steps in this article. Few roses will up and die without added fertilizer as they might if they receive inadequate sunlight or water. This is especially true of species roses and non-remontant hybrids.
There are two kinds of roses: those that need feeding and those that don't. Of course, roses fall into a wide range of categories: some do best with no feeding, some with modest feeding, and some require quite a bit of feeding. Some roses are completely happy on thin, sandy soil. Others really do better with a good bit of clay in the soil. All roses do better with organic material and healthy soil flora and fauna. When they do better they bloom more.
The simplest way to look at it is that if roses bloom heavily and repeat they will require heavier feeding. If they are aggressivly pruned they will need heavier feeding. If, on the otherhand, they live in fertile soil, are rarely or just lightly pruned, and if they bloom just once per year, feeding is not so important.
Non-repeating roses such as gallicas, albas, and most species roses will blossom but one time per year. In good soil they are capable of going years without feedings. Annual mulching might completely suffice. Their performance can be enhanced by applying some slow-release fertilizer to the soil in early spring as the plant comes out of dormancy. By the time the fertilizer reaches the roots, the plant will be at a point where that feeding will really pay off.
Most roses will flower eight to twelve weeks from leafing out, then spend the rest of the season storing up energy for next season's display. A tiny bit of fertilizer after the blossoms go away probably won't hurt anything, but don't fertilize after midsummer.
Roses use up a lot of resources in making stems and blossoms, so roses that repeat will tend to use up more food than those that do not. For this reason feeding of remontant roses: Hybrid Teas, Floribundas, Bourbons, Hybrid Perpetuals, and remontant Shrub Roses and Climbers is essential.
Apply a slow-release fertilizer in the spring as the roses start to leaf out. Then, when the first flower opens add some more fertilizer. If you live in a hot part of the country and the rose stops producing blossoms over the summer, then it is a good idea not to keep dumping on fertilizer. But when the roses start blooming again in September you can add just a bit more.
If you live in zone 5 or cooler, it's probably not a good idea to fertilize roses after the first of September. The roses need to 'harden off' for the winter and fertilizer promotes soft new growth that is prone to freezing.
Feeding to meet Special Needs
People who live in the southwest may need to add soil acidifier and supplemental iron from time to time to prevent the spiraling alkaline level from making iron unavalable and causing chlorosis. If one keeps the surface well mulched, the mulch will tend to moderate the pH of the soil, and additional chemicals may not be required as often.
Some people add greensand, which happens to be a rich source of a number of minerals. People on clay soils add gypsum as a soil ammendment to break up clay soil. It is a great source of calcium and it does not alter the pH of the soil as would limestone.
People who raise exhibition roses may use foliar sprays that contain fertilizers. The assumption is that if you feed the leaves exactly what they need, it saves the plant the trouble of doing it for itself. The technique evidently works well enough to have a lot of followers; at least if used in moderation it does little harm.
The more you press your roses for blooms by pruning and watering, the more you will need to worry about soil nutrition.