Dennis Patton says:
Want more flowers on your roses? If so then you need to provide an extra boost through fertilization. Proper fertilization develops strong, vigorous canes that will end in big fat, plump buds with nice flowers.
Most local soils have plenty of phosphorus and potassium so the recommendation of using a balanced fertilizer such as 13-13-13 or 10-10-10 is outdated. The old recommendation was to apply about 1/2 cup of this type of fertilizer per plant three to four times per growing season between mid-April and mid-August.
Newer recommendations are more environmentally-friendly, as it reduces the use of unnecessary fertilizer containing phosphorus and potassium. Nitrogen, the first number on a bag of fertilizer is the key nutrient for established roses. Using a higher rate of nitrogen and low amounts of phosphorus and potassium are best. Examples of fertilizers to use are 27-3-3 or 25-5-5.
Hybrid tea roses have the highest need for fertilization. Instead of using the half cup rate, use only about 1 to 2 tablespoons per plant using the same schedule. This supplies the same amount of nitrogen as the traditional application but does not build up high and unnecessary levels of phosphorus and potassium. If your roses are grouped in a bed then you will want to reduce the rate as the roots are all sharing the same soil. Maybe use the above rate for each two closely spaced plants.
Landscape or shrub roses may really not need any additional fertilizer. These plants are more vigorous than the old hybrid tea types. As a result they will normally flower all summer long on naturally occurring nutrients in the soil. If you like, they can be given a boost with a mid-April application to get the shrub roses off to a good start.
For the organic growers blood meal and alfalfa is an excellent rose fertilizer. These are higher in nitrogen while being low in the phosphorus and potassium. You can apply about 1 cup or so each time to provide about the same rate as man-made fertilizers.
Fertilizer placement should be made out and around the plant at the drip line. Avoid placing the fertilizer at the base of the plant. Rose roots are out and around the plant. Be careful when working the fertilizer into the soil. Do not dig deeply into the soil as this will damage the rose roots. If the plants are mulched, pull it away, apply the fertilizer and then replace. Fertilization is not complete until the nutrients are watered into the soil.