There are so many different fertilizers and so many people saying you should feed roses at such and such time it gets a little confusing.
Here are some of the products I have in my garage:
Composted cow manure (0.5-0.5-0.5)
MiracleGro Shake N Feed (Slow Release 9-18-9)
Blood meal (12-0-0)
Fish Emulsion (5-1-1)
How and when should I be using them?
Most roses are what are known as “heavy feeders” which means that in addition to providing them with high quality soil, amended with compost and well-rotted manure (which you have on hand), you should be applying fertilizer to help them grow vigorously and to bloom well. The easiest way to do this is to use a granular fertilizer suitable for roses. These can be found at most garden centres. Your slow release fertilizer should work well – the package directions will tell you whether it is recommended for roses. It goes without saying that you should follow the directions on the package when applying any fertilizers.
Knowing when and how to fertilize can be confusing, since plants have different needs at different times in their growth cycle. Experts recommend fertilizing established roses in early spring, at which time a formula high in nitrogen (the first number of the three on your packages) is appropriate to encourage leaf growth. Blood meal and fish emulsion fall into this category. A second feeding in June before your roses bloom and a third feeding in July are recommended, this time with a formula similar to the slow release fertilizer you have on hand. The higher middle number refers to phosphorus, which promotes blooming. After July, roses should not be fertilized since you don’t wish to encourage delicate new growth before the cold season sets in.
The American Rose Society has a clearly written guide on fertilizers for roses: https://www.rose.org/single-post/2018/03/20/a-fertilizer-primer-what-s-in-that-rose-food that you may find helpful. It is important not to over-apply any fertilizer, and particularly blood meal, as leaf scorching can result. Watering before and after fertilizing is important.