Fertilization begins after you've pruned your roses. Mid-February is the time to begin pruning. After you've done this, fertilize established roses. But do not fertilize when you are planting new roses in your garden. Give them time to settle in.
Some gardeners use a granular fertilizer that becomes available to the plants when the soil warms.
Others use this as well as organics. And, of course, others use only organics. Don't forget to water. Many gardeners apply Epsom salt, which contains magnesium and sulfur, after they have pruned. They've found it encourages lush growth and pretty flowers.