No fertilizer should be applied after August 15 so as not to encourage soft, succulent growth that could be easily winter-damaged. Roses can be fall fertilized after the plants have gone dormant. Applying fertilizer at this time will not encourage growth but will be available as the plants start to grow in the spring. Also by using a fertilizer high in potassium winter hardiness tends to be increased. Nutrient release is dependent on the soil moisture and temperature.
Several forms are commercially available.