It is ideal for fertilizing iris plants just before the flowering season. Many iris varieties bloom in the late spring, it's ideal to start fertilizing them as soon as the weather warms up, or about six to eight weeks before the flowering period begins. Although the flowering phase is over, many varieties will require fertilization at some point in the future. Therefore, it is important to maintain track of the soil nutrient status and product instructions, such as how often to apply the product.