During the spring and summer, which are the active growing months for citrus trees, they need fertilizing one to two times. During a young tree's dormant period in the fall and winter, it is only necessary to fertilize every two to three months, according to Gardening Know How.
As a tree matures, it is not necessary to fertilize during the dormant period. When a citrus tree's appearance is healthy, with full, dark green foliage and no dropping fruit, fertilize it less often because over-fertilizing may result in a poor quality fruit. Fertilize the citrus trees with a nitrogen-rich mixture that contains micronutrients, such as magnesium, iron, copper and boron.