Generally, you should fertilize your shrubs in either the spring or fall, according to the North Carolina Cooperative Extension. This recommendation applies only if the condition of your shrub indicates it need fertilization. The extension does not advise an annual fertilization based on the calendar only. Set out any spring application before the start of new growth. Apply in autumn after the first frost. Avoid late summer applications, which may promote new growth that won't survive the winter.
New shrubs typically need fertilization to help them become established. If you place new plants in the ground in autumn, wait until spring to fertilize. If you set new plants in the soil in spring, wait six to eight weeks before applying a fertilizer. A slow-release fertilizer gives the best results for new plants. Give only a light application of fertilizer for your new shrubs.
Let a soil test serve as a general guide when deciding how much fertilizer your shrubs need. Check with your county extension to see if it performs soil tests. How much rainfall your location receives can also determine when you should apply fertilizer. During a rainy season, your soil may require more fertilizer. Dry seasons or period of drought stress your shrubs, and they do not need fertilizer spurring new growth and adding even more stress. Once you decide how much fertilizer your shrubs need per year, consider splitting the application between early spring and late fall.
Do not assume that it's time to fertilize your shrubs because of wilting leaves, yellow leaves or poor growth. Other conditions besides a lack of fertilizer might cause these problems. Inadequate moisture, adverse climate, poor aeration and disease could be the culprits. Spurring new growth by adding fertilizer will not necessarily solve the problem and might make it worse. Always determine the source of your shrub's failure to flourish before deciding it's time to fertilize.