Answer: Evergreens are usually fertilized in the spring. With evergreens, you would not want to fertilize in fall as it might encourage late season growth that would then suffer more severe winter damage.
Deciduous trees and shrubs are sometimes fertilized in the late fall so their roots can take up the nutrients prior to spring growing season. The reason this works is their roots continue to grow until the soil turns cold even as they drop their foliage.
Also, when thinking about fertilizing your shrubs, consider whether or not they need to be fed. If they are growing adjacent to a lawn that is fertilized, they may not need any additional fertilizer on their own.
If in doubt it is better to underfertilize than over fertilize. You can certainly top dress with compost once or twice a year and use an organic mulch. The mulch will break down slowly over time to feed the soil as well. I hope this helps.
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