Shrubs, including flowering shrubs can be fertilized in the spring or fall. Fall applications should be done after leaf drop. But if the shrubs are healthy, there usually is no need to fertilize. Following are a few links to University Extension sites with more info about fertilizing trees and shrubs:
https://extension.unh.edu/resource/fertilizing-trees-and-shrubs-fact-sheet

https://hgic.clemson.edu/factsheet/fertilizing-trees-shrubs/

Early spring, late March to early April when new growth starts to emerge is the best time to fertilize perennials. Be careful, because too much fertilizer can result in leggy growth and few flowers. Following are links to University Extension sites with more info about fertilizing perennials:
https://www.canr.msu.edu/news/fertilizing_established_perennial_gardens_feed_em_and_weep

https://ag.umass.edu/cafe/fact-sheets/fertilizing-flower-gardens-avoid-too-much-phosphorus