Please see our website for information on fertilizing perennials http://extension.umd.edu/hgic/flowers/maintenance-annuals-and-perennials
Established trees and shrubs usually do not benefit from fertilization. Mature trees also do not need fertilizer because as trees age and mature the growth process slows down. In the landscape, woody plants receive nutrients from lawn fertilizer if their roots are adjacent to or growing in turf areas. And nutrients are provided by decomposing organic matter such as fallen leaves, decaying mulch, and minerals in the soil. Overfertilization can make the plants susceptible to sucking insects.
A reason to fertilize is if you notice poor growth, leaf yellowing, etc. Fertilize according to a soil test. Results will give pH and nutrient deficiencies. http://extension.umd.edu/hgic/soils/soil-testing