When do I fertilizer my mature oak tree?
Maintaining maturing trees and shrubs
As trees and shrubs mature and growth rate naturally slows down, the need for N drops. The low maintenance level in Table 3 should be used for established trees and shrubs. The purpose of this low maintenance level is to maintain landscape plants in a healthy condition without excessive vegetative growth.
When to fertilize
Most trees experience a single flush of growth during spring followed by slower growth throughout the summer and fall. Because of this single flush of growth, it is desirable to have nutrients available to the tree as this growth is about to occur. The most beneficial time to apply fertilizer is from when the ground is workable in the spring until just before trees start growing in early May. On sandy soils, applications should be split, half in early spring and half in mid- to late May.
Courtesy of the NYBG Plant Information Service