Answer: As a rule of thumb, deciduous trees can be fertilized in mid fall or early spring, evergreens typically in spring. A topdressing of good quality compost can be applied in fall, spring, or early summer as well. The compost combined with an ongoing layer of organic mulch may be sufficient without additional fertilizer. Whether or not you need to feed them at all (most mature plants do not require it, for example) would best be based on the results of soil testing and the plants' performance to date.
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