According to the University of Minnesota Extension, the best time to fertilize evergreens is in early April, prior to the expansion of new growth. Fertilizer may be applied until about mid-July.
Applications after midsummer stimulate growth too late in the season to harden before the cold arrives, and such growth is more likely to suffer injury during winter months. Slow or timed-release fertilizers, however, can be added later in the season, as a light, late summer or early fall application helps stressed trees survive the winter in better condition. Mid to late fall applications of slow-release fertilizer may also prove beneficial.