Question: I’m not sure when I should fertilize my shrubs and trees. I used to think that spring was the best time, but I see people fertilizing in summer, and recently I was told that fall is the best time. What do you suggest?
Late summer and early fall are the best times to fertilize shrubs and trees. At this point in the year, woody plants make the most efficient use of the fertilizer's nutrients, which are absorbed when the roots are actively growing and shoot growth has ceased. Research has shown that early-spring growth depends almost exclusively on nutrients that were absorbed and stored the previous year.
Spring is the second best time, after new shoot growth has ceased and the leaves have reached mature size. The concern used to be that late-season applications of fertilizer would trigger new growth that might not survive the winter. This has proven not to be an issue in the case of plants such as conifers and most hardwoods. However, with species that are characterized by indeterminate growth—multiple flushes of shoot growth during the growing season, as seen in apples and crape myrtles—you will need to use caution in determining the amount of fertilizer to apply in the fall. For indeterminate growth species, split applications of fertilizer—one in early summer and the other in the fall—are probably best.
This post is excerpted from the September/October 2004 issue of Horticulture.
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