Fall Fertilization of Trees

There is considerable discussion about the proper time to apply fertilizers to trees. The practice of applying N-P-K fertilizer to trees in the fall is common in the arborist and landscape industries, even in the northern zones. Is this practice a good one, or not?

P and K, Yes; but only “Low or Slow N” in Late Fall

Application of phosphorus (P), potassium (K) in the fall actually helps the tree prepare for winter, by inducing a proper hardening-off of this year’s foliar tissues. This practice is common in forest tree nurseries. However, high levels of soluble nitrogen can encourage foliar tissues to remain young (juvenile), and to delay their preparation for frost. Therefore, application of high levels of soluble nitrogen (N) fertilizer to trees within 6 weeks of anticipated first frost will increase the risk of early frost damage to the foliage. Such nitrogen-induced foliar damage is more prevalent in hardwoods rather than in conifers. Slow-release forms of nitrogen are safe for use in the fall, since these will not have the same effect in promoting top growth as the fully soluble forms of N. For this reason, most commercial applications of nitrogen in the fall utilize “low and/or slow” Nitrogen.

Arborists and landscapers in northern zones who fertilize in the fall, particularly in the late fall, we encourage applications of lower, slow-release N, like Roots PHC for Trees SRN 11-22-22. This product contains 50% slow release nitrogen and a comprehensive selection of micronutrients. If you already fertilize in the spring, and you want a second fall treatment, consider Roots PHC BioPak Plus 3-0-20. Since BioPak Plus will not encourage untimely foliar growth, it has no winter restrictions, so it is ideal as the product of choice, even for late fall application, and is compatible with PHC Injectable and other chemical fertilizers. Remember, these two products can be used in any season when injections are done: spring, summer, or fall. But high to moderate levels of soluble nitrogen fertilization should be restricted to spring, summer, to avoid frost damage. If your fertilizer schedule is approaching mid to late fall, then use Roots PHC BioPak Plus.