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Ash trees should be fertilized once or twice a year. However, not just any fertilizer will do. Ash evolved in very specific forest conditions and are susceptible to insects and disease. They therefore have very specific fertilizer requirements that will not be met with a standard "one-size-fits-all" lawn and turf fertilizer. In particular, fertilizers that release large amounts of Nitrogen quickly into the soil can encourage structurally weak growth that could cause problems. Aside from the formulation, the method of fertilization differs from that of your grass. In order to give your trees the most benefit, the fertilizer must be placed below the grass roots.
To find out how to obtain fertilizer specially selected for use on ash trees, click here.