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The ABCs of fertilizing trees

Q: Should I be fertilizing my trees annu­ally? If so, do I fertilize both young and mature trees, and what is the best way to fertilize them?

Phil Albertson, Brattleboro, VT

Apply fertilizer at the base of a tree to the drip line every two to three years. Apply fertilizer at the base of a tree to the drip line every two to three years. Photo/Illustration: Allison Starcher

A: Wayne Cahilly, an arborist and manager of the Institutional Mapping Department at the New York Botanical Garden, replies: While it is good to be concerned for the health of your trees, they do not necessarily need to be fertilized on an annual basis. There are two key questions you should ask yourself when evaluating the need for a tree-fertilization program:

Is the tree showing any signs of a nutrient deficiency, and are there any fertilizers that are being added during other landscape-management practices and, if so, how much?

Many times, trees will tell you if they need more nutrients. The most common sign is yellowing of the leaves on deciduous trees, also known as chlorosis. This can be a symptom of several nutrientdeficiencies. To pinpoint the culprit, it’s best to have a soil test done. An important step in any good fertilizer program, soil tests should be done every year or two. They can tell you about the nutritional elements that are available in your soil and about the nutrients your plants may require. Nitrogen is most often the missing nutrient that trees need.

Once you know what the soil is lacking, consider what else is being fertilized nearby. Grass and trees share root zones, so whatever fertilizer you are applying to your turf is being applied to your trees as well. Be sure to factor this into your overall tree-fertilization program.

In general, mature trees often benefit from fertilization every two to three years. Fertilizing a mature tree is not an attempt to make a big tree bigger but to ensure adequate nutrition during its natural cycles. Fertilizing a young tree can involve an every-other-year application of fertilizer containing nitrogen, phosphorus, and potas­sium in a ratio appropriate for your soil type and gardening conditions. Application rates will vary from region to region, but younger trees usually can tolerate and utilize a higher rate of fertilization than older trees.

One way to fertilize a tree is to broadcast granular fertilizer evenly over the area under the tree, extending to the drip line. Be sure to apply the fertilizer to moist soil or water it in immediately after application to avoid fertilizer-burn damage to the roots.

From Fine Gardening 95