Question Corner

Q. QI assume that trees need to be fertilized just like any other plant. When is the best time to fertilize the trees in my lawn?

A. aThe ideal time to fertilize trees is in the late fall, soon after leaves have dropped. This includes evergreen species as well. At this time, roots are still actively growing, absorbing and storing nutrients, and priming the plant for vigorous growth in the spring.

Although trees can grow adequately with no applied fertilizers, a fertilizer can stimulate natural growth and help trees that are struggling in growth. A good gauge of whether trees need fertilizer is new shoot vigor and length.

Depending on species, trees less than 10 to 15 years of age may grow 8 to 12 inches or more a year; for older trees, adequate growth may be only a few inches at best.

Nitrogen in fertilizers, when broadcast on the soil surface, will move downward (leach) to tree roots over a short period of time. But if turfgrass is present, it will always intercept a portion of the nitrogen, resulting in over-fertilized turf and poorly fertilized trees. Alternatively, nitrogen applied for turfgrass will often leach down to tree roots.

Phosphorus and potassium tend to tie up in the top soil layer when broadcast, especially in heavy clay soils. Thus, these nutrients should be applied deeper using injection or holes in the soil. In general, phosphorus and potassium do not need to be applied as often as nitrogen, perhaps every three to five years at best.

A soil test will help determine the need for these two nutrients. Young trees tend to respond to annual nitrogen application better than older trees. Older trees have a better ability to extract natural soil nitrogen and mobilize stored nitrogen from woody tissue as needed. If your lawn is regularly fertilized, often there is adequate leached nitrogen for large trees in the landscape.

In general, 2 to 6 pounds of nitrogen per 1,000 square feet is recommended for tree fertilization. This rate is too high for turfgrass. Thus, fertilizer for trees is often best applied by liquid injection or drilling holes in the surrounding soil and filling with dry fertilizer. Urea (45%N) or ammonium sulfate (21%N) are cost effective, “single-grade” nitrogen-only dry fertilizers that require less per area than a “complete” 12-12-12 fertilizer.

Starting several feet from the trunk, make holes in the soil with a rod or drill auger 12 to 18 inches deep. Use a grid pattern, and place holes about 2 to 3 feet apart, extending around the tree and half the radius of the tree canopy beyond the drip line. For example, for a tree with a 10-foot radius in spread from the trunk, make holes 5 feet beyond the drip line. The holes should be filled partially to within 4 to 6 inches of the top with fertilizer to reduce turfgrass access.

While the hole method is not as easy to calculate as broadcasting, square footage can be estimated, and needed fertilizer split evenly between the holes made. Nitrogen can also be injected in the soil using liquid formulations, and this is a common method used by tree care companies.

The pattern of injection is similar to dry application. In general, tree spikes are expensive and difficult to insert below the turf root zone, though they can be useful for new trees. As an added benefit, the holes that are made have been shown to aerify clay soils and improve growth, even without fertilizer.

While fertilization is inexpensive for small trees, it can be expensive, time consuming, and possibly unneeded for larger trees. For larger, established trees, consider a hole-punch application of a complete analysis fertilizer, such as 12-12-12 every three to five years. This will provide all three nutrients and meet the lesser nitrogen requirements of an older tree.