How And When To Fertilize Trees

A number of factors need to be considered before you invest time and money in fertilizing your trees. The general condition and color of the plant needs to be noted, and you need to know what the “normal” coloration should look like. If trees have poor growth or pale green leaves, fertilizer may make them grow faster and give them more vibrant color. If trees are under attack by canker-causing fungi, fertilizer may make them more vigorous and less subject to these troubles.

But remember that feeding a plant can actually make problems worse if an inappropriate strategy is used.

Tree or shrub health can be easily determined with observations of the terminal bud scars. Bud scales enclose and protect buds on the ends of twigs during the winter and leave scars that encircle the twig after the scales fall in the spring. These scars remain evident for several years on many tree species.

From the tip of the branch to the ring of bud scale scars nearest the tip is the current season’s growth. The growth of the previous years can be determined by observing the distance from bud scale scars to bud scale scars as they occur down the twig.

By observing the length of growth for the preceding 3 or 4 years on several twigs, it’s possible to estimate whether the growth rate is satisfactory or unsatisfactory, increasing or decreasing. At the very least, you’ll be able to second-guess the tree care company that comes in and tells you that all of your trees absolutely, positively need to be fed.

The growth rate will vary with the tree species, soil type and environmental conditions. You should expect trees in the villages, where the soil is richer and deeper, to grow faster than an identical tree found near the Pine Barrens, in Shinnecock Hills or in dune areas where the sub-soil is thin and lacking in available nutrients and the ability to retain moisture.

As a general guide, terminal twig growth on most trees can be 9 to 12 inches or more a year, but it’s dangerous to generalize here. Trees approaching maturity may show only 6 to 9 inches of growth annually, and often less.

A second method used to determine tree growth rates is one that most homeowners won’t want to try. This procedure involves measuring the width of annual wood rings produced in the trunk and is accomplished with an increment borer or increment hammer. Both tools should be used by a trained arborist who can compare cores of wood from “sample” trees to determine their growth characteristics.

Knowing the condition of the tree, however, is not enough. It’s also important to know the condition of the soil. In most instances, the best tool for such use is a soil profile tube or soil corer, but a spade or trowel can be used for taking soil samples.

Several factors affecting the condition of the soil should be considered.

First, topsoil depth is important. The greater the depth, the greater the volume of soil with physical, chemical and biological characteristics favorable for root growth.

Second, soil texture should be noted. Is it composed predominantly of sand, silt or clay?

Third, soil structure is best determined when the soil is moist. Does it stick together to form a tight ball or, more desirably, remain in crumbs that can be sifted through the fingers?

Fourth, is the subsoil tight clay (unlikely in these parts, but it can happen), stony or gravelly?

Finally, has the soil been disturbed? Soil compaction, usually from construction, a change in drainage, the removal of a layer of topsoil or a fill of clay above the original topsoil often reduces plant vigor and growth.

The ideal soil we usually look for has a deep topsoil, silty loam texture, aggregate structure, high organic matter content, high nutrient content, good aeration, moderately high water-holding capacity, and a subsoil slowing internal draining. Remember, this is ideal soil—and there are very few places on Long Island where you’ll find it.

The soil can be tested for nutrients, structure and content, but most homeowners, unless they are starting a personal arboretum or an orchard, will forgo most of these tests.

The advantages usually outweigh the disadvantages in fertilizing trees, but certain points should be kept in mind.

Fertilizing trees or shrubs, when done on the cheap, may stimulate more lawn growth than tree growth, resulting in frequent mowing and little growing, to say nothing of the green rings around the base of the trees. Unless regularly pruned, small ornamental shrubs, through fertilization, may become too large for their situations in a few years.

Heavy nitrogen applications tend to increase twig growth but reduce flower and fruit production in many ornamentals. Some woody species may become tall, spindly or succulent and develop a weeping appearance after prolonged fertilizing. In addition, some research done in the 1960s indicated that American beech, white oak, and some crab apple varieties may be injured by fertilizer applications containing nitrogen, phosphorus and potassium.

And, to add one more element of confusion, there are those who believe that the only good tree fertilizer is one that’s 100 percent organic. There also is a new camp that claims that even more important than fertilizers are biostimulants and soil composts that naturally stimulate tree and shrub growth without adding potential pollutants to the soil and water table.

As for the “when,” nitrogen fertilizers should be applied annually if the need is indicated. Little available nitrogen remains in the soil from year to year, as most of it is used up by plants or carried away by water.

Nitrogen fertilizers can be applied in April or May but are probably best added in October and November. Phosphorus and potassium fertilizers are chemically bound in the soil and become available slowly through several growing seasons, and are generally added every three to five years along with the nitrogen in October or November. In fact, though, most of the time when trees are fed they will be given all three nutrients at once, unless a test reveals the immediate need for one particular element.

There are several ways of getting the fertilizer to the parts of the tree that will absorb them—generally, the roots. A granular fertilizer can be spread under the tree; holes can be drilled, or “punched,” into the soil around the tree, which are then filled with granular fertilizer. A band of fertilizer can be run around the tree at the drip line, but this is impractical where lawns encircle the tree.

Liquid fertilizers can be “injected” into the soil by an arborist using high-pressure equipment, and homeowners can get hose attachments that can do the same and may be useful on small properties with smaller trees and shrubs. But on larger properties, these devices are impractical.

One of the easiest methods for both large- and small-property owners are fertilizer spikes, which are about 4 inches long and an inch in diameter. They are placed around a tree’s drip line and hammered into the ground. In quantity, this method will cost about $8 to $12 for an 8-inch diameter tree.

This method is not without controversy, though, as it’s entirely dependent on soil moisture to break down the spikes, and the nutrient distribution is not nearly as good as injection methods.

These are just the basics. If you think your trees need feeding, there are a number of weeks left this fall to take care of it. More detailed instructions are available online and through the Cornell Cooperative Extension Office in Riverhead.

Keep growing!