When and How Often 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 need 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 a darker green color. If trees are attacked by an insect like the Hemlock woolly adelgid and the hemlock is fertilized — this encourages the needle-feeding insect by providing more lush needles to feed on. This is also true of privet hedge that’s been attacked by the Prunicola scale. In all of these cases, feeding the plant actually makes the insect problem worse by increasing the availability of certain nutrients within the plant that the insects thrive on.

One of the best indicators of tree or shrub health can be easily seen on plants with 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 three or four 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 who comes in and tells you that all of your trees absolutely positively need to be fed. This is rarely the case.

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, Shinnecock Hills or in green belt and dune areas where the sub-soil is thin and lacking in available nutrients and 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 only show 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 — available at “better” garden supply stores — 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? Thirdly, 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, removal of a layer of topsoil or fill of clay above the original topsoil often reduces plant vigor and growth.

The ideal soil that 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 now profess that the only good tree fertilizer is one that’s 100 percent organic. There is also 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 “whens?” 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, 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. Granular fertilizers can be “punched” into the soil or added to the soil in holes drilled around the tree which are then filled with fertilizer. You need to know how much and what formulation though. A band of fertilizer can be run around the tree at the drip line, but this is impractical where lawns encircle the tree as the fertilizer leaves a persistent green ring.

Liquid fertilizers can be “injected” into the soil by an arborist using high pressure equipment, and you can get hose attachments that allege to do the same. On a small scale, they do, but on larger properties these devices are impractical. One of the easiest methods for small- and medium-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 $10 for an 8-inch diameter tree.

Depending on the spikes being used — organic ones are available — Jobe’s is the most recognizable name in tree fertilizer spikes, but generic ones are available as well. This method is not without controversy as it’s entirely dependent on added soil moisture to break down the spikes. For a small orchard, though, this could be an easy solution.

The cost of having an arborist do a fertilizer injection will inevitably be greater, but the results may be noticeably better. On the other hand, every tree does not need to be fed every year, possibly with the exception of fruit trees.

Compost teas have also become very popular in recent years as a tree fertilizer as well as a biostimulant. These are generally applied by a tree care company using a soil needle that allows the tea to be injected into the soil. Based on a few years of experience with this method on older estate trees I can highly recommend such an application but ask what’s being used, how and where it’s produced and how long a particular “brew” has been around.

These are just the basics. If you think your trees need feeding there are a number of weeks this fall to take care of it. Also keep in mind that fruit trees generally perform much better when they’re on a fertilizer schedule. Keep growing.