Fertilizing Trees and Shrubs

Objectives

Fertilizers improve the appearance and condition of ornamental trees and shrubs and enable plants to resist specific diseases and insects. Fertilizer response varies with the plant as well as the environment. Soil fertility, aeration, drainage, exposure to sun and wind, temperature of the site, and proximity to buildings, walks, and streets are but a few of the many factors that influence plant growth.

Best growth depends upon the landscape use of the plant. The largest plant is not necessarily the best. Fertilizer comes second to water in producing color and size, two attributes of good growth.

The extent to which fertilizers are applied for established plants depends upon the fertility of the soil and the growth desired. If plants are growing well and look good, you may choose not to add more fertilizer. For additional growth, more fertilizer would be added. This is also true where malnutrition is evident, as indicated by poor foliage color and short and weak growth not caused by lack of moisture or fungus or insect attack.

General Plant Nutrient Needs

Essential elements for plant nutrition include nitrogen, phosphorus, potassium, calcium, zinc, copper, molybdenum, magnesium, iron, sulfur, manganese and boron. They come from the soil and from applied nutrients. Carbon, hydrogen and oxygen are obtained from the air or through the soil.

Certain elements such as boron, zinc, manganese, iron, copper, and molybdenum are spoken of as trace or minor elements because plants require very small amounts of them. However, they are just as essential for plant growth as elements required in larger amounts like nitrogen, phosphorus and potassium.

Types of Fertilizers for Woody Plants

For convenience and adequate fertilization of most woody ornamental plants, home gardeners use a complete fertilizer, that is, a fertilizer containing all three of the major fertilizer materials. The law requires that every package of fertilizer be labeled to show the guaranteed minimum percentages (or grade) of the three major fertilizer nutrients. For example, a 10-6-4 fertilizer contains at least 10 percent nitrogen, 6 percent phosphoric acid and 4 percent potash. Many grades of complete fertilizers are available.

Inorganic types:
Many complete fertilizers are composed of simple chemicals quickly absorbed by plants. These inorganic fertilizers, some of which are readily soluble in water, are the least expensive and can be bought at any garden-supply store, but they require some care for safe application during the growing season. In addition, inorganic fertilizers are sold in liquid form (see below).

Organic types:
Natural organic fertilizers and synthetics (urea-forms) release their nutrients somewhat more slowly. Many of these organic or synthetic forms are incorporated in other all-purpose fertilizers, which are widely available. These will appear on the label as water-insoluble nitrogen (WIN); for example: urea-form, IBDU and sulfur-coated urea. Processed fertilizers having nutrients derived from various manure sources are becoming more widely available. These fertilizers will have the guaranteed minimum percentages of the major nutrients listed on the labels. The natural and synthetic organic fertilizers may be more expensive, but the danger of burning plants is reduced provided the recommendations on the package are followed explicitly. Barnyard manure is relatively low in nutrients and usually contains many weed seeds. Weed-free fertilizers are preferable even when manure is available without cost. Commercially dried manure products are very expensive for the benefits received.

Readily soluble fertilizers:
Readily soluble fertilizers are high analysis (for example 20-20-20), dry, concentrated fertilizers. Most of their components are derived from inorganic sources. Usually the recommendations are to dissolve a specified number of ounces or teaspoonfuls in a particular volume of water. When recommended rates are used, they can be applied safely to growing plants. In borders, the ease of application is a major advantage for more uniform distribution of the fertilizer can be made than by using dry materials.

One of the concerns in using readily soluble fertilizers is that nitrogen can be readily leached. To alleviate this concern, formulations are available that combine soluble forms with controlled-release forms of nitrogen.

Complete liquid fertilizers:
Complete liquid fertilizers are similar to readily soluble fertilizers. The only difference is that they are liquid concentrates. Usually a small amount of the concentrate is diluted in a larger volume of water to make a working solution. The manufacturer's recommendation should be followed to avoid plant injury. Complete liquid fertilizers can be used in the same manner as readily soluble fertilizers. The advantages of ease of application and uniform distribution are the same as for readily soluble materials.

Foliar fertilizers:
Foliar applications of readily soluble fertilizers offer homeowners advantages as well as disadvantages. Advantages include convenience and ease of application, correction of minor element deficiencies and "green-up" of yellow, nutrient-deficient foliage. Foliar-applied nutrients may improve the appearance and growth of plants that do not receive an adequate nutrient supply through the roots. However, foliar applications of fertilizers should generally be looked on as a supplement to, not a substitute for, soil applications of fertilizers. Foliar nutrition may effectively supplement root nutrition throughout the growing season, but it is generally not economical for homeowners to attempt to provide all the nutrient requirements of a plant through the foliage.

The problems occasionally associated with foliar applications of fertilizer are mainly a result of not following the directions of the fertilizer manufacturer. Only those materials that give specific recommendations for use as a foliar spray should be used, and directions should be closely followed. Failure to follow the manufacturer's directions may result in plant injury. Spray on cloudy days or in the evening.

Chelates:
The application of chelated iron to the soil produces a longer-lasting effect than spraying the foliage. Repeated applications are often necessary to maintain attractive green foliage. The addition of chelated iron is supplemental to regular fertilizer practices for certain plants. Chelated iron is now available in garden-supply stores. Carefully follow the manufacturer's recommended application rates. Spray on cloudy days or in the evening.

Timing, Methods, and Rates

The application of fertilizer for trees and shrubs should be made in early spring at the first signs of spring growth. Applications made at recommended times for turfgrass will also benefit those trees and shrubs growing in the turfgrass areas. An additional application of fertilizer may be made in autumn when the trees are dormant, generally starting in October and continuing until December while the soil is still warm (40o F.). Apply the fertilizer in autumn after a several-day period when the average temperature has been under 500 F. each day.

Several methods can be used to fertilize trees and shrubs: broadcasting the dry or liquid fertilizer on the ground, subsurface application of either granular or soluble fertilizers or injection.

Broadcasting
The broadcasting method is the least expensive and, in certain instances, can be just as effective as subsurface techniques. It is the method most commonly used for shrub or small tree fertilization. In broadcasting where lawn areas surround the tree, it is suggested that the fertilizer be applied over the entire lawn area to stimulate and improve both lawn and tree. Adjoining garden areas of shrubs or ground covers should be fertilized as well. Complete fertilizers, of such composition as 10-6-4 or 5-10-5, can be used effectively for the general range of trees and shrubs on soils that are neutral to acid in reaction (pH 7.0 or less). Whenever possible test soil for nutrients and follow the recommended rates of fertilizer suggested based on the test results (contact Cornell Cooperative Extension - Suffolk County for information on having soil tested). If soil test results are not available these fertilizers should be broadcast at the following rates:

For the general range of shrubs
Apply 5-10-5 at 2 to 3 pounds per 100 square feet or 10-6-4 at 1 pound per 100 square feet. Fertilizers of similar analysis could be substituted and used accordingly.

For trees where fertilizer is broadcast over the whole lawn area
Starting at a distance 2-1/2 feet away from the trunk, evenly apply 20 pounds of 5-10-5 or 10 pounds 10-6-4 to each 1,000 square feet of established lawn. Fertilizers of similar analysis could be substituted and used accordingly. Lawn grass must be dry at the time of application; and immediately after the fertilizer is spread, the lawn should be thoroughly watered.

For trees where fertilizer is broadcast in non-lawn areas, including areas where ground covering plants are located
Measure the trunk diameter of the tree 4-1/2 feet above the ground. Then apply fertilizer, starting at a distance 2-1/2 feet away from the trunk and extending beyond the spread of the branches. To determine how far to extend beyond the spread of the branches do the following: Measure the distance from the trunk to the edge of the spread of the branches. Divide this distance by 3 (or 1/3). This is the distance to extend beyond the spread of the branches. Use the following amounts:

Trees up to 3 inches in diameter
5-10-5: 2 lbs. per inch of trunk diameter
10-6-4: 1 lb. per inch of trunk diameter

Trees over 3 inches in diameter
5-10-5: 5 lbs. per inch of trunk diameter
10-6-4: 3 lbs. per inch of trunk diameter

If a rate indicated is in excess of 20 pounds of 5-10-5 in 1,000 square feet or 10 pounds of 10-6-4 in any 1,000 square feet, the fertilizer should be divided into two or more portions and applied at intervals of from 4 to 6 weeks. Where roads, patios and sidewalks take up a large proportion of the space under the trees, the area to which fertilizer can be applied is reduced. Rates of application should not exceed 20 pounds of 5-10-5 or 10 pounds of 10-6-4 per 1,000 square feet at any one application. Fertilizers of similar analysis could be substituted and used accordingly.

Subsurface methods
Subsurface applications are recommended over broadcast techniques in some instances. These include situations where limited space results in rates of fertilizer applications greater than can be tolerated by ground-covering plants (i.e. lawns), where slope areas foster the runoff of fertilizers applied to the surface of the soil, or where large trees are present. The crowbar method involves the placement of granular fertilizer in holes that are punched at 18-inch intervals, to a depth of 8-18 inches for deciduous plants (8 inches for evergreen plants), starting at 2-1/2 feet from the trunk and extending beyond the spread of the branches. Determine how far beyond the spread of the branches to apply fertilizer and the fertilizer rate to apply as described in the previous paragraph. Divide the rate of fertilizer evenly by the total number of holes made. Pour this amount of fertilizer into each hole; then fill the hole with water. Do not exceed one cupful of 5-10-5 or one-half cupful of 10-6-4 in any one hole. After the water drains away, fill the hole with good topsoil. Fertilizers of similar analysis could be substituted and used accordingly.

Commercial firms that manage trees (tree specialists or arborists) inject solutions containing fertilizers beneath the soil surface. These solutions usually contain soluble fertilizers and controlled- release forms of nitrogen suspended in solution. Such combinations provide both immediately available nutrients as well as nutrients that will gradually become available over an extended period of time. Fertilizers used in this manner are now available from horticultural supply firms and should be used according to the manufacture's instructions.

Subsurface applications of fertilizer by either means should be made at the first signs of growth in the spring. If such applications are made at other times of the year, it is advised that a major portion of the nitrogen be of a controlled-release nature. This would avoid forcing of late plant growth as the result of applications of totally soluble forms of nitrogen; it also reduces the risk of leaching of nitrogen during late autumn and winter.

Trunk injection of nutrients
This technique is employed by commercial firms as treatment in extraordinary situations (i.e. chlorosis in pin oaks or in extremely limited growing areas where it is difficult to make above- or in-ground applications of fertilizer). Observations to date indicate varying degrees of success with trunk injection, and in some cases injury to plant parts has resulted.

Fertilizers and the Environment

Fertilizers are a necessary input for landscape plantings; yet when fertilizers are applied in excess or improperly, various nutrients escape from the horticultural system and damage the environment. Nutrients escape from the horticultural system in various ways depending upon the chemical and biological nature of the element.

Nitrogen, regardless of the chemical form added, converts to nitrate and is lost in the soil water or by erosion in the soil organic matter. Thus, nitrogen may be a problem in groundwater as well as in surface waters.

Phosphorus is usually bound tightly to the soil particles with only very small amounts in the soil water. It may also occur in organic materials. Some of these organic materials are water soluble. Phosphorus is usually lost by surface runoff and erosion. A number of techniques help prevent nutrient loss to the environment. The most effective method is to add the amount of nutrients needed, but not to exceed crop needs. Don't over- fertilize. The second technique is to apply the fertilizers in a manner to achieve efficient plant uptake.

Preventing soil erosion is also a necessary part of protecting the environment.

Reprinted from: Suggested Practices for Planting and Maintaining Trees and Shrubs, A Cornell Cooperative Extension Publication, Information Bulletin 24, 10/89.