How To Mainten Trees and Shrubs

Application Methods

Fertilizers can be applied either directly or indirectly to plants. When turf is fertilized, tree and shrub roots that extend into the turf area absorb some of the fertilizer, and are therefore indirectly fertilized. Turf fertilization rates should be supplemented only if trees and shrubs are showing symptoms of nutrient deficiency.

Direct application of fertilizer may involve incorporation into the backfill soil or placement in the planting hole at planting time. However, the most common form of direct fertilizer application, broadcasting, is generally the most effective, especially relative to cost. Simply broadcasting the desired fertilizer over the soil atop the tree and shrub roots and watering it in is usually adequate. Compacted soil should first be aerated or raked. Table 2 describes other direct fertilizer application methods.

Fertilizer Placement:

Fertilizer should not be concentrated around the stem or trunk of a tree or shrub, but should be applied over as much of the plant's root zone as possible. For trees and shrubs, fertilizer should be applied over an area twice as large as the crown spread or drip-line. Since most landscape plant roots grow in the top foot of soil, surface or shallow, but not deep application, is recommended.

Factors Affecting Fertilizer Uptake

Numerous factors affect how easily and well trees and shrubs absorb fertilizers. The most important uptake factors are:

• Fertilizer form (inorganic, fast release, or liquid forms are absorbed faster than organic, slow-release, or dry forms)
• Soil type (clay particles and organic matter adsorb or bind more nutrients than sand, so fertilizer application needs to be more frequent in sandy soils, but with lower rates each time due to leaching potential).
• Soil moisture content and soil temperature (nutrient uptake is faster in moist warm soils).
• Plant vigor (plants under stress are less able to take up available nutrients due to damaged or reduced root systems).

Fertilizer Rates:

Fertilizer use rates should be based on plant type, with younger trees and shrubs generally receiving higher rates of nitrogen (N) than mature plants. Fertilizer rates for trees are no longer based on trunk size or caliper, but on root system spread, calculated by doubling the area of crown coverage (crown coverage = 3.14 x radius2).

In general, use one to six pounds of actual nitrogen for every 1,000 square feet of root zone. Evergreen shrubs and trees need less (1 to 3 pounds) while deciduous trees and shrubs commonly need more (3 to 6 pounds). Reduce the rate when plants are growing in restricted areas (sidewalk cuts, parking lot islands) or where roots of multiple plants overlap.

If applied fertilizer will go over a turf area, do not exceed 1 1/2 pounds of nitrogen for every 1,000 square feet for any one application to avoid over stimulating or burning the grass. Use split applications a few months apart if higher rates are needed. If a soil test shows that phosphate (P) or potash (K) is needed, apply at the rate of 1 to 2 pounds and 1 1/2 pounds nitrogen for every 1,000 square feet, respectively. If a complete fertilizer is used, the ratio of N-P-K should be 3-1-2 or 3-1-1.

Additional information on fertilizer analysis and formulation.

Application Timing:

Fertilizer should be applied when plants need it, when it will be most effective, and when plants can readily take it up. Late summer and early fall fertilization may stimulate new growth that is not winter hardy, and summer drought may interfere with nutrient uptake, but spring, fall, and winter applications are acceptable. A split application may be beneficial, applying half the yearly rate in early spring and the rest in the fall as or after plants go dormant.

If water is unavailable, do not fertilize at all - plants will be unable to absorb the nutrients. (During a dry season, fertigation - application of fertilizer through an irrigation system - can be beneficial.)

Summary:

Tree and shrub fertilization is only one part of total plant maintenance. Fertilization may not benefit a plant if it is under stress from poor soil aeration or drainage, saturated soil, insufficient light or space, or excessive pest problems. All factors influencing plant growth should be kept at optimum levels to ensure plant vigor.