Trees will grow without fertilization in most soils once they are established. However, they will grow with greater health and vigor if they are fertilized annually. Shrubs will respond to proper fertilization with lush growth, greater resistance to pests, and less winter damage.
For shrubs growing in cultivated beds, fertilizer may be applied in late March or early April. Depending on the plants involved, each 100 square feet of bed area should receive between one and three pounds of a low-analysis, complete fertilizer. The fertilizer should be distributed uniformly over the soil beneath the shrubs, with most of the fertilizer under the outer edge of the shrub where the fibrous roots that absorb the nutrients are located. The fertilizer should not be allowed to touch the foliage, or foliar burn (a reaction to the chemicals) may result. If the soil is dry, the fertilizer should be worked into the soil with a hoe. If the weather has not been abnormally dry, the fertilizer can be left untilled and the next rainfall or irrigation will wash it into the soil.
Trees are fertilized in different ways depending on the species, the age of the plant, the adjacent plants and terrain, and the equipment available to do the job. Small trees may be fertilized to promote their growth and ensure their health. Mature trees may be fertilized to sustain their health but with no concern for size expansion. Where trees stand in open lawn areas, fertilizer can be applied to the surface using the same spreader that would be used for lawn fertilization. In settings where trees are crowded by structures or other plants, or are on sloped terrain where runoff would prevent movement into the soil, fertilization may be by direct incorporation into the soil or directly into the tree. With only a few exceptions, the method used makes no difference in the trees’ reaction. As long as the fertilization provides a correctly balanced nutritional supplement, the delivery system is not significant.
Most important is delivering the fertilizer where it can be taken up by the tree. The take-up of nutrients by trees occurs at the outer extremes of the root zone and within the top six to eight inches of the soil. The outer edge of the root zone in established trees was long believed to correspond to the edge of the foliage canopy, termed the drip line. Recent knowledge indicates that tree root systems commonly extend far beyond the drip line. Therefore, placing fertilizer close to the trunk or limiting its application just to the drip line may miss the most absorbent roots entirely.
If fertilizer is being applied to the soil in holes drilled with augers or by injections made with high pressure hydraulic sprayers (Figure 11-1), the holes or injections should extend from halfway between the trunk and drip line to a point approximately a quarter of the radius outside the drip line. Drilled holes are best for dry fertilizer application. They should be six to eight inches deep and spaced 18 to 24 inches apart throughout the zone of application. A dry mixture of 50 percent high analysis, complete fertilizer and 50 percent sand as a carrier, is poured into the holes.
High-pressure injection uses concentrated liquid fertilizers. The spacing is greater than for dry applications, with the objective being to apply about a gallon of fertilizer with each injection and enough injections to deliver approximately 200 gallons of fertilizer within each 1,000 square feet of the zone of application.
Direct application of nutrients into a tree’s vascular system is possible using the same technology as that used to apply systemic pesticides (see Chapter 6). However, from a purely practical standpoint, it is only of relevance when a tree is in need of micronutrients that are unavailable to the tree for specific reasons such as local soil conditions.
The one most common mistake in the fertilization of landscape plants is the application of fertilizer too late in the growing season. The result is often a flush of vegetative growth in response to the nitrogen that leaves the plant ill-prepared for winter. Great damage can result to plants from well-meaning but ill-timed fertilization.