Healthy trees and shrubs increase the value of your property and are more resistant to drought, disease and pests. The proper fertilizer can increase the vigor of your valuable landscape plants, but the wrong fertilizer can damage plants, encourage disease and pollute groundwater. To determine the right fertilizer for your needs, conduct a thorough soil test. The tests, available at nurseries, home centers or through your local county extension, might show that you do not need to apply any fertilizers at all.
All plants need certain nutrients to grow and thrive. According to Erv Evans, consumer horticulturist at N.C. State University, there are 17 elements needed for growth. Plants acquire oxygen, carbon and hydrogen, which are found in the air, and micro- and macronutrients that are absorbed from the soil. Macronutrients, those required in larger amounts, include nitrogen, potassium, magnesium, calcium, phosphorous and sulfur. Micronutrients include iron, zinc, molybdenum, manganese, boron, copper, cobalt and chlorine.
Commercial fertilizers list the macronutrients nitrogen (N), phosphorus (P) and potassium (K) in a numeric ratio expressing the percentage of each ingredient by weight. Your soil test and the specific needs of your particular trees and shrubs will determine your NPK formulation.
Fertilizers come in a variety of forms. Foliar sprays are applied directly to the leaves of a tree or shrub. Available in liquid or powder, they are mixed with water and applied with a hose end or pressure sprayer. Nutrients are absorbed through the leaves.
Granular fertilizers are distributed around the base of trees and shrubs. When applying granular fertilizers, pull back surrounding mulch to reduce runoff.
Fertilizer spikes are a convenient way to apply fertilizers. Widely available in nurseries and home centers, these slow-release formulations are hammered into the ground around trees and shrubs.
Quick or SlowRelease
The nitrogen in these fertilizers may be “quick release” or “slow release” or a combination of the two. Look for the initials WIN (water insoluble nitrogen) or WSN (water soluble nitrogen) on the label. WIN indicates a slow-release form of nitrogen while WSN indicates a quick-release.
N.C. State University horticulturists state that “a slow-release fertilizer releases nutrients at a rate that makes them available to plants over a long period. Slow-release fertilizers need not be applied as frequently as other fertilizers and there is less potential of leaching into ground water.”
Spring is the optimum time to fertilize fall-planted trees and shrubs. If you are planting trees or shrubs in early spring, apply a small amount of slow-release fertilizer 6 to 8 weeks after planting. For established trees and shrubs, an early spring application of fertilizer based on your soil test is best.
If your soil is sandy, you may need to make an additional application, especially during a wet growing season. Do not fertilize during periods of drought because it will encourage water-demanding new growth.