That sumac along the driveway presents dozens of offspring every year, but the Japanese maple your daughter planted as a school project falters every summer. The reason is often quite simple: trees and shrubs seldom need extra cultural care in their native habitats. Fertilizer is seldom needed for native species but may be necessary to replace nutrients missing in our soils for ornamentals that we add to our landscapes.
Have your local state university agricultural extension guide you in conducting a soil test to determine your plants' needs. The test will tell you whether your soil has the right structure and acid-alkaline balance (pH) to support the tree and shrub varieties growing in it. Even native species will suffer during dry periods or suffer in soil that contains too much silt or clay.
Non-native species are susceptible to weakness due to improper soil pH. In addition to soil amendments to lighten soil and adjust pH your county agent may also be able to suggest beneficial mulch, how much water your plants should receive and what percentages of nutrients fertilizer should contain.
The Nature of Fertilizer
According to the University of Massachusetts’ Agriculture and Landscape Program, nitrogen, phosphorus and potassium are the nutrients most needed by plants but nitrogen is easily depleted as well as the one most necessary for plant growth. Other major building blocks—carbon, hydrogen and oxygen—come from water and air. Whether chemical or from natural sources like manure, compost or sludge, fertilizers should contain high percentages of water-insoluble nitrogen (WIN) to avoid “burn”, a process of die-off and desiccation caused by an “overdose” of nitrogen.
The percentage of nitrogen is the first number in the “N-P-K” (nitrogen, phosphorus, potassium) number on packages and should always be the largest. A soil test will determine whether nutrients like sulfur, calcium, magnesium, iron, manganese, boron, copper and zinc should be included for individual cultivars.
When to Fertilize
Fertilize trees and shrubs in the fall to encourage root development. They may also be fertilized in very early spring before they break dormancy and begin to bud; waiting too long will result in premature budding that may make the plants susceptible to late frosts. Unlike garden plants, trees can’t just send up new shoots. Add phosphate and potassium when planting new trees and shrubs since neither nutrient moves very little in the soil. Superphosphate, potassium phosphate and rock phosphate can be placed in the planting hole as the plant is set so they will be available to the root ball.
Incorporate granular fertilizer into mulch; it will leech gradually into the soil with moisture. Keep fertilizer away from direct contact with trunks and branches to avoid burn. Professional arborists use liquid injection around the “drip line” because of its convenience; and older method using granular fertilizer in a field of foot-deep holes in concentric circles is a time-consuming alternative. Dry “spikes” are popular but spacing makes nutrient delivery irregular. Fast delivery of minor nutrients like calcium or iron calls for spray application (foliar feeding) in early spring or direct injection into trunks.
Trees and shrubs often do not need to be fed annually unless they are stressed or in a temporary location such as a nursery where conditions may be cramped. Many trees and shrubs located in lawns will never need fertilization—they happily share nutrients with turf grasses.