LATE SUMMER and EARLY FALL is an excellent time of year to fertilize your shade trees, large pines and large spruces. SPRING (April and May) can also be a good time to fertilize. We suggest fertilizing only when a need for fertilizing is identified. “Routine” fertilizing is not warranted. On this page, we will discuss some of the facts about fertilizing trees and indications for its use.
LARGE TREES in natural settings survive well without supplemental fertilization. However, in a landscape setting there are many factors that may limit the ability of the tree to absorb enough nutrients:
1. The root system is often compromised by paving, building, digging for water lines, gas lines, sprinkler systems, etc.
2. There is competition from turf grass, shrubs, flowers, and other trees that are often crowded into small urban lots.
3. Leaves and grass clippings that would return some nutrients to the soil are usually removed and not allowed to decompose beneath the trees.
4. Fertilizers applied to the surface often are taken up by grass and other surface plants. If large amounts of fertilizer are applied, surface vegetation can be damaged. Even without surface plants, it is difficult to apply enough fertilizer to the surface to satisfy the needs of a large tree.
LACK OF NUTRIENTS, in particular nitrogen, can show up in several ways:
1. Smaller than normal (stunted) leaves or light green to yellow coloration of normally deep green leaves.
2. Thinning of leaves in the crown of the tree.
3. Dieback and stressed appearance of the tree.
4. Increased susceptibility to pests and diseases.
TREES WITH DISEASES such as needlecast diseases often benefit from fertilizing at the start of treatment. The fertilization encourages healthy new growth that is then protected with fungicides over a several year period until the disease has been controlled or cleared.
TREES STRESSED BY PEST INFESTATIONS such as scale or borers or defoliators will often benefit from fertilization. Sometimes systemic insecticides are added to the mix for control of insect pests.
TREES WITH SPECIFIC NUTRIENT DEFICIENCIES including chlorosis from iron, zinc, or manganese deficiency may need specific additives and ancillary systemic injections into the trunk or root flare. Additives may include chelated micronutrients, mycorrhizae fungi, or microfine sulfur for acidification.
DEEP ROOT FERTILIZATION is done with a hydraulic sprayer, injecting the fertilizer in solution or suspension with large amounts of water. The mixture is injected into the soil to a depth of 12 to 18 inches. We inject 5 gallons per trunk diameter inch, delivering ¼ to ½ pound of elemental nitrogen per trunk diameter inch. A large portion of the nitrogen is in a slow-release form. (Sometimes, if injection area is limited or the ground is particularly difficult to inject, we use higher strength solution and inject the same amount of fertilizer with less water).
AFTER WE HAVE FERTILIZED: You may notice green spots in the lawn where fertilizer solution has bubbled up around the injection holes. You may see holes, sometimes with small mounds of gravel or sand which has been pushed up by the water pressure. You may see some green or blue fertilizer residue on the ground or on surfaces near an injection point. If it is objectionable, it should rinse off easily with a garden hose.