Homeowners over-fertilize trees, encouraging rapid, succulent growth. This growth is usually not as winter-hardy as normal growth, and thus more susceptible to winter injury. A another drawback to over-fertilization is that it will cause trees and shrubs to grow too large. Common tree and shrub fertilizers include 16-20-10 and 10-30-10. These are high analysis fertilizers that will satisfy both the nitrogen and phosphorus requirements of the plants.
Fertilizers do not directly provide food for trees. Trees produce their own food (sugars, carbohydrates and starches) in their leaves, through the process of photosynthesis. Fertilizers provide the source of the nutrients required for photosynthesis. Not all trees require fertilizer, but all trees require the nutrients available in fertilizers. In the forest, many of these nutrients are recycled as the leaves fall and decay back into the soil.
Fertilizers encourage growth, and therefore all fertilizers should be applied in early spring just before growth occurs. Basically all forms of fertilizer are suitable, although slow release fertilizers may continue to release nutrients well into the summer when it is not recommended. Late summer fertilizing is not recommended as it will encourage succulent growth which will be more prone to winter injury. Fall fertilizing is also not recommended, as leaching of nutrients is possible in both fall and spring.
Consider the specific diseases a plant can tolerate in relationship to the site in which it will be planted. Use specific varieties of trees compatible with the planting site. Selecting the right tree for the right location is critical to the survival of that tree. Understand how shade promotes specific diseases when combined with excessive rain and determine which type of irrigation will work best for a specific type of plant. Recognize the effects of overcrowding and the need for increased air circulation. Sometimes it is necessary to actually remove a tree to promote air circulation and healthy growth.