Ask the Arborist a Tree Care Question


Should I fertilize my trees?

Trees often require fertilization at some point in their life span. Remember, most trees' natural environment is the forest. There nutrients are continually recycled as leaves and other plants, and even animal material, fall and decompose on the forest floor. When you put a tree in a landscape setting it is usually deprived of these nutrients.

One thing you can do is try to recreate the tree’s natural environment by providing mulch rings. (See Mulching.) Most people, however, will not be able to mulch their whole yard! Fertilization is needed by many trees to replace the nutrients they are missing.

Like any other landscape plants, shade trees will respond to fertilization. Most shade trees exist in nature without much care, but transplanting trees into urban areas or man-made conditions can create problems. Often these trees will be growing in restricted root zone areas, be surrounded by pavement or compacted soil or even be physically damaged by construction activities. One should realize that the root system is just as important (and delicate) as the above ground parts. Fertilizer alone will not improve the health of a tree stressed by one of these environmental conditions. Fertilizer is only one factor in the complex formula of plant requirements. Here’s how to detect if your tree needs fertilizing. Symptoms of a nutrient deficient tree include:

  • a slow rate and low amount of annual growth on twigs and trunk, 
  • smaller than normal foliage, 
  • off-color foliage, 
  • increased numbers of dead branches,
  • tip-die back in branches,
  • and increased rates of disease and insect problems. 

Trees that possess these symptoms generally would respond to a fertilization treatment. One should make sure that nutrients (or lack of) are the problem before fertilizing. Other common tree disorders to be aware of in urban areas would include poor planting techniques, moisture problems, construction damage, girdling roots, or utility leaks from a natural gas line or sewer line.

How does the arborist determine when to fertilize? Why does my tree need a soil test?

Arborists have a number of tools at their disposal. The best is the arborist’s knowledge of local soil and environmental conditions. The arborist may be able to make a fertilization recommendation simply by examining your tree and yard. This saves the client time and money. On other occasions the problem may be more complex and the arborist will recommend a soil and/or foliar analysis to determine the problem.

Your arborist does know that most shade trees growing in landscape settings will need nitrogen. Nitrogen is not stable in the soil. It is easily washed away by water and may periodically need to be replaced.