Spring and summer give us the best opportunities to identify tree health problems, since a cursory inspection can tell whether the tree "looks" healthy compared to previous years or nearby trees of the same species. Diagnosis of the actual cause of the tree malady is a tricky business best left to an expert. As with human illness, prompt detection and treatment can be critical.

Most pest management activities have a very specific and narrow window of treatment that coincides with when the pest is active on the plant and/or vulnerable to the treatment.

Fertilizers are best applied when the plant roots can actively uptake the nutrients. Of course, the use of fertilizers on shade and ornamental plants should be restricted to situations in which there is a nutrient deficiency.

Some will argue that, in temperate areas, fall and winter are the best times to prune. When the tree is bare, its branch architecture is easier to observe, so problems can be more readily corrected. Proper pruning at other times of the year will generally do no harm to trees, but there are exceptions. For instance, pruning an American elm when the beetle that carries Dutch elm disease is busy flying from infected to healthy host trees greatly increases the elm’s chances of infection. Pruning of deadwood only does not have the same effect and can be carried out any time.