The first general rule of thumb when fertilizing trees is… less is more. Over fertilization can lead to excessive growth which makes trees vulnerable to disease and breakage. If your trees have been looking healthy and growing steadily, they probably don’t need anything more than some good mushroom or other organic compost.
If your tree seems unhealthy and there is no clear sign of plant disease, you may choose to try a fertilizer and see if that helps. Signs of nutrient distress may be truncated growth, yellowing of leaves, and poor foliage and/or flower production.
When adding fertilizer, do so as soon as the ground is workable in early spring. That way the nutrients will be available to them as they start to grow and bloom.
The most common type of nutrient deficiency in trees is low nitrogen. Signs can include the yellowing of older and younger leaves. The tips and margins of leaves can also appear scorched over time. Please note that these signs could also be a warning of other nutrient deficiencies such as sulfur. A good way to determine what you are dealing with is to consider what mulch you have been using. Many organic types of mulch, especially wood varieties will deplete soil of nitrogen as it breaks down.
The good news is… almost all fertilizers contain at least a minimal amount of nitrogen. If you continue using wood chips, bark or any other nitrogen depleting material, be sure to amend your soil with blood meal or a nitrogen based fertilizer before laying down your mulch. It is highly advised to use aged wood for mulching as the decomposition of aged wood uses up less nitrogen in the soil.
You can also periodically mix in compost; manure or ground-up scrapes from fruits and veggies. These will naturally add nitrogen to the area and help keep your soil safe from depletion.
If you don’t suspect any of the above as your tree issue, you may consider testing your soil to combat the problem. And it never hurts to do an internet search about the soil in your particular region as some areas are notorious for being either high or low in certain nutrients and other soil properties.