Letís discuss fertilizing trees. As trees get older they need less nitrogen. If trees are stressed, such as dry weather, insects, disease, there is research, which shows positive results from fertilizing. The best time to fertilize is early spring when the leaves start to emerge, the next best time is early fall. Use approximately 1.5 # nitrogen/1000sq ft of lawn area under the drip line of the tree. The drip line is out to the edge of the branches. Over fertilization, especially of older trees, can be very detrimental to the health of the trees.
Many lawn fertilizers contain weed and feed formulations that may be harmful to your trees. The same herbicide that kills broadleaf weeds in your lawns is picked up by the tree roots and can harm or kill broadleaf trees if applied incorrectly.