Here’s a question that tends to divide folks right down the middle. When it comes to the wisdom of feeding trees, expert opinion is sharply — in some cases acrimoniously — divided, but the weight of modern practice is increasingly in favor of the dictum that less is more.
A small amount of fertilizer is fine. It will help compensate for the absence of natural fertility that tends to distinguish lawns (where all the leaf litter gets raked up and there is no understory to speak of) from woodlands (where the trees are nourished by lots of decayed plant material).
But before you go out and buy tree food, remember that the small amount needed is likely to be already present as a by-product of fertilizing the lawn. Once you get into adding more than that, it’s likely you will do more harm than good — if you do anything at all. The harm comes because fertilizer pushes the tree into making lots of tender, soft growth. It’s lush, it’s green, it’s very impressive, and it’s also highly vulnerable t insect attacks, climate stress, and the myriad fungus diseases that would be thwarted by tougher tissue.
The doesn’t-do-anything-at-all situation results from putting the food where the tree can’t get at it. Most of a tree’s feeder roots are in the top 10 to 18 inches of soil, and most of them start near the out edge of the canopy (the drip line) and spread outward from there. That means using an injector to put the eats down deep is not going to do much except pollute the groundwater. And spreading the tree’s meal close to its trunk will be just as fruitless.
The bottom line: keep feeding to a minimum unless the tree is in a container where it cannot possibly find nourishment on its own. And if you do use fertilizer on a landscape tree, spread it in a wide band that works out from the drip line.