Of the many ways to deliver fertilizer to trees, one of the best is with a hose-end root feeder-a simple, widely available screw-on device in which a one-metre-long syringe is attached to the end of a garden hose. Water flows past fertilizer cartridges in the syringe to make a solution, which then passes through a hollow needle inserted 20 to 30 centimetres deep into the ground along the periphery of the tree's canopy. The advantages of this system are many. The tree is irrigated as the fertilizer solution is applied and nutrients are delivered directly to the feeder roots. Also, nearby plants remain undisturbed and no moisture is lost to evaporation. While good soil stewardship (regular applications of organic matter such as compost and shredded leaves) is one of the best ways to keep trees healthy, root feeders are especially valuable for getting young trees off to a good start or for rejuvenating mature ones that have been neglected.
Keep These Hungry
Never fertilize trees that are under stress due to dehydration, insect infestation or disease. Horticulturists also deem newly planted trees (in their first growing season) to be “stressed out”-keep them well watered but don't fertilize until the second year.
Trees that are growing in a naturalized environment where leaves are allowed to drop undisturbed (forming a mulch) and grass isn't mowed also don't require supplemental fertilizing.