But all you want to do is push it far enough to bypass the surface root systems of the surrounding turf or other ornamental plants. Research has shown that no matter how big that plant is, almost all of the roots of trees and shrubs that actually absorb the water are within the top 12 inches of the soil. Trees and shrubs have roots that go deeper, but they are thicker roots whose job it is to stabilize the plants, not absorb water and nutrients. It is also important to remember that the fertilizers used in root feeders are water soluble and are used up quickly. Nutritionally, they aren't much different than using a product such as Miracle-Gro.
Organic fertilizers are naturally slow release.
Products such as Osmocote are not well suited for trees and shrubs because they would still be fertilizing late in the growing season which isn't advisable in our climate. There are slow-release fertilizers that can be injected into the soil by professional arborists that do work well. During the first year after they are planted, they should just be watered or given a very weak solution, such as Start-Up. Once they are established, trees and shrubs will need to be fertilized yearly. In cold northern climates they can be fertilized either in the spring as soon as they show signs of new growth or in fall as soon as they have gone dormant.