Gardeners should not add any fertilizer to lawns, gardens, trees or shrubs until a soil test has been done. It is possible that there are adequate nutrients (perhaps even extremely high levels) in the soil and it may not need fertilizer at all (in Colorado this is most commonly true for potassium and phosphorus). If the test results show that the plants will benefit from fertilizer, then the best time to fertilize trees is in the spring or early summer. Late summer fertilization for woody plants is not recommended because it promotes late-season growth that is susceptible to injury from early frost. Remember that roots of mature trees and shrubs extend well beyond the canopy in all directions. Tree spikes only come in contact with a very small segment of the vast root system. A more appropriate fertilizer for woody plants is a granular fertilizer, spread evenly around the tree from the canopy's edge and beyond (roots of established plants may extend three to five times the height of the tree). Be sure to water in the granular fertilizer soon after it has been applied. Also remember that if trees are located within a lawn area and the lawn is being fertilized on a regular schedule, no more fertilizer is needed for the trees.

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