Fertilizer tree spikes contain slow-release fertilizer and are designed to be inserted into the soil around trees to provide the appropriate amount of fertilizer over an extended period. Although more expensive than granular fertilizer, many gardeners prefer the convenience of installing the spike to the effort of calculating fertilizer needs and applying the fertilizer manually. However, the nutrient levels in spikes may not be high enough for some trees, such as evergreens. In such cases, granular fertilizers may be the best option costwise, according to Deborah Brown, extension horticulturalist for the University of Minnesota.
Assemble the tree spike by placing the plastic cap over the top of the spike.
Locate the drip line of the tree. The drip line corresponds to the edge of the canopy when foliage is present.
Measure the circumference around the tree, following the drip line.
Refer to the manufacturer's recommendations to determine the number of spikes required for the area around the type of tree being fertilized.
Push the spike into the soil so that the top of the spike is 2 inches below the surface of the soil. Repeat the placement at equal intervals around the drip line of the tree.