Tips from the Field: Tree and Shrub Fertilizing
By Shaun Price
The health and appearance of trees and shrubs can greatly benefit from fertilizing, which supplies nutrients that may not be readily available from normal soil conditions. Tree fertilizer applications may reduce (but cannot eliminate) environmental stresses such as low water availability, soil conditions, and competition with turf and other plant material.
Trees and shrubs have their greatest need for fertilizer and nutrients in the spring, so the best time to fertilize is anytime after leaf drop but before they leaf out in the spring. The nutrients are stored in the roots and become immediately available when needed for new growth in the spring. Applications can improve plant health and vigor until July; applications after July or too early in the fall will stimulate growth late in the season that may not have time to harden off before cold temperatures arrive. This soft tissue growth is much more likely to suffer winter injury and dieback. Additionally, applications should not be done if the plant material is under drought stress.
A complete fertilizer with nitrogen, phosphorus, and potassium is typically recommended; however different settings and plant types will affect what nutrients need to be applied. Trees and shrubs in a lawn area or natural setting will require less fertilizer than trees and shrubs surrounded by parking lots and concrete. Any plant material that shows signs of a deficiency should be noted through properly integrated pest management practices, such as continuous scouting. Signs to watch for include foliage color, short shoot growth, or an overall stunted growth appearance. If the plant material continues to struggle, a soil test can be done to determine if the soil is lacking nutrients that are necessary for optimum growth. The report can also tell if the soil pH levels are off, preventing nutrients from being taken in by the plant material.
The easiest and most economical way is to apply a granular fertilizer over the soil and water it in. Another method is to make holes in the soil for compressed fertilizer spikes. The spikes must be correctly spaced and the proper number of spikes must be used. Foliar applications can be made where nutrients are absorbed through the leaves; however, this method is less efficient for larger plant material and trees. The optimal method is a liquid application through a hose and root feeder probe, which gets the fertilizer right down to the root zone where it is needed. Applying fertilizer in a liquid state eliminates the extra time needed to break down dry fertilizer materials by water sources so it can be absorbed.
Fertilizing your trees and shrubs will create healthy, beautiful plant material that will enhance your landscaping for years to come.