Just like people, trees have nutritional needs, which they meet by producing energy in their leaves using sunlight and extracting mineral nutrients from the soil with their roots. Just like your pantry needs periodic restocking, soil also runs low on nutrients over time and needs a boost. Plant food or fertilizer encourages growth and tree health by replenishing depleted nutrients. If you plan to enhance your landscape with evergreens, proper fertilization will help your trees grow quickly and remain healthy for many years.
Evergreens require 13 minerals from the soil in order to survive. Commercial fertilizers generally provide three of these, called primary macronutrients: nitrogen, phosphorus and potassium. When you purchase a bag of fertilizer, the three numbers indicate the proportion of those three nutrients, in that order. In nature, nutrients leach into the soil from decaying plant and animal matter.
Evergreen trees do not need fertilizer added during planting, according to the University of Wisconsin Extension, unless it is slow-release packets or tablets added to the planting hole. However, evergreens do generally prefer loose, well-drained soil, so enhancing soil with organic matter prior to planting not only loosens soil but also provides a low dose of essential nutrients to help your tree get started.
Apply fertilizer to evergreen trees in the early spring or late fall. Evergreens generally require less fertilizer than deciduous trees, and the University of Wisconsin Extension recommends 1/3 lb. per foot of height or spread of the tree, whichever is greater. Work the fertilizer into the soil around the tree and water well after application. Do not bring fertilizer into direct contact with any part of the tree.
Fertilizer encourages rapid growth of the tree, so the University of Wisconsin Extension recommends using it only when you want the tree to grow larger. Once the tree has reached desired dimensions, fertilizer applications are no longer needed.
Trees absorb fertilizer through their roots, so it is important to apply fertilizer across the entire root zone. Evergreen roots extend one and a half to two times larger than the diameter of the branch spread, according to the University of Missouri Extension.
Composted manure works well as a fertilizer for evergreen trees, if you prefer to garden using organic methods. Apply it as surface mulch or dig it into the soil around the tree. Mulching with compost also replenishes all of the nutrients the tree needs in a slow-release form. Your evergreen will shed part of its needles each year, leaving them on the ground recycles nutrients back into the tree.