Evergreens, which keep their leaves or needles throughout the year, often have fertilizer requirements that differ from those of their deciduous cousins, which loose their leaves in fall. Although you may think of Christmas trees when describing evergreens, some shrubs and smaller plants also fall into the evergreen category. In general, the best fertilizer for evergreens has a high nitrogen content.
All plants require 17 nutrients in order to grow normally. Nitrogen, phosphorous and potassium rank among the most important nutrients for normal evergreen growth. Some evergreens, however, don't require fertilizer because they receive sufficient nutrients from their environment. Evergreens that receive too much fertilizer tend to have an open growth habit with widely spaced branches.
Signs of Deficiencies
Inspecting your evergreens will help you determine whether or not they need fertilizer. Look at the color of their leaves or needles. Nutrient-deficient evergreens may have light-colored or yellow leaves or needles. Consider the plants' growth. Evergreens lacking nutrients may grow more slowly than normal and be shorter than expected. Performing a soil test on the soil around the evergreens will determine whether or not nutrient deficiencies exist. Leaf tissue analysis of evergreens also identifies nutrient deficiencies, especially for some micronutrients, such as iron.
Using a fertilizer that provides the nutrients in which your evergreens are deficient is important. If you don't have the benefit of a soil test or leaf tissue analysis, then use a fertilizer with a high nitrogen content. Both natural and synthetic fertilizers provide nutrients to evergreens. Natural fertilizers such as compost, bone meal and manure release nutrients more slowly than synthetic fertilizers such as ammonium nitrate. Natural fertilizers, however, may improve soil structure.
Amount of Fertilizer
In general, evergreens need 2 to 4 pounds of nitrogen per 1,000 square feet of soil surface every two to four years to maintain growth. Mature or slow-growing evergreens require about 1 pound of nitrogen for every 1,000 square feet of soil surface every two to four years. Calculate the amount of fertilizer to apply by finding the fertilizer grade on the fertilizer package. For example, a 10-8-6 fertilizer contains 10 percent nitrogen, 8 percent phosphorus and 6 percent potassium. A 100-pound sack of that fertilizer contains 10 pounds of nitrogen. Consider using a slow- or time-released fertilizer, which provides nutrients over a period of months. For a more simple calculation of how much fertilizer to use, apply 1/3 pound of a commercial, high-nitrogen fertilizer such as 20-10-5 or 12-6-4 per 1 foot of tree height or canopy spread. For example, a 6-foot-tall evergreen tree requires about 2 pounds of the fertilizer. An evergreen with a 3-foot-wide canopy needs about 1 pound of the fertilizer.
Timing of Application
The fertilizer should be applied in early spring or late fall. Avoid fertilizing in late summer because the fertilizer may encourage growth that suffers winter injury. Don't fertilize during drought conditions or when water is unavailable because the evergreens will be unable to absorb the nutrients.
Placement of Fertilizer
Apply dry fertilizer to the soil around the base of each evergreen tree or shrub, spreading the fertilizer on top of the soil and then watering the site well. An alternative application method begins with making 8- to 12-inch-deep holes that are 2 inches in diameter and 2 feet apart around each evergreen, using a soil auger, crowbar or rod to create the holes. Start making the holes 1 1/2 feet from the trunk of young trees and 3 feet from the trunk of larger, mature trees. Fill each hole to the top with fertilizer.