The invigoratingly fragrant, cone-bearing pines, junipers, firs and spruces that grace so many Christmas cards are the first trees many of us associate with the word "evergreen." Some broadleaved trees, however, qualify as evergreen because they retain their leaves through the winter. Hollies, magnolias and live oaks are broadleaf evergreens. Both kinds of evergreens benefit from fertilizer. The types of fertilizers and methods of application, however, vary according to circumstances.
Needled Evergreen Fertilizer
All fertilizer labels display their percentages of nitrogen (N), phosphorous (P) and potassium (K)--in that order--as three prominent numbers. Needled evergreens perform best with a fertilizer that has a nitrogen concentration higher than its phosphorus and potassium concentrations, says University of Minnesota Extension horticulturist Deborah Brown.
Broadleaf Evergreen Fertilizer
Broadleaf evergreen trees need acidic (pH below 7.0) soil. Many commercial fertilizers are available for acid-loving trees. The nitrogen in these fertilizers comes from urea or ammonium instead of nitrate, say Ohio State University extension agent Jane Martin and master gardener Robert Staut. Broadleaf evergreens don't perform well with nitrate-based fertilizers.
Frequency of Application
Needled evergreens retain a single set of needles from three to six years, notes MSU associate professor of horticulture Dr. Bert Cregg. As a result, they need fewer fertilizer applications than broadleaf trees that replace their leaves each year. Needled evergreens in nitrogen- or iron-deficient soil develop generalized needle yellowing. Soil low in phosphorus causes purple needles. Either discoloration is a sign that it's time to fertilize the trees. Broadleaf evergreens, on the other hand, benefit from an annual application of fertilizer formulated to raise soil acidity.
Timing of Application
Feeding needled evergreens in early spring before their needle buds open maximizes the fertilizer's impact, says Brown. Fertilizer applications as late as mid-July are acceptable. After that new growth is too tender to survive freezing temperatures. For the same reason, early June is the latest advisable time to fertilize broadleaf evergreen trees. If a hard freeze has already occurred and the broadleaf trees are dormant, a late-autumn application is acceptable.
The appropriate amount a fertilizer for a needled evergreen depends on the square feet of ground the tree covers. Measuring the tree at its widest branch spread and multiplying the result by itself will produce the square footage. A tree with a 20-foot spread covers 400 square feet (20 x 20). Fertilizer labels indicate the number of pounds to apply for each 1000 square feet.
Broadleaf evergreens benefit from annual fertilizer applications that provide 1 1/2 to 2 1/2 lbs. of actual nitrogen per 1,000 square feet, says OSU Extension professor Dr. Mary Ann Rose. Multiplying the weight of the fertilizer by the N number on the label determines how much actual nitrogen the bag contains.