Fertilizing Shade Trees
- When to Fertilize:Fertilizer can be applied any time from late September through early April.
Newly planted trees should not be fertilized until they have become well established. This usually is not until after their first growing season.
Young, rapidly growing trees should be fertilized annually. Mature trees need fertilizer every 2-3 years to maintain good foliage and vigor.
- Use Complete Fertilizer!Use a complete fertilizer for trees. Analysis of fertilizer shows on the bag in three numbers, for example, 10-10-10. The first number gives the percentage of Nitrogen (N); the second indicates the percentage of Phosphorus (P); the third is the percentage of Potassium (K). Thus, 100 pounds of a 10-10-10 fertilizer would contain 10 lbs. N, 10 lbs. P, and 10 lbs. K.
Tree growth is limited more often by a deficiency of Nitrogen than by lack of any other element. As a result, trees respond best to fertilizer with 2-1-1 or a 3-1-1 ratio. Commonly available fertilizers with a 2-1-1 or similar ratio are: 12-6-6, 10-6-4, 10-8-6, and 10-8-4.
If the desired ratio is not readily available, add 12 oz. of Hi-Yield Ammonium Nitrate (33-0-0) to each pound of fertilizer such as 12-12-12 to make a 3-1-1 ratio.
- What is the Best Way to Fertilize a Tree?A 12-year study was conducted at USDA Nursery Crops Research Nursery in Delaware, Ohio to evaluate fertilizer placement methods. The study, begun in 1969, compared the effects on tree growth of surface applied fertilizers and fertilizer placed in holes.
‘Select’ linden, ‘Snowdrift’ crabapple, and ‘Sentry’ sugar maple trees received 0, 3, 6, or 9 lbs. actual N/1000 ft2 applied as ammonium nitrate either to the surface or 2-inch holes 12 inches deep (20 holes per tree). The fertilizer added to the holes was mixed with calcined clay. Trees were fertilized initially and then every 3 or 4 years with nitrogen treatment plus six pounds of actual P and K. Most trees were 6-8 in caliper at the end of the experiment.
The results were surprising. As expected, all nitrogen treatments caused significant growth increases compared to the unfertilized controls. However, after 12 years, there were no differences in caliper among trees given 3. 6. 9 pounds N; nor was there any measurable difference between trees given surface applications and those fertilized by the hole method.
Most surprising was, after 12 years, Linden trees given the hole treatment with no fertilizer were 2 inches larger in caliper then control tress and were not significantly different from those given any of the nitrogen treatments.
Crabapple trees also showed a dramatic response to the hole treatment after six years, but no effect was evident after nine years. The maples did not respond to the hole drilling. The authors (Eldon Smith and Sharon Treastor) concluded that surface application is as effective as hole placement. they also concluded, in poorly drained soil, aeration does as much good as fertilizing with 3 pounds N/1000 ft2.
- Fertilize Total Root Area!Fertilize shade trees throughout the area occupied by the root system. This usually will include the soil beneath the branch spread and slightly beyond. However, roots of columnar trees expand well beyond the branch spread.
In general, large evergreen trees can be treated the same as deciduous trees with respect to fertilizing. Evergreens normally have roots that extend beyond the branch spread.
- How Much Fertilizer?
- Tree Trunk Diameter Method: Measure the trunk diameter 4½ ft above the ground. Deciduous trees less than 6 inches in trunk diameter should get 1-2 lbs. of fertilizer annually for each inch of trunk diameter. For deciduous trees more than 6 inches in diameter, apply 2-4 lbs. of complete fertilizer for each inch of trunk diameter.
Fertilizers containing 15% or less nitrogen should be used at the higher rates listed. Those with greater than 15% nitrogen should be used at the lower rates listed.
These rates are based on an unrestricted area of soil for root growth. If half the root area is covered with pavement, use half the amount of fertilizer. Also, adjust the rates to compensate for overlapping root areas of trees in groups.
Thoroughly soak soil beneath the tree the day before fertilizing. Drive a 1 inch diameter rod in the damp soil or use a soil auger to make holes. A heavy-duty electric drill can also be used. Make 10 to 15 holes for each 1 inch of trunk diameter. The holes should be 12-15 inches deep and about 2 ft apart in concentric circles around the trunk. The outer circle should be slightly beyond the limit of the branch spread and the inner circle should be at least 2 ft away from the trunk.
Apply 1/4 – 1/2 cup of fertilizer to each hole. The fertilizer can be mixed with sand, pea gravel, peat moss, or some combination of these materials to improve soil aeration. After the fertilizer has been applied, soak the area thoroughly with a lawn sprinkler. The holes may then be filled or left open as desired.
- Tree Root Area Method: For nitrogen, stake off a square or rectangle area that includes the entire branch spread of the tree. Determine the square feet in the area by multiplying the length times the width.
Apply 6 lbs. of nitrogen annually per 1,000 square feet of tree root area in three separate applications. Make the first in the spring (March or April), a second in early fall (mid – late September), and a third 4-6 later (mid – late October).
To avoid plant injury, do not apply more than 2 lbs. actual nitrogen per 1,000 square feet. If the lawn under the tree has been fertilized, subtract the amount of fertilizer applied to the lawn from the amount required for the tree fertilization.
If part of the nitrogen in the fertilizer is from a synthetic organic source such as Urea-formaldehyde, 3 lbs. of actual nitrogen may be applied at one time without injury to trees or turf. When using this method, apply 3 lbs. of actual nitrogen in the spring (March or April) and the remaining 3 lbs. in the fall (September or October). This amount of nitrogen should also meet the needs of turf growing beneath the trees. Apply the fertilizer with a lawn fertilizer spreader; then sprinkle the area to wash fertilizer off the grass. In dry weather, thorough watering helps move the fertilizer down into the root zone.
- With Phosphorus and Potassium: Surface applications may not reach tree roots in adequate amounts. To satisfy tree needs, these materials must be placed in holes rather than on the surface.
Apply Phosphorus and Potassium fertilizers every 3-5 years at the rates recommended. Use the application techniques described under Tree Trunk Diameter Method.