The purpose of fertilizing landscape plants during the first year or two after transplanting is to increase height, width and caliper. Once the plants are established and growing well, however, the function of fertilizing is to continue satisfactory growth and health but not necessarily to produce maximum height or caliper.
After Planting Research has shown that about 3 lbs. of actual nitrogen, the element most responsible for vegetative growth, per 1,000 square feet per year is all that is needed to maintain the health of woody plants in most landscape situations. If foliage color, annual growth or general vigor is not normal, collect foliar samples, have them analyzed and follow the recommendations that come back with the results. Otherwise, use the suggested rate as a guide.
To calculate the surface area under the branch spread of a tree, multiply the radius times itself and then multiply that by 3.14 (surface area = Radius2 x 3.14). (The radius is the distance from the trunk to the edge of the branch spread.) As an example, a 6-inch diameter trunk with a total branch spread of 36 feet would have a radius of 18 feet. The area, according to the formula would equal 18 x 18 x 3.14, or 1,017 square feet. Following the recommendation of 3 lbs. of actual nitrogen per 1,000 square feet, you would apply about 9 lbs. of 33-0-0 fertilizer (3 divided by .33 = 9 lbs.).
Woody plants respond well to fertilizers with a 3-1-2 or 3-1-1 ratio, such as 24-8-16, 18-6-12, 18-5-9, 15-5-5, 12-4- 4 or similar formulations. An application of 3 lbs. of actual nitrogen per 1,000 sq. ft. applies 1 lb. of P2O5 and 2 lbs. of K2O when using a 3-1-2 ratio.
The trend in recent years has been for fertilizer formulators to use higher analyses in the fertilizer package. Often the nitrogen content is 30 percent or more and four or five times the phosphorus level. These formulations, though promoted for turf, can be satisfactorily used around woody plants. In fact, plants with root zones beneath lawn areas that are fertilized at least three times per year do not need additional fertilizer applications. The use of fertilizer and herbicide combinations around landscape ornamentals increases the chance of herbicide injury on the ornamentals.