Tree Fertilization

How long has it been since trees in your landscape have been fertilized? Soil testing is one way to determine if fertilization is needed. Another way is to look at the tree. Symptoms of a nutrient deficient tree include a slow growth rate, smaller than normal foliage, off-color foliage, increasing dead branches, tip die back, and increased insect and disease problems. It is important to make sure that a lack of nutrients is the problem before fertilizing.

Avoid fertilizing when a new tree is planted except to correct nutrient deficiencies identified by a soil test. After the first growing season, trees can be fertilized annually to promote growth; however, an established tree may only need fertilizer every five years. In years between soil testing, a fertilizer with a ratio of nitrogen to phosphorus, and potassium of 3:1:2 or 4:1:2 can be used. Examples would be 12-4-8, 16-4-8, or 18-6-12. The months of November through March have active root growth and are good for fertilizing trees.

Fertilizer should be placed where the tree can get it, near the feeder roots. The feeder roots begin several feet from the trunk and can extend 2 to 3 times the width of the tree canopy. The method of fertilization depends on the plant cover under the tree, soil conditions, and the amount of time and labor involved in fertilizing the tree. The first method is the drill hole method. Holes are drilled or punched 8 to 12 inches deep in the soil spaced about three feet apart. The recommended amount of fertilizer should be distributed evenly among the holes. The fertilizer should be mixed with peat moss, pine bark or compost to backfill the holes. This technique works well in compacted soils since the soil is aerated while the fertilizer is placed near the feeder roots. This also prevents over-fertilizing lawns or fertilizing them at the wrong time of year.

Feeding needles inject liquid fertilizer into the soil. The advantage is to provide immediately available nutrients and the moisture to help take them up. This technique does not work well with normal home water pressures and is usually done by tree care professionals. This technique also places nutrients below lawn roots.

Surface application is the simplest and fastest way to fertilize trees. Just spread the fertilizer on the surface under to just outside the branch spread. If trees are in lawns do not surface apply the fertilizer to warm season lawn areas this time of year.

There are a couple of ways to determine how much fertilizer to apply for deciduous shade trees. The easiest is to apply 1 pound of one of the fertilizers discussed earlier (12-4-8, 12-6-6, 16-4-8, 18-6-12) or similar fertilizer for each inch of trunk diameter measured at 4 1/2 feet above the ground.

The second fertilizer determining method is based on crown spread area. Apply 2 pounds of actual nitrogen per 1000 square feet of soil surface area under the spread of deciduous tree branches. For trees less than eight inches in diameter, use 1/2 the recommended rate. For tree root zones obstructed by sidewalk, driveway, or street reduce the fertilizer rate by the percentage the obstruction reduces the root area or use the soil surface area where the tree is growing. For evergreen trees over ten inches in diameter, use the same calculation used for deciduous trees. For evergreens between four and ten inches, use 1/2 the rate recommended for deciduous trees of the same size.

For evergreens between one and four inches in diameter, use 1/4 pound of fertilizer per foot in height or 1 pound of fertilizer per 100 square feet of crown area using one of the fertilizers listed previously.

If trees on your property have root systems were grass is fertilized, it is unlikely that additional fertilizer should be applied to them. However if soil tests indicate a need and you have a warm season grass then use the drill hole method of fertilization to place the fertilizer below the root system of the grass to prevent damage. Always use fertilizer responsibly to protect the environment. Do not concentrate heavy amounts in small areas.

If you have questions about caring for your landscape call the Extension Master Gardener Volunteers of Pitt County at 252-902-1705 or email them at Volunteers are in the office on Mondays and Thursday from 10 am to 12 pm between now and February. If you are a gardener or know a gardener then call 252-902-1709 to find out how to get tickets for our Peter Hatch Garden Event on January 31st, 2015. Peter Hatch will be speaking about the gardens at Thomas Jefferson’s Monticello at this event hosted by the Pitt County Extension Master Gardeners and I. Event information can also be found at