Dieback, poor twig growth, and off-color, smaller leaves that drop early in the fall are signs that a tree or shrub might need fertilizer. Most of the oxygen in the soil is in the top 10-14 inches; that’s where the main roots of trees and shrubs absorb nutrients. When fertilizer is applied to the surface, irrigation or rainfall carries it to the roots.
Fast-release, water-soluble fertilizer leaches quickly into the soil. A few inches of rain or irrigation in well-drained sandy soil can cause fast-release fertilizer to move past the roots. Leaching will be slower in clay and finely-textured soils, but more will run off and not benefit the roots.
There is less chance for fertilizer damage or burn with slow-release fertilizer, half of which is water-soluble nitrogen. Use slow-release fertilizer on newly-planted trees and shrubs, in compact soil or clay, or on slopes where the runoff is high.
Organic composted sewage and cow manure provide nitrogen and other nutrients slowly. They contain less nitrogen, phosphorus and potassium, but they also provide small amounts of iron, zinc and other minerals.
Fertilizers labeled “weed-and-feed” formulated for lawn grasses may contain herbicides that can damage trees and shrubs.
Unless soil test reveals that phosphorous and potassium are not needed for trees and shrubs, a complete fertilizer containing all three minerals should be used such as 12-6-6, 12-4-8, or 16-4-8. Those numbers stand for the ratio by weight of nitrogen, phosphorus and potassium in a fertilizer.
Why and When to Apply
Make sure that a tree or shrub is not suffering from insects, diseases, compacted soil or bad weather. If they are growing normally, there is likely no need to fertilize.
Apply fertilizer when trees and shrubs are actively growing and there is adequate soil moisture. Early spring is good. Do not apply when plants are stressed by drought or when water is unavailable; water is needed to carry the nutrients to the roots.
Where to Apply
Apply fertilizer to the area occupied by the roots of a tree. This root zone area, a rough circle with the tree in the center, extends 1 ½ times beyond the outermost branches of a tree. If the distance from the trunk of the tree to the drip line, or crown radius, is 8 feet, the roots that can absorb fertilizer extend an additional 4 feet. That means the root zone where you apply fertilizer extends in a circle 12 feet from the trunk.
How to Apply
Spread dry or liquid fertilizers over the root zone, then water it. If a tree is surrounded by turf, drill 1- to 2-inch holes 8 to 10 inches deep about 2 feet apart. Don’t go deeper than that. It might put the fertilizer beyond the roots. Divide the fertilizer among the holes and water.
How Much to Apply
Give trees and shrubs 2-4 pounds of nitrogen per 1,000 square feet of root spread; this is the root zone. The root zone is 1 ½ times the area occupied by the spread of the crown.
To calculate the area of crown spread, multiply Pi (3.14) times the radius squared. If the radius of the crown spread measured from the trunk is 12 feet, multiply 3.14 x 12 x 12 = 452.16 square feet.
To determine the root zone of a bed of shrubs, multiply the length times the width. If the bed is 30 feet long and 10 feet wide, the root spread is 30 x 10 = 300 square feet.
Deciduous trees need more fertilizer than evergreen trees.
Do not use any fertilizer that contains weed killers. They can damage trees and shrubs. If trees or shrubs are planted in lawns do not use more than 1 pound per 1,000 square feet of root zone.