Around trees and shrubs, avoid using "weed-and-feed" lawn fertilizers containing herbicides. Sensitive species may suffer from repeated exposure to these chemicals.
Some testing labs analyze leaves to accurately determine a plant's nutrition needs. For more information, contact your state's cooperative extension agent or a tree service.
Photography by Suzanne DeJohn/National Gardening Association.
Most landscape trees growing in their natural environments rarely need fertilizer. However, trees growing in infertile soil along roadsides, in urban areas, and around new homes may need extra nutrients to keep growing strong. Apply in early spring or autumn when roots are actively growing.
Determine need for fertilizer. Compare trees to others of the same kind: look at leaf size and color, and the length of new twig growth. Small, pale leaves and stunted growth may signal fertilizer need, but first rule out disease, insects, physical damage, and environmental stress such as flooding or drought. To determine which supplemental nutrients your tree needs, send a soil sample to a testing lab. Find a lab near you by checking in your telephone directory, or by calling your local cooperative extension office.
Choose a fertilizer. Granular fertilizers are the easiest to apply. Choose one especially formulated for the type of tree, such as fruit or evergreen, or apply an all-purpose formula such as 10-10-10.
Calculate the size of the root zone. Tree roots grow at least twice as far from the trunk as the branches do. To calculate the root radius, measure in feet the distance from the trunk to the end of the longest branch. To calculate the size of the root zone in square feet, multiply (root radius) x (root radius) x 3.14.
Determine the required amount of fertilizer. You can safely apply up to 1 pound of nitrogen per 1,000 square feet per year. A 20-pound bag of 10-10-10 fertilizer, enough to cover 2,000 square feet, contains 10 percent or 2 pounds each of nitrogen, phosphorus, and potassium. Multiply your tree's root zone by the application rate per square foot to find the total number of pounds to apply.
Apply fertilizer. Measure out the amount of fertilizer you need. Mark the outside boundary of the root zone with a garden hose or a circle of flour or lime. Also mark a circle 3 to 4 feet from the trunk. Evenly spread the fertilizer between the two circles, avoiding application close to the trunk. If the tree is in a lawn, apply when grass is dry. Water to moisten the soil and distribute the fertilizer to a 12- to 18-inch depth.