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Fertilizing Trees and Shrubs

Source: NDSU Extension Service Horticulturist

Spring is the time to give your trees and shrubs the help they need to produce strong growth during summer or to revive plantings damaged over the winter months or through previous neglect.

The best way to determine when plants need fertilizer is to observe the color of the leaves and rate of new growth. Fertilizer is probably needed if trees and shrubs have light green or yellow-green leaves, short annual twig growth, excessive dead wood or sparse foliage. If the leaves are dark green and twig growth excessive, delay application for at least a year or two. Fertilize trees and shrubs in early spring when the buds begin to swell. Do not feed in midsummer as heavy vegetative growth may occur late in the season and result in winter injury.

Before fertilizing trees, it is good to know their growth habits. The feeder roots of nearly all trees are in the top foot of soil and mostly located at the drip-line or crown of the tree and beyond. With this in mind, the feeder root area can be fertilized on lawn areas by making imaginary circles at 2 foot intervals starting at the branch pattern extension--or drip line--and going another 2 feet to 4 feet beyond.

Use a mixed grade of fertilizer such as 16-8-8 or 20-10-5. Shade trees are fertilized at a rate of 1 pound (1 pint) per inch of trunk diameter measured 4 feet above ground level. Flowering trees should be limited to pound per inch of trunk diameter. Evergreens should have 1/3 pound per foot of height or spread of plant, whichever is greater. Shrubs should receive 1/4 pound per foot of the shrub's height or spread. Where shrubs are planted in beds on bare soil, apply 2 pounds per 100 square feet of bed area. For shrubs, fertilize while plants are small and rapid vegetative growth is desired. As plants reach desired size, reduce or eliminate fertilizer to limit growth.

For future reference, you may want a copy of Extension bulletin H-1035, "Fertilizing Trees," which is available from your county office of the NDSU Extension Service.

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