Spring is a good time to fertilize young trees and shrubs

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Last Updated: 
April 30, 2008

CORVALLIS - Trees and shrubs often are forgotten when it comes time to fertilize the yard in the spring. Young trees, especially those with a trunk diameter of less than six inches, can benefit from regular applications of fertilizer.

When young trees receive nitrogen fertilizer, they grow faster, develop a denser canopy and stay green longer into the fall. It might not be necessary to fertilize large, established trees or shrubs growing in or near lawns or groundcovers that are fertilized regularly.

Tree root systems extend for a long distance and they absorb nutrients when the area above them is fertilized. Additionally, as trees mature, their roots develop associations with fungi called mycorrhizae. These beneficial fungi help the tree and other elements from the soil.

Before you fertilize, take a look at your trees and ask these questions to help you decide if your tree needs additional nutrients:

How much annual growth do you see? Most young trees average about a foot of new shoot growth each year; older trees have significantly less. Is your tree growing less than expected?

Has the color, size, or amount of foliage changed over the past few years? Has the tree recently had disease or insect problems?

If you answered yes to any of these questions, the tree might benefit from fertilization.

The best time to fertilize is in the spring. If you fertilize in the fall, you run the risk of shocking the plant into becoming metabolically active right when cold weather hits.

Most woody plants begin the new year's growth with elements stored from the year before. An application of fertilizer in the spring gives an additional boost to this new growth.

Garden references vary about how much fertilizer to apply to trees and shrubs. Ross Penhallegon, horticulturist with the Oregon State University Extension Service, has a general rule of thumb for fertilizing trees and shrubs—use 1/4 to 1/2 pound of nitrogen per inch of diameter for trees six inches or more in diameter at breast height. Use 1/4-pound actual nitrogen per inch on smaller trees. This is roughly two to four pounds of complete fertilizer per inch diameter on the larger trees and half that dosage on smaller trees.

"As time goes on, you will be able to tell by the condition of tree or shrub, whether or not it needs more fertilizer," said Penhallegon. "Typically, healthy trees and shrubs have 12 to 18 inches of branch growth per year. Their leaf color should be dark green, with yellowish new growth."

Apply the fertilizer along the drip line of the tree, the area with the majority of the roots. If the fertilizer is applied to the soil surface only, much can be washed away or will not filter into the soil to the root zone.

For better absorption, Penhallegon recommends using a punch or probe to make holes 12 to 18 inches deep, and then filling the holes with fertilizer. Then be sure to water deeply.

Another way to fertilize is to "pepper" the ground with fertilizer as you walk around the drip-line of the tree. This method should also provide an adequate amount of fertilizer, said Penhallegon. It is best to apply the fertilizer in this manner right before it rains, so the fertilizer will be washed into the root zone. Or water the fertilized area for an hour after application.

For more information on "Fertilizing Ornamental and Shade Trees," FS 103, visit our on-line catalog. Our publications and video catalog at: http://extension.oregonstate.edu/catalog shows which publications are available on the Web and which can be ordered as printed publications.

Author: Carol Savonen
Source: Ross Penhallegon