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Do your trees and shrubs need fertilizing this spring?

Kathy Zuzek

The most beneficial time to fertilize trees and shrubs is from early spring when soil becomes workable until the time in April or early May when woody plants move into active growth. Fertilizing at this time provides woody plants with nutrients just as they put on their main flush of growth during spring.

But do your woody plants really need fertilizing? Not necessarily.

The nutrients needed by trees and shrubs are divided into two groups called macronutrients and micronutrients. Most of Minnesota’s soils contain adequate amounts of the micronutrients that plants need in very small amounts. Oftentimes deficiencies of micronutrients within plants have more to do with high soil pH than low levels of nutrients in soil.

Macronutrients are needed in larger amounts and three of them – calcium, magnesium, and sulfur – are usually found in adequate amounts in Minnesota soils. We are all familiar with the three remaining macronutrients that are found in most landscape fertilizers: nitrogen, phosphorous, and potassium. Lawn and landscape soils in Minnesota are usually high in phosphorous. Potassium levels are adequate in most Minnesota soils, although sandy soils may be deficient. The most common reason for fertilizer applications to trees and shrubs is a nitrogen deficiency. When this occurs applying a fertilizer that will provide 0.1-0.2 pounds of nitrogen per100 square feet is appropriate.

How do you know if your trees and shrubs need a shot of spring fertilizer?
    Terminal bud scars indicate where
    branch tips were in previous years.
    Photo:  K. Zuzek
  • Use your knowledge of your landscape and the practices used to maintain it. For example, trees and shrubs growing in yards that are fertilized for turf on a regular basis rarely need any additional fertilizer unless deficiency symptoms occur. On the other hand, trees and shrubs growing in non-turf areas on sandy soils may need regular fertilizer applications. Sandy soils are often low in organic matter content and have low nutrient-holding capacities.
  • Look at last year’s growth rate on tree branches by measuring growth between the terminal bud scar and the tip of the branch. If shoot growth was 6 inches or more, fertilization is
    A decrease in shoot growth rate from 
    one year to the next may indicate a
    need for fertilizer.  Photo:  University
    of Illinois Extension
    not necessary. Shoot growth between 2 and 6 inches may indicate a need for need for fertilizer. If shoot growth is less than 2 inches, fertilizer should be applied.
  • Compare annual growth rates of tree and shrub branches over the past 2-3 years by measuring growth between terminal bud scars. A decline in shoot growth from one year to the next may indicate a nitrogen deficiency in trees and shrubs.
  • Foliage that appears yellow or off-color during the growing season may also indicate a deficiency of nitrogen or other nutrients
If you suspect a nutrient deficiency, the best management practice is to have your soil tested. The soil testing lab at the University of Minnesota tests soils, identifies specific nutrient deficiencies, and provides fertilizer recommendations to correct deficiencies.

More information on fertilizing trees and shrubs can be found here.

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