How should I fertilize my trees and shrubs?

A common question I receive is, “How should I fertilize my trees or shrubs?” The answer, as with many gardening questions, is “It depends.” Factors such as soil conditions, plant age, and the specific needs of each species all play a role. It also depends on what your tree is telling you. Is it showing signs of stress? Are those symptoms caused by a nutrient deficiency, environmental conditions, or pests?

This article will cover when to fertilize, how to apply fertilizers effectively, and other common causes of tree and shrub stress.

Soil Conditions

Before applying fertilizer, it’s important to test your soil to determine nutrient availability, soil pH, and any underlying issues. You can contact your local County Extension Office for soil testing information.

In Eastern Idaho, many soils are alkaline, making some nutrients unavailable to plants. For example, iron chlorosis (yellowing leaves with green veins) occurs when high soil pH prevents plants from absorbing iron. To correct this, you can lower soil pH by applying elemental sulfur or ammonium sulfate.

Your soil test results will include nutrient recommendations based on the trees and shrubs in your landscape. When submitting soil samples, include details about the plant’s age and current health. This information helps customize the report to your specific needs.

Plant Age

Fertilizing trees and shrubs requires a careful balance between promoting growth and maintaining plant health.

• Young Trees & Shrubs: Focus on root growth and establishment rather than rapid top growth. Apply organic mulch (such as bark or wood chips) in a 3–4 inch layer around the base of the tree. This provides nutrients naturally and reduces the need for synthetic fertilizers.
• Established Trees & Shrubs: They require enough nutrients for steady growth but not excessive fertilization. While fast-growing trees may seem desirable, rapid growth leads to weaker wood, making trees more susceptible to damage.
• Choosing a Fertilizer: If additional nutrients are needed, use a balanced fertilizer with a 3:1:1 ratio (Nitrogen:Phosphorus:Potassium).

Recognizing Plant Symptoms

Many people assume fertilization is needed when they notice something unusual with their trees. However, symptoms may be caused by factors unrelated to nutrition. Below are ways to assess whether a problem is nutrient-related, environmental, or pest-related.

Nutrient Related Symptoms

Generally, if there is a nutrition issue with the plant, the indications I look for are what is happening to the leaves and the new growth. Leaf color is the first thing I look at.

Environment Related Symptoms

Many times, the environment is causing the problems you see in your trees and shrubs. Often, when you notice the symptoms related to the environment you may be too late to correct the problem.

Drought stress can display itself in a variety of ways, early leaf drop, yellowing of leaves, browning of leaves, sun scald, malformed fruit, or small buds. Drought stress over a long period of time can lead to decline of the health of the plant, making it susceptible to disease and insect problems. I recommend watering trees and shrubs deep (2 inches) intermittently throughout the year. Ensure the water is being applied at the drip line of the tree or shrub (where the outer branches are). That is where the roots will be below the ground.

Both heat and cold can impact trees and shrubs. Always refer to the Plant Hardiness Zones when selecting what plants to purchase for your property. If you inherit plants that are not thriving, consider replacing them with plants that are better suited to our climate. Symptoms of cold stress include cracking branches or trunks, browning of evergreen needles, bronzing of leaves, freezing buds. Heat symptoms may be discolored fruit, stunted growth, scorched leaves, and early leaf drop.

Salinity refers to the salt accumulation in the topsoil. Salt can build up from a variety of reasons, road salts or de-icing materials, fertilization, poor drainage, high water table, low rainfall and high evaporation, or use of manure as compost. A soil test will indicate the salinity level found in your soil.

Pest Related Symptoms

Pest related problems include insects and disease. On evergreen trees, the browning of needles can be for a variety of reasons including insects, fungi, bacteria, and environmental conditions. To determine the reason for symptoms, inspect for insects, or signs of insects. Many times, sticky leaves are an indication of aphids. The chances of pest related problems increase when the tree or shrub is unhealthy. Keep a close eye for pests if you are seeing abnormalities in your trees and shrubs.

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