Tree feeding, or fertilization, supplements the nutrients naturally available in the soil. While forest trees generally thrive without intervention, urban and residential trees often face compromised soil conditions, including compaction, altered pH levels, and the removal of natural organic matter. Providing targeted nutrients helps offset these limitations, promoting better health and resilience against pests and diseases. This guide offers practical instruction on when, what, and how to properly fertilize trees.
Assessing the Need and Timing for Fertilization
Yellowing leaves (chlorosis), small leaf size, and reduced vigor are common indicators that a tree may be struggling to absorb sufficient nutrients. Observing premature leaf drop or shortened annual twig growth also suggests a possible deficiency, signaling a need for intervention. These symptoms often reflect a lack of mobile nutrients like nitrogen or iron, which are necessary for chlorophyll production.
A more precise way to determine a tree’s nutritional status is through a professional soil test, which provides a detailed analysis of nutrient levels and soil pH. This test is the most reliable diagnostic tool, identifying specific deficiencies rather than relying on generalized visual symptoms. Soil testing allows for the calculation of exact amounts and types of fertilizer required, preventing the unnecessary application of nutrients already present.
The most effective time to fertilize established trees is generally either in late fall or early spring, coinciding with periods of maximum root growth. Applying fertilizer after the leaves have dropped in the autumn allows the roots to absorb and store nutrients before the ground freezes. These stored nutrients are then readily available for the burst of growth that occurs the following spring.
Early spring, before the active growth of the canopy begins, is also an appropriate window. Avoid fertilizing during the mid-summer heat or late in the growing season, as feeding a tree too late can stimulate succulent, weak growth highly susceptible to damage from early winter frosts.
Selecting the Right Nutrient Formulation
Commercial tree fertilizers are indicated by the Nitrogen-Phosphorus-Potassium (NPK) ratio, displayed as three numbers on the product label. Nitrogen (N) primarily supports above-ground growth, promoting new leaves and shoots and contributing to overall green color. Phosphorus (P) is involved in energy transfer and is important for healthy root development and the formation of flowers and fruits.
Potassium (K) supports the regulation of water movement and enhances the tree’s ability to withstand environmental stresses, such as drought or temperature extremes. For established trees, a fertilizer with a high nitrogen content, such as a 3-1-1 or 3-2-1 ratio, is recommended to support canopy growth. Young, recently transplanted trees may benefit from a more balanced formulation that includes higher phosphorus to encourage strong root establishment.
Fertilizers are available in both synthetic and organic forms, each offering different release characteristics. Synthetic options, such as water-soluble liquids or granular pellets, offer a readily available and predictable concentration of nutrients. They can be formulated for slow release, where nutrients are encased in a coating that breaks down over time, providing a steadier supply.
Organic sources, including composted manure, bone meal, or cottonseed meal, release nutrients more slowly as soil microbes break down the material. While organic options improve soil structure and microbial activity, their nutrient release rate is less predictable and highly dependent on soil temperature and moisture. Selecting a slow-release product, whether synthetic or organic, helps prevent nutrient runoff and provides sustained feeding.
Essential Application Techniques
Proper application begins with accurately identifying the target area, known as the critical root zone (CRZ), where absorbing roots are located. The CRZ extends outward from the trunk, typically reaching a diameter two to three times the spread of the canopy or dripline. Since absorbing roots are concentrated in the top 6 to 18 inches of soil, fertilizer must be placed within this zone.
One common method is surface application, or broadcasting, where granular fertilizer is spread evenly over the soil surface within the CRZ. This technique is simple and effective where the soil is not compacted and has minimal turf competition. However, when dense grass is present, surface-applied nutrients may be absorbed primarily by the turf, preventing them from reaching the tree’s roots below.
A more efficient approach for established trees, particularly in compacted urban soils, is the drill-hole method, also known as deep root feeding. This technique involves using an auger or a soil probe to create a series of holes 1 to 2 inches in diameter and 6 to 12 inches deep throughout the CRZ. Holes should be spaced in a grid pattern, typically 2 to 3 feet apart, beginning away from the trunk.
Once the holes are created, the pre-measured granular fertilizer is divided and placed into the openings, which are often backfilled with organic material or left open to settle naturally. Placing the nutrients deeper bypasses competing turf roots and delivers the formulation directly to the area where the tree’s feeder roots are most active. This method also provides aeration benefits to the surrounding soil.
Pre-packaged fertilizer spikes offer a convenient, measured dose of nutrients designed to be hammered into the soil around the dripline. While spikes are easy to use and minimize surface runoff, they deliver a highly concentrated dose of fertilizer to only a few isolated points in the CRZ. This localized concentration means that a large portion of the tree’s root system may not receive any benefit, making them less effective than a broadcast or drill-hole application.