University of Maryland Extension

Fertilizing Trees and Shrubs

Back to Trees/Shrubs - After Planting Care

Fertilizers are materials that supply chemical nutrients required by plants. Although plants need 17 major and minor nutrients to grow properly, most landscape fertilizers usually contain only three nutrients: nitrogen, phosphorus, and potassium. These are the ones most often found deficient in landscape soils.

Before you fertilize

Fertilizer is not a remedy for any and all landscape problems. Although a deficiency in one of the essential nutrients will cause plant problems, such problems are rare in relation to problems caused by drought, inadequate soil aeration, root injury, or pests and diseases. Furthermore, fertilizing a declining or injured tree will not necessarily return it to good health. Always determine a nutrient deficiency is the cause of a problem before you apply fertilizer as a cure. Click here for help diagnosing woody ornamental problems.

• To maximize nutrient availability, maintain the proper pH based for the type of plants grown. Most trees and shrubs will grow well in a 5.5 - 7.0 pH range. Ericaceous plants like mountain laurel and rhododendron require a soil pH of 4.5 - 5.5.

Test your soil and correct deficiencies of phosphorus and potassium during the soil preparation stage. In established landscapes, have the soil tested every three to five years and adjust the type of fertilizer to correct any nutrient deficiencies.

Newly planted trees and shrubs do not benefit from fertilization. Although this seems contrary to common sense, research studies show most of a plant’s energy is directed at root growth during the establishment period. The application of nitrogen during this period seems to suppress root growth rather than enhance it. Newly planted trees do not require fertilization for one to three years. Shrubs do not need fertilization until their second growing season.

• In most landscapes, healthy, mature trees and shrubs do not require fertilizers. Woody plants receive nutrients from lawn fertilization if their roots are adjacent to or growing under the turf area. And they pick up nutrients from decaying mulches and leaves and from the minerals in soil.

Fertilizing guidelines

• Fertilizing woody ornamentals is warranted when soil testing indicates a need for additional nutrients or you observe pale green leaves and very limited new growth in the spring.

• If you do fertilize, do it in the fall, between late October and early December, or in late winter or early spring between late February and early April.

Never fertilize in late summer or early fall because the available nutrients will stimulate new growth at a time when trees and shrubs are preparing for dormancy.

Excess nitrogen fertilization can produce long succulent shoots that are attractive to various sucking insect pests, like aphids.

Fertilizer Types

Complete analysis granular - These fertilizers are the most commonly used. They are easily available, easy to apply, and relatively inexpensive. They are known by their analysis numbers, such as 10-6-4 or 5-10-5. Select an analysis that supplies the nutrients your plant needs without over-supplying unnecessary nutrients. If your soil test indicates levels of phosphorus and potassium are adequate you only need to apply nitrogen. However, it is often difficult to find a nitrogen-only granular fertilizer. As an alternative, select a fertilizer with low amounts of phosphorus and potassium, such as 10-5-5 or 10-6-4.

Liquids - This is a popular type of fertilizer in the commercial tree and landscape business. The nutrients are mixed in water and injected into the soil. Products that utilize a water-soluble fertilizer applied with a garden hose attachment are available to the homeowner. In general, liquid application fertilizers are more expensive than granular types for the same amount of nutrient applied.

Slow Release Nitrogen - This form of nitrogen is supplied in a low but continuous rate. It is usually more expensive than the more common water-soluble (fast release) nitrogen, but less likely to contribute to nutrient pollution of our water resources. Slow release nitrogen is available at a uniform rate over the entire season. Fast release types, when applied in late fall or early spring, are available in spring when growth begins, but dissipate by the end of summer.

Fertilizer Application Methods

Surface Broadcasting - The most practical fertilization method is broadcasting fertilizer on the soil surface, covering as much of the root zone as possible. Be aware that certain broadleaf herbicides, such as dicamba (found in several weed-and-feed products) can be absorbed by woody plant roots growing in lawns and injure them if applied under the drip line of these plants.

Drilled Application - With this method you put fertilizer into holes drilled into the soil throughout the root zone. Make the holes six to 10 inches deep and one to one and a half inches in diameter. Place holes in a regularly distributed pattern at two-to three-foot intervals, starting at the drip line and extending to within several feet of the base of the plant. 

Fertilizer Spikes - A popular method of fertilizing trees is to use compressed fertilizer spikes. Drive the spikes into moist ground with a hammer. While relatively easy to use, the spikes concentrate the fertilizer into small areas and do not cover a large portion. Which is why we do not recommend this method.

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