https://hgic.clemson.edu/

Fertilizing Trees & Shrubs

Fig. 1. Determine the need to supply nutrients to landscape shrubs and trees before applying fertilizer.

Fig. 1. Determine the need to supply nutrients to landscape shrubs and trees before applying fertilizer.
R. F. Polomski ©2023 Clemson Extension

Trees and shrubs are living investments that grow in value each year. The foundation of their longevity starts with selecting and siting adapted plants and proper planting. After the plants become established, appropriate and timely maintenance, such as pruning, watering, fertilizing, and mulching, further contribute to their growth in the landscape. Not all landscape plants need to be fertilized. Mature trees and shrubs growing in favorable soil conditions require little or no fertilizer, just as established drought-tolerant plants require little or no supplemental watering during dry periods.

Objective-Driven Fertilizer Applications

Fertilizers contain essential elements or nutrients necessary for growth, flowering, and fruiting. When the soil does not provide these nutrients in sufficient quantities, the nutrients can be supplied to plants with fertilizer. Fertilizers should be used with a defined objective and not applied routinely and for no reason. Consider the following reasons to fertilize shrubs and trees:

  1. Treat a nutrient deficiency determined by a soil test or plant tissue analysis.
  2. Encourage growth, particularly in young, established trees.
  3. Maintain growth of older, more mature trees, especially when the leaves are removed, resulting in little or no recycling of organic matter.
  4. Improve flowering and fruiting of fruit- and nut-producing plants, such as blueberries, figs, pawpaws, and pecans. Fertilizer replaces the nutrients removed in the harvested fruits and nuts.

Avoid fertilizing plants exhibiting symptoms of poor growth. Do not assume that fertilizer will correct it. Lighter green than normal leaves, reduced annual twig or shoot growth, and decreased flowering and fruiting could occur for various reasons. For example, roots damaged by pests, drought, or excess water can affect a plant’s ability to absorb adequate water and nutrients despite their availability in the soil. Also, a soil pH that is either too low (acidic) or too high (basic) can cause nutrients to become unavailable or “tied-up” and can hamper the activity of beneficial soil microorganisms that support plant growth. Adding more fertilizer will not correct this problem until the soil pH is in the appropriate range. For more information, see HGIC 1650, Changing the pH of your soil.

Frostproof gardenia (Gardenia jasminoides) showing nutrient deficiency.

Frostproof gardenia (Gardenia jasminoides) showing nutrient deficiency.
R. F. Polomski ©2023 Clemson Extension

Fig. 2. A neglected Frostproof gardenia (Gardenia jasminoides) displays visual symptoms of a nutrient deficiency. A soil test or plant tissue analysis will determine a need for fertilizer.

Fig. 2. A neglected Frostproof gardenia (Gardenia jasminoides) displays visual symptoms of a nutrient deficiency. A soil test or plant tissue analysis will determine a need for fertilizer.
R. F. Polomski ©2023 Clemson Extension

Observe plants for nutrient deficiency symptoms and get a soil test or plant tissue analysis to determine fertilizer needs. Soil test results list the soil pH, the amounts of essential nutrients in the soil, and recommendations regarding the type and amount of fertilizer(s) required and when to apply it. Soil test results do not explain why nutrients are low, sufficient, or excessive. For more information, refer to HGIC 1652, Soil Testing. Contact the Clemson Extension Home & Garden Information Center for help interpreting soil test results.

A plant tissue analysis provides information on the mineral content of the particular shrub or tree (see Plant Tissue at the Agricultural Service Laboratory). It is most often used by commercial nursery growers and orchardists.

Once the need for nutrients has been established, follow the 4Rs of proper fertilization to maximize plant uptake and minimize losses to the environment: apply (1) the right kind of fertilizer, (2) in the right place, (3) in the right amount, and (4) at the right time. For more information, see HGIC 1299, Fertilizers and the Environment.

Choose the Right Kind of Fertilizer

Follow soil test or plant tissue analysis recommendations to select the appropriate grade or guarantee of fertilizer. As discussed in HGIC 1228, Reading a Fertilizer Label, the fertilizer grade or guarantee is comprised of the three prominent numbers on a fertilizer label. These three numbers stand for nitrogen (N), phosphorus (P), and potassium (K), which are often abbreviated as NPK. Technically, NPK refers to the amount (in percent) by weight of nitrogen (N), phosphoric oxide (P2O5), and potassium oxide or potash (K2O) in the fertilizer. For example, in a 12-4-7 fertilizer, 12% (0.12) of the weight of the fertilizer is actual or total N, 4% (0.04) is P2O5, and 7% (0.07) is K2O. If the net weight of the 12-4-7 fertilizer is 30 lbs., then there are 3.6 pounds of N, 1.2 pounds of P2O5, and 2.1 pounds of K2O. The remaining weight (the total must add up to 100% or 30 pounds) is comprised of inert material or nutrient carrier that aids in dispersing the nutrients.

The fertilizer guarantee is also a ratio between N:P:K. To determine the fertilizer ratio, divide the percentage of nitrogen, phosphate, and potash by the lowest percentage of the three nutrients. For example, a 16-4-8 fertilizer has an N-P-K ratio of 4:1:2. Generally, landscape shrubs and trees prefer a fertilizer ratio of 3:1:1 or 3:1:2. If a soil test report does not recommend phosphorus, then the recommended fertilizer ratio would be 3:0:1 or 3:0:2, such as a 30-0-10 fertilizer.

Fertilizer recommendations are based on nitrogen because it is required in the highest concentration relative to the other nutrients and has the greatest influence on growth and development. The tree care industry provides guidelines for woody plant fertilization in their American National Standards Institute (ANSI) A300 (Part 2)-2018 Soil Management. For fertilizers with slowly available nitrogen, the ANSI A300 standards recommend 2 to 4 lbs. of total nitrogen per 1,000 square feet per year and not to exceed 6 lbs. per 1,000 square ft. per year. Note that these recommendations refer to the weight of nitrogen in the product and not the amount of fertilizer. For fast-release nitrogen fertilizers, apply 1 to 2 pounds of total nitrogen per 1,000 square feet (assuming the absence of turfgrass and groundcovers in the application area), but not to exceed more than 4 pounds of nitrogen per 1,000 square feet annually. Due to concerns with potential leaching, runoff, and environmental contamination, slow- or controlled-release nitrogen fertilizers are preferred.

Avoid applying too much fertilizer, which may injure or kill plants, resulting in “fertilizer burn.” Excessive nitrogen stimulates leafy shoot growth at the expense of flowers. This lush, rampant growth is more susceptible to cold injury, drought, and pests. Fertilizer not absorbed by the plant roots may contaminate ground- and surface water. See HGIC 1299 Fertilizers and the Environment for information regarding the appropriate use of fertilizers to protect natural resources.

With so many types and forms of fertilizers, selecting the right one can be challenging. Refer to HGIC 1230, Choosing a Fertilizer, to choose an appropriate fertilizer that satisfies the plant’s nutritional needs.

Fig. 3. Apply fertilizer evenly on mulched and unmulched surfaces up to the drip line or outermost branches of the canopy.

Fig. 3. Apply fertilizer evenly on mulched and unmulched surfaces up to the drip line or outermost branches of the canopy.Image courtesy of International Society of Arboriculture. Artist: Bryan Kotwica

Apply Fertilizer in The Right Place

The root zone area, the space beneath the canopy of the shrub or tree, contains most of the plant’s fine or “feeder” roots that absorb water and nutrients. It extends from he trunk (outside of the buttress or structural roots at the root flare) to the outermost branches (Fig. 3).

Fig. 4. Columnar or fastigiate cultivars, such as upright European hornbeam, have narrow crowns and reduced root zone areas.

Fig. 4. Columnar or fastigiate cultivars, such as upright European hornbeam, have narrow crowns and reduced root zone areas.
R. F. Polomski ©2023 Clemson Extension

These nonwoody roots (< 2 mm in diameter) create an extensive network to absorb water and nutrients and develop associations with soil microorganisms. Most of these roots only live for a few days to weeks until they die and are replaced by new ones. Others become woody, supportive roots that contribute to the structural framework of the root system.

Trees with narrow canopies have a reduced root zone area. They include upright European hornbeam (Carpinus betulus’ Fastigiata’), ‘CCSQU’ Palisade® American hornbeam (Carpinus caroliniana ‘CCSQU’ ), Slender Silhouette sweetgum (Liquidambar styraciflua’ Slender Silhouette’), Pringreen pin oak (Quercus palustris ‘Pringreen’ Green Pillar®), Nadler hybrid oak (Quercus × warei’ ‘Nadler’ Kindred Spirit®), and columnar lacebark elm (Ulmus parvifolia ‘BSNUPF’ Everclear®).

Trees whose natural growth has been cropped with architectural pruning techniques, such as pollarding, pleaching, espalier, and topiary, may not produce roots that extend to the dripline. To determine the root zone area of these specimens, estimate the root zone area from the trunk diameter. Measure the diameter in inches at 4.5 feet above the soil level (technically called diameter at breast height or dbh) and multiply it by either 1 or 1.5 to get a number expressed in feet. This number is the radius of the root zone area or area to be fertilized. So, a 12-inch diameter columnar tree, depending on the multiplication factor, will have a radius of the fertilization area of 12 to 18 feet.

1. Diameter at breast height (dbh) in inches × 1 or 1.5 = Radius of root zone area to be fertilized in feet.

Radius of fertilizer application area in feet
Root zone area to be fertilized

Fig. 5. For trees with naturally narrow or columnar canopies or trees whose canopies have been reduced with specialized pruning techniques, calculate the radius of the fertilizer application area based on the diameter of the trunk at breast height (dbh). Measure the dbh in inches (measured 4 ½ feet above the ground) and multiply it by either 1 or 1.5 to determine the radius of the root zone area in feet.

Fig. 5. For trees with naturally narrow or columnar canopies or trees whose canopies have been reduced with specialized pruning techniques, calculate the radius of the fertilizer application area based on the diameter of the trunk at breast height (dbh). Measure the dbh in inches (measured 4 ½ feet above the ground) and multiply it by either 1 or 1.5 to determine the radius of the root zone area in feet.
Image courtesy of International Society of Arboriculture. Artist: Bryan Kotwica

Fig. 6. For closely planted trees, the root zone area (striped area) beneath the crowns of the entire group of trees is the area to be fertilized.

Fig. 6. For closely planted trees, the root zone area (striped area) beneath the crowns of the entire group of trees is the area to be fertilized.
Image courtesy of International Society of Arboriculture. Artist: Bryan Kotwica

When trees and shrubs grow in groups, beds, or borders with several closely spaced or overlapping canopies, treat them as one single canopy to determine the root zone area (Fig. 6). Calculate the root zone area as a circle (area = 3.14 × radius × radius) or square or rectangle (area = length × width) (Fig. 7).

Fig. 7. For a border planting of Magland Oakland® holly growing in a lawn that receives no fertilizer, view the planting bed as a rectangle and multiply the length and width to determine the root zone area to be fertilized.

Fig. 7. For a border planting of Magland Oakland® holly growing in a lawn that receives no fertilizer, view the planting bed as a rectangle and multiply the length and width to determine the root zone area to be fertilized.
R. F. Polomski ©2023 Clemson Extension

Apply the Right Amount of Fertilizer

Soil test and plant tissue analysis results include recommendations on the kind and rate of fertilizer to apply. They are usually given in pounds of a particular fertilizer grade applied to a specific area. For example, it could be 2 pounds of 12-6-6 fertilizer per 100 square feet for shrubs, or 30 pounds of 15.5-0-0 per 1,000 square feet for trees.

Apply Fertilizer at the Right Time

Actively growing plants absorb fertilizer, typically from budbreak or when new growth begins, up to late summer and early fall. Avoid fertilizing drought-stressed trees and shrubs during the summer months because adequate soil moisture is necessary for the uptake of nutrients.

Instead of applying the annual recommended amount of fertilizer at one time, split or divide the total into a few smaller applications. Providing nutrients in smaller quantities, especially nitrogen, improves the chances that they will be taken up and not lost to the environment. For shrubs and trees growing in sandy soils, which are more subject to leaching than other soil types, the lower rates of split fertilizer applications reduce the potential for losses.

Time the fertilizer applications from spring to early summer and in late summer to early fall when it coincides with plant demand and favorable temperature and moisture conditions. Late summer and early fall fertilizer applications take advantage of actively growing roots when shoot growth has ceased. Research indicates that early spring growth depends almost exclusively on nutrients absorbed and stored the previous year. When those reserves are used, and the new leaves expand and mature, then the tree acquires nutrients from the surrounding soil.

Fig. 8. These red maple trees growing in a fertilized lawn will acquire nutrients from the applications made to the turfgrass. Any additional fertilizer applied to the trees should be based on specified objectives. Note: when mulching trees, follow the rule of 3s: apply a 3-inch layer of mulch 3 inches from the trunk and up to the dripline or outermost branches of the tree.

Fig. 8. These red maple trees growing in a fertilized lawn will acquire nutrients from the applications made to the turfgrass. Any additional fertilizer applied to the trees should be based on specified objectives. Note: when mulching trees, follow the rule of 3s: apply a 3-inch layer of mulch 3 inches from the trunk and up to the dripline or outermost branches of the tree.
R. F. Polomski ©2023 Clemson Extension

In the past, there were concerns that fertilizer applications in the latter stages of the growing season would trigger soft, new growth that may not harden-off or acclimate to impending cold temperatures and be damaged or killed during the winter. Those claims are unfounded, particularly with conifers and most hardwoods. For more information, see Best Management Practices: Tree and Shrub Fertilization in the list of references at the end of this fact sheet.

Shrubs and trees that produce multiple flushes of growth during the growing season have the potential of being stimulated by late season fertilizer applications. These indeterminate species include azaleas, hollies, hydrangeas, and red and sugar maples. But, not all indeterminate species respond the same to late-season fertilizer applications. It is best to apply only portions of the total amount of fertilizer at this time of year rather than the entire recommended annual amount.

For shrubs and trees growing in fertilized lawns, follow the recommendations for the turfgrass (Fig. 7). See HGIC 1201, Fertilizing Lawns, for more information. Shrub and tree roots can intercept nutrients from fertilizer applications made to lawns.

Depending on the formulation, applications exceeding 2 pounds of total nitrogen can overstimulate or burn the grass. Some trees or shrubs growing in fertilized lawns may show nutrient-deficiency symptoms that indicate a need for additional fertilizer, space the fertilizer applications a few months apart to avoid exceeding the total yearly amount of nitrogen required by the lawn grass. Visually evaluate the plant’s response to the application and conduct follow-up soil tests or tissue analyses to determine whether the tree’s fertility requirements are met.

When planting shrubs and trees, water, and not fertilizer, is the most important element for successful establishment. In situations where infertile soil is nutrient-deficient as determined by a soil test or plant tissue analysis, mix the required amount of slow-release nitrogen fertilizer into the backfill or apply it just outside of the rootball. For additional fertilizer instructions for newly planted shrubs and trees, see the fact sheets, HGIC 1052, Planting Shrubs Correctly and HGIC 1001, Planting Trees Correctly.

Fertilizer Application Methods

Fig. 9. Use a cyclone spreader to evenly broadcast the recommended fertilizer over the root zone area of shrubs and trees.


Fig. 9. Use a cyclone spreader to evenly broadcast the recommended fertilizer over the root zone area of shrubs and trees.
R. F. Polomski ©2023 Clemson Extension

An economical and effective method of fertilizing trees and shrubs is to broadcast a granular or pelletized fertilizer over the entire root zone area. Use a drop-type or cyclone spreader to distribute the recommended amount of nutrients evenly (Fig. 9). Follow the manufacturer’s instructions on the label. Although mulch, turfgrass, or groundcovers in the root zone area can interfere with the movement of nutrients to the roots, do not make allowances and apply more than the recommended amount. Fertilizer applications should be guided by objectives, observations, and soil tests or plant tissue analyses.

Disperse fertilizer over the top of shrubs and groundcovers when the leaves are dry to avoid injury called leaf burn or scorch. Since fertilizers are salts, moisture will be drawn out of the leaf, resulting in brown dead spots that appear along the edges or other parts of the leaf. Some plants, such as azaleas and gardenias, may collect fertilizer granules in the whorls of their leaves. Use a leaf rake or broom to gently remove the fertilizer from the plants.

Whatever fertilizer or method of application you choose, irrigate soon after their application to wash any fertilizer from the leaves and to help nutrients dissolve and penetrate through the mulch and soil to the roots. Without irrigation or rainfall, some of the nitrogen may evaporate and be lost to the atmosphere without benefiting the plants.

Fig. 10. Subsurface liquid injection uses pressure to apply the fertilizer 4 to 8 inches deep within range of absorbing roots.

Fig. 10. Subsurface liquid injection uses pressure to apply the fertilizer 4 to 8 inches deep within range of absorbing roots.
R. F. Polomski ©2023 Clemson Extension

Surface-applied ready-to-use liquid or dissolved dry formulations of fertilizer are applied with hose-end sprayers. These liquid fertilizers contain fast-release nitrogen, which is quickly available to plants. When using liquid fertilizers, evenly cover the root zone area and use the proper dilution rate to apply the recommended amount of nutrients. Slow-release nitrogen fertilizers are best for fertilizing woody plants; however, consider fast-release nitrogen fertilizer when slow-release fertilizer will not provide a sufficient response.

Fertilizer can be applied in liquid form to the leaves of shrubs and trees. Foliar applications provide a temporary solution to nutrient deficiencies in existing leaves, with the best results achieved in the spring. For example, use liquid fertilizer to correct micronutrient deficiencies such as iron chlorosis or yellowing in azaleas. Young yellow leaves with green veins are a symptom of azalea iron deficiency.

However, leaf fertilizer application will not eliminate the micronutrient deficiency. Have the soil tested to find the underlying problem. In the case of azalea iron chlorosis, improper soil pH is often the cause. Until the soil pH is corrected, foliar fertilizer applications must be repeated to keep the leaves green.

Professional arborists use a subsurface liquid injection method to inject liquid fertilizer into the soil under pressure to a depth of 4 to 8 inches (Fig. 10). Injections are made 1 to 3 feet apart and evenly spaced throughout the fertilizer treatment area. Injecting liquid fertilizer into the soil delivers the nutrients to the fine absorbing roots of shrubs and trees. This delivery of nutrients into the soil makes it practical for use on slopes to reduce the chances of runoff. It is also helpful when there is a need to penetrate the competing layer of turfgrass or groundcover roots.

Summary

  • Apply fertilizers to shrubs and trees when the soil does not provide these nutrients in adequate amounts that allow for normal growth and development.
  • Use visual inspections and a soil test or plant tissue analysis results to determine the need for fertilizer
  • Practice the 4Rs of fertilizing plants to maximize plant uptake and minimize losses to the environment: apply the right fertilizer, in the right place, in the right amount, and at the right time.
  • Slow-release nitrogen fertilizers are best for woody plants, with one-third to one-half of the nitrogen in a water-insoluble, slowly available or slow-release form.
  • Shrubs and trees absorb fertilizer during their active growth period: from spring, when growth begins, to late summer and early fall. Water must be available to dissolve the nutrients so they can be absorbed by roots.
  • Broadcasting granular or pelletized fertilizers over the root zone area is an economical and effective method of fertilizing trees and shrubs.
  • When shrubs or trees are growing in fertilized lawns, it may be unnecessary to add additional nutrients. Visually inspect the shrubs and trees to see if fertilizer is necessary.

References:

  1. American National Standard – ANSI A300 (Part 2)- 2018 Soil Management a. Assessment, b. Modification, c. Fertilization, and d. Drainage. Tree Care Industry Assoc., Inc. Londonderry, NH.
  2. Bruulsema, T.W., F. Garcia, and T. Satyanaryana. 2012. The 4R Nutrient Stewardship Concept, p 2-1—2-7. In: Bruulsema, TW, PE Fixen, GD Sulewski (eds). 4R Plant Nutrition: A manual for improving the management of plant nutrition. International Plant Nutrition Institute. Norcross, GA.
  3. Day, S. D., P. E. Wiseman, S. B. Dickinson, and J. R. Harris. 2010. Contemporary concepts of root system architecture of urban trees. Arboriculture & Urban Forestry 36(4):149–159.
  4. Rose, M. A. 1999. Nutrient use patterns in woody perennials: implications for increasing fertilizer efficiency in field-grown and landscape ornamentals. Vol. 9(4):613–617 https://doi.org/10.21273/HORTTECH.9.4.613
  5. Smiley, E. T., L. Werner, S. J. Lilly, and B. Brantley. 2020. Best management practices: Tree and shrub fertilization. 4th ed. International Society of Arboriculture, Atlanta, GA.

Document last revised 11/23 by Robert F. Polomski

Originally published 05/99

If this document didn’t answer your questions, please contact HGIC at hgic@clemson.edu or 1-888-656-9988.

Factsheet Number

Newsletter

Categories

Pin It on Pinterest

Share This