Lawn fertilizers are often a great choice for trees and shrubs because they contain the ideal ratio of nitrogen, phosphorus, and potassium. A complete fertilizer with an NPK ratio of 16-4-8, 12-4-8, or 12-6-6 should work well for most plants. However, it's wise to do a soil test before selecting the fertilizer to identify any significant deficiencies in the soil. Some fertilizers also have added micronutrients such as zinc and iron, so if a soil test reveals that your soil is lacking these nutrients, you can buy a fortified formula.
For trees and shrubs on the lawn, apply fertilizer at the right time and amount for the lawn. Fertilizers should not be used as a cure for sick plants when unadapted or unhealthy plants are chosen, planted carelessly, or improperly watered. Trees and shrubs that grow on lawns should be fertilized at the right time and rate for the lawn (see the section Amount of fertilizer to apply). For example, the radius of the fertilization area of a 12-inch diameter tree would be 12 to 18 feet, depending on the multiplication factor that was used. However, trees and shrubs that grow in seedbeds may need to be fertilized, especially on sandy soils with little or no organic matter.
While plants absorb some nutrients through their leaves, large shade trees are not fertilized with this method. For newly planted shrubs and trees, or in areas where the potential for runoff is very high, such as slopes or compacted soils, slow-release fertilizers are a good option. If trees or shrubs growing on fertilized lawns show symptoms of nutrient deficiency that indicate the need for additional fertilizer, space fertilizer applications a few months apart, not exceeding the total annual amount of nitrogen required by the lawn (follow the dosage and time of application of the grass). This formulation allows it to slowly penetrate the soil, providing a continuous source of nutrients for plants for several months. 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.
Trees and shrubs should be fertilized in early spring, and a light fertilizer can be applied in early summer if conditions are conducive to plant growth (i.e., reasonable temperatures and soil moisture). For instructions on fertilizers for new shrub and tree plantings, see the fact sheets HGIC 1052, Planting Shrubs Correctly and HGIC 1001, Planting Trees Correctly. For shrubs and trees in the lawn, follow the recommended dosage and time of fertilization for the lawn. While liquid fertilizers will still provide nutrients to trees and shrubs as a quick solution, they will drain through the ground much faster, especially if the plants are at the top of a slope or hill. For example, if the distance from the trunk of the tree to the drip line (which is called the radius of the crown) is 8 feet, then “feeder” or mineral-absorbing roots can extend 4 feet beyond this drip line. When selecting a fertilizer for your trees and shrubs it is important to consider your soil type and any deficiencies that may exist.
A complete fertilizer with an NPK ratio of 16-4-8, 12-4-8 or 12-6-6 should work well for most plants. If your soil test reveals any deficiencies in micronutrients such as zinc or iron then you may want to opt for a fortified formula. Additionally, slow release fertilizers are recommended when planting new trees or shrubs or when there is potential for runoff due to slopes or compacted soils. It is also important to consider when you apply your fertilizer. Trees and shrubs should be fertilized in early spring with a light application in early summer if conditions are conducive to plant growth.
Never fertilize late summer or early fall as this will stimulate new growth at a time when plants are preparing for dormancy. When applying fertilizer to trees and shrubs growing on lawns follow the recommended dosage and time of application for your lawn.