Just like people need a balance of protein, carbohydrates and vitamins, plants need “food” in the form of nitrogen, phosphorus, potassium and other minor nutrients. And it comes in the form of fertilizer. Different types of plants – annuals, perennials, trees, shrubs and lawns – have different fertilizer needs. Let’s take a closer look at the fertilizer application needed for trees and shrubs by answering three basic questions.

Fertilizing trees and lawn

Trees that border the lawn will likely get enough nutrients from the lawn fertilizer application.

Photo Credit: Daniel Overcash

Fertilizing deciduous tree

Young plants especially need fertilization every year to promote root growth and establishment.

Photo Credit: Daniel Overcash

  1. How much fertilizer should I apply? This is a difficult question to answer. In some cases, plants display symptoms of a nutrient deficiency, such as yellowing leaves. But recognizing those symptoms requires careful observation and an in-depth knowledge of plant physiology. Luckily for homeowners, there’s an easier way.

    The amount of fertilizer that a plant needs varies based on many factors, like the age of the plant, soil type, pH level and much more. The most accurate way to determine the amount of fertilizer a plant needs is to do a soil test. Soil tests are usually performed by your State Department of Agriculture or Extension Service for free or for a small fee. The analysis sent to you by the soil-testing lab will provide a detailed report of your plants’ nutrient needs, as well as the type of fertilizer that will best suit your specific situation.

    With that said, if you decide not to get a soil test done, here are two general recommendations to go by:

    • For small trees (one with a trunk diameter less than 1 inch at 4 feet above the ground) and all shrubs, use ½-1 cup of 10-10-10 fertilizer per plant. (Note: The ½ cup rate is more for newly planted trees, and the 1 cup is more for established trees that are still less than 1-in diameter.)
    • For larger trees, measure the tree’s trunk diameter at 4 feet above the ground, then apply 1 pound of 10-10-10 for each inch in diameter.
  2. When should I apply fertilizer? Trees and shrubs should be fertilized once a year, either in spring or fall. If applying in the spring, fertilize one month before the leaves appear for deciduous plants and one month before new growth occurs on evergreens. Observation from year to year and garden journaling may be helpful in determining approximately when one month before leaves and new growth occurs, but admittedly, it’s a difficult guideline to follow. As a general rule, you can make your spring fertilizer application around the date of the last frost of the cool season, and fall applications can be made one month after the first frost.
  3. How do I apply fertilizer? When “feeding” trees and shrubs, spread the fertilizer evenly over each plant’s entire root zone. (Remember, for older trees with extensive root systems, the entire root zone can be up to three times the area past where the branches extend.) There’s no need trying to get the fertilizer into the ground or below the mulch because that first watering will quickly move the fertilizer into the soil.

Avoid these common mistakes in fertilizer application:

  • Do NOT dump all the fertilizer in one spot; spread it evenly. If fertilizer is left in one pile, the roots beneath that spot will be damaged from too much of it. (Plant damage caused by too much fertilizer is call “fertilizer burn.”)
  • Do NOT allow fertilizer to stay on foliage. If some fertilizer gets stuck on the plant, rake it off or use a hose to wash it off. Fertilizer is intended to be taken up by the roots. If it’s left on the leaves, this can result in fertilizer burn, causing yellow or brown spots on an otherwise healthy leaf.
  • Do NOT use fertilizer stakes. These tools may seem convenient, but you simply cannot evenly distribute the nutrients by this method.

Without proper fertilization, trees and shrubs may not grow well, the leaves may turn an unattractive light green, and the plants may not produce many flowers or fruit. Following these general guidelines will help you maintain a healthy landscape – and plants – year after year.