The Importance of Fertilizing Trees and Shrubs

As a lover of gardening and someone with years of experience, I can attest to the importance of fertilizing trees and shrubs in maintaining their health and vitality. At Tips Tree Planting, we understand that the key to a thriving landscape lies in maintaining the vigor of your plants. In this article, we will dive deep into the topic of fertilization, exploring its benefits, when and how to fertilize, and the best fertilizers to use.

Is Fertilizer Necessary?

Trees and shrubs in your landscape may not always require supplemental fertilization if certain conditions are met. Proper site selection, the use of nutrient-rich mulch, and the presence of surrounding fertilized turf areas can contribute to the overall health of your plants. However, it is crucial to recognize that fertilization is not a cure-all solution for careless planting, improper watering, or poor drainage. To ensure optimal growth and resistance to environmental stressors and pests, it is essential to choose trees and shrubs that are well-suited for your climate and location.

Signs of Nutrient Deficiencies

In many cases, landscape trees are planted in soils with low fertility. Conducting a soil test before planting can help determine if the soil at the planting site is deficient in phosphorus or potassium. These nutrients do not easily move through the soil when surface-applied, so amending the soil to recommended levels during planting can promote better tree establishment. Avoid using high rates of nitrogen fertilizer on newly planted trees, as this can lead to excessive leaf growth at the expense of root development, making the tree more susceptible to drought stress. Instead, a light surface application of nitrogen fertilizer one month after planting will aid in the establishment process.

Furthermore, in most landscape situations, fallen leaves are often removed to facilitate turf growth. However, this practice disrupts the natural nutrient cycle that occurs in tree species’ natural habitats. As a result, fertilization may be necessary to replenish the nutrients that are being lost through leaf removal. Signs of low nutrition include poor tree growth, pale green or yellow leaves, mottled patterns between the veins, dead spots, stunted leaves, or premature leaf loss.

To gauge tree growth, measure the current season’s growth by observing the distance between the tip of a twig and the first ring of bud scale scars. Previous year’s growth can be determined by the space between the first and second bud scale scars. A healthy young tree should exhibit 9 to 12 inches or more of twig growth per year, while mature trees may only grow 4 to 6 inches per year.

When to Fertilize?

If fertilization is necessary, the best time to apply it is from late September through early April. The optimal period for fertilization is after the first hard freeze in October but before the soil freezes in December. During this time, the soil temperature is still warm enough for roots to absorb and store minerals for spring growth. Fertilizing during winter can result in leaching or runoff, leading to fertilizer loss. The second-best time to fertilize is in March or April, just before new growth begins. It is crucial to avoid fertilizing after mid-July, as late growth stimulation can be damaged by an early freeze.

Newly planted trees should be lightly fertilized until they have become well established, which is typically after their first growing season. For young, rapidly growing trees, annual fertilization is recommended to promote rapid establishment. Mature trees, on the other hand, may only require fertilization every two or three years to maintain vibrant foliage and vigor.

Choosing the Right Fertilizer

Fertilizers can be classified as organic or inorganic. Organic materials are derived from plant or animal sources and release nitrogen slowly through microbial conversion. In contrast, inorganic fertilizers contain readily available forms of nitrogen. Slow-release inorganic fertilizers are commonly used in turf fertilizers and provide a more controlled release of nutrients. When choosing a fertilizer, look for a 2-1-1 or 3-1-1 ratio of nitrogen, phosphorus, and potassium (N-P-K), as it is most beneficial for tree growth. Commonly available fertilizers with this ratio include 18-6-12, 12-6-6, 10-6-4, 10-8-6, and 10-8-4. If the desired ratio is unavailable, you can add ammonium nitrate (33-0-0) to achieve the desired ratio. It is important to note that continuous fertilization with phosphorus and potassium, regardless of soil levels, can lead to nutrient imbalances.

Avoid fertilizers containing broadleaf weed killers when fertilizing trees, as these can harm the trees. Turf fertilizers often work well for trees since they contain the desired nitrogen-phosphorus-potassium ratio and provide slow-release nitrogen.

Fertilize the Entire Root Area

To effectively fertilize shade trees, it is important to cover the entire area occupied by the root system. In most cases, this includes the soil up to 1.5 to 2 times the diameter of the branch spread. Approximately 95 percent of a tree’s roots are found within 18 inches of the surface. Fertilizing the turf surrounding a tree can often supply the necessary minerals to keep the tree healthy. If leaves are raked, it is advisable to apply fertilizers or compost containing phosphorus and potassium 12 to 14 inches deep every few years to compensate for nutrient depletion and ensure the fertilizer reaches below the turf roots. Alternatively, you can use a mulching mower to chop the leaves in place, allowing the nutrients they contain to be recycled into the turf.

For large evergreen trees, the same fertilization methods used for deciduous trees can be applied. It is important to note that evergreens typically have roots that extend beyond the branch spread.

How much Fertilizer is Needed?

Ideally, soil test results should be used to determine the appropriate fertilization rates, especially for phosphorus, potassium, and micronutrients. If test results are not available, fertilizing with a 2-1-1 or 3-1-1 ratio fertilizer at a rate of 2 to 4 pounds of actual nitrogen per 1,000 square feet per year should promote healthy plant growth. If applying more than 2 pounds of quickly available nitrogen, it is recommended to split the application into two, preferably in April and October. Certain plants, such as broadleaf evergreens, dwarf conifers, and alpine plants, should receive approximately half the rate of most deciduous plants. Over-fertilization of pines can create large gaps between branch whorls.

Methods of Fertilizer Application

Surface Application: In most cases, surface application of fertilizers is an efficient and effective way to deliver nutrients to tree and shrub roots. Fertilization of surrounding turf often provides adequate nutrients for the trees. Nitrogen from granular fertilizer easily moves into the root zone of woody ornamentals. To determine the amount of fertilizer to apply, measure out a rectangular area that covers most of the root zone, extending 4 to 6 feet beyond the branch spread. Calculate the area in square feet by multiplying the length by the width. For example, if the area to be fertilized is 20 feet square and you wish to apply 2 pounds of nitrogen per 1,000 square feet, you would apply 0.8 pounds (12 ounces) of nitrogen. Using an 18-6-12 fertilizer, you would need approximately 4.4 pounds of fertilizer to achieve this.

Hole Method: If leaves are being raked, it is advisable to use the hole method to place fertilizer, compost, or materials containing phosphorus and potassium into the soil. This compensates for nutrient loss due to leaf removal and places the fertilizer below the turf roots. Holes should be at least 1-inch in diameter, 6 to 12 inches deep, and spaced about 2 feet apart. Compost or coarse sand can be added to the holes along with the fertilizer, or compost alone can be used in a technique called vertical mulching.

Another method is injecting liquid fertilizer directly into the soil using a pressurized “soil needle.” Injections are typically made 2 to 3 feet apart and can be used for soluble fertilizers or suspensions of slow-release materials.

At Tips Tree Planting, we value the health and vitality of trees and shrubs in your landscape. Fertilizing is a crucial step in maintaining their vigor and resilience against environmental stressors and pests. By understanding the importance of fertilization, choosing the right fertilizers, and applying them correctly, you can ensure the long-term success of your trees and shrubs.

For more tips and insights on tree planting and maintenance, visit Tips Tree Planting.


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