The Secret to Nurturing Trees & Shrubs for Longevity

Trees and shrubs are like living investments that appreciate in value year after year. To ensure their long and healthy life, it’s important to choose the right plants and provide proper care, including pruning, watering, mulching, and fertilizing. Not all plants require fertilization, especially mature ones growing in favorable soil conditions. However, when fertilization is necessary, it should be done with a clear objective in mind. Let’s explore the objectives and best practices for fertilizing trees and shrubs.

Objective-Driven Fertilizer Applications

Fertilizers contain essential nutrients that promote growth, flowering, and fruiting. When the soil doesn’t provide enough of these nutrients, fertilizers can supply them to the plants. However, it’s crucial to have a specific objective when applying fertilizers, rather than using them routinely. Here are some reasons to fertilize trees and shrubs:

  1. Treating nutrient deficiencies identified through soil tests or plant tissue analysis.
  2. Encouraging growth, especially in young, established trees.
  3. Maintaining growth in older, mature trees, particularly when organic matter recycling is limited due to leaf removal.
  4. Improving flowering and fruiting in fruit and nut-producing plants by replenishing the nutrients that are removed during harvest.

It’s important to note that fertilizing plants with poor growth symptoms may not solve the underlying issue. Lighter green leaves, reduced twig growth, and decreased flowering and fruiting can occur due to various reasons, such as pest damage, drought, or imbalanced soil pH. Adding more fertilizer won’t address these problems until the root causes are addressed. Observing plants for nutrient deficiencies and conducting soil tests or plant tissue analysis are crucial for determining the need for fertilization.

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Choosing the Right Kind of Fertilizer

Soil test results or plant tissue analysis will provide recommendations for the appropriate fertilizer grade. Fertilizer labels indicate the ratio of nitrogen (N), phosphorus (P), and potassium (K) in the product. For example, a 12-4-7 fertilizer contains 12% nitrogen, 4% phosphorus, and 7% potassium. The fertilizer ratio is determined by dividing the percentage of each nutrient by the lowest percentage among them. Landscape shrubs and trees generally prefer a ratio of 3:1:1 or 3:1:2, while a 30-0-10 ratio may be recommended if phosphorus is not needed.

Nitrogen is the most important nutrient for plant growth and development, so fertilizer recommendations are mainly based on nitrogen content. Slow-release nitrogen fertilizers are preferred to prevent excessive growth and potential environmental contamination. Applying too much fertilizer can lead to “fertilizer burn” and stimulate excessive leafy growth at the expense of flowers.

Applying Fertilizer in the Right Place

It’s essential to apply fertilizer in the root zone area, which extends from the trunk to the outermost branches. Fine roots within this zone absorb water and nutrients. The root zone area may vary depending on the tree or shrub’s canopy size, and it can be calculated based on the trunk diameter. When multiple trees or shrubs are closely spaced, their root zones can be treated as a single canopy.

Applying the Right Amount of Fertilizer

Soil test results or plant tissue analysis will provide recommendations for the amount of fertilizer to apply. Recommendations are usually given in pounds per specific area. It’s important to follow these recommendations to avoid over-fertilization, which can lead to plant damage, excessive growth, and environmental pollution.

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Applying Fertilizer at the Right Time

Fertilizers should be applied when plants are actively growing and able to absorb nutrients effectively. This is typically from spring to early fall, avoiding the summer months when plants may be stressed by drought. Splitting the total recommended amount of fertilizer into smaller applications can improve nutrient uptake and minimize environmental losses. Late summer and early fall applications are particularly beneficial as roots are active, and shoot growth has ceased.

Fertilizer Application Methods

A cost-effective method for fertilizing trees and shrubs is to broadcast granular or pelletized fertilizers over the entire root zone area. It’s important to follow the manufacturer’s instructions and avoid excess application, even if mulch or groundcovers are present. Irrigation after fertilizer application helps dissolve nutrients and prevent loss through evaporation.

Liquid fertilizers can be applied to the leaves of shrubs and trees, providing a temporary solution to nutrient deficiencies. However, addressing the underlying issue, such as soil pH, is necessary for long-term correction. Subsurface liquid injection is another method used by professional arborists to deliver nutrients directly to the roots.

In summary, fertilizing trees and shrubs requires a targeted approach based on specific objectives. It’s crucial to choose the right fertilizer, apply it in the right place and amount, and at the right time. Following these practices will help promote healthy growth and longevity for your precious plants. For more information and guidance, visit the Ames Farm Center.