Tree Fertilization | How to Feed Trees and Shrubs Made Easy

350 million years ago, the first trees sprouted across an ancient landscape. Over the course of eons, those sprouts have evolved into more than 60,000 different species.

The endurance of these many varieties is rooted in their ability to adapt to their environment. In doing so, they’ve become ‘the lungs’ of our planet. Inhaling carbon dioxide from the atmosphere and exhaling oxygen, which allows us to breathe.

Trees improve soil quality, temper local heat levels by providing shade, and filter air contaminants. For these reasons, including native species in your landscaping projects, is key to long-term success.

When planting in poor soil, knowing how to feed trees and shrubs is a must. But, since trees and shrubs are living, breathing investments that appreciate in value over time, it’s important that your fertilization plans are both effective and beneficial.

tree fertilization.

Establish the Need for Tree Fertilization

Landscape design often includes both native and non-native trees. Due to unfamiliar soil and climate conditions, non-native species are more likely to display symptoms of nutrient deficiency than local ones.

For this reason, occasional fertilizing will help non-native varieties become better established and display healthy growth. Amending the soil with well-aged compost and fresh topsoil can also create an inviting base whether your choice is native or otherwise.

To encourage your investment’s long-term success, a quality soil test will indicate what nutrients need to be added. The type of tree you’re planting will dictate how much needs to be applied.

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Test Soil Nutrients and pH

soil test kit

Soil test kits range from simple pH strips to those that detect specific nutrient and mineral deficiencies. Target results can vary too, so choosing the right test will save you time and money.

Tests that determine levels of available macro and micronutrients are the best option and it is advisable to opt for laboratory test services. Generally, test kits come in the form of a simple home test kit allowing you to take a soil sample and mail it off to a laboratory for testing. You will then receive a report showing the soil analysis and often recommendations for soil amendments.

Another measure of interest is soil pH. Most trees and shrubs thrive in a soil pH of 6.0 – 7.0, with the exception of acid-lovers like hydrangeas and azaleas.

These prefer a 4.0 – 5.5 pH range. Anything outside these ranges would block nutrient absorption completely, putting your trees at risk of dying or at least not producing healthy growth or blooms.

Condition of Growth

If you have already-established trees and shrubs on your property, there are things you can do to gauge any existing nutrient deficiency. One is to simply look at them. Observe how they’re growing and look for any signs of poor growth. These might include:

  • Fading foliage color in spring and summer
  • Abnormal leaf size or shape
  • Stunted growth
  • New twig or leaf drop in spring and summer

These symptoms aren’t necessarily due to nutrient deficiency. They may also be caused by compacted soil, pests, disease, or adverse weather. Determining the exact cause will lead to proper action.

Tree Age

This is an easy way to know how much, how often, and what type of tree and shrub fertilizer to use. 

Starting in the second year after planting, extra nourishment will support newly-established roots. This will accelerate growth and assist young trees in nicely filling their places in your landscape. 

tree stage of growth

The older these garden monoliths get, the more adapted to their new environment they become. The more ‘at home’ they feel, the less fertilizing they will need.

  • Fertilize Newly Planted Trees and Shrubs

Saplings started in fertile soil will experience robust, root growth. But no extra fertilizer should be added for the first year. Doing so will force new growth that distracts from healthy root development. New growth will be weak and vulnerable to pests and disease.

  • Rapid Growth Stage

‘Rapid’ tree and shrub growth are triggered by increasing spring temperatures. This growth will appear faster and more vigorous than any displayed in fall or winter. For this reason, spring is the best time to apply a single dose of tree fertilizer, which can last all year.  

  • Mature Trees

Older trees need less fertilizer if any. But, this isn’t a ‘one size fits all rule. Different trees absorb soil nutrients at different rates. And may eventually show signs of nutrient deficiency. In this case, trees should be fertilized every three to four years to maintain soil fertility.

Indirect or Secondary Fertilization

An important consideration in any tree fertilization regimen is where they’re planted. Those planted in lawns will naturally absorb whatever fertilizer is applied to those laws. This is called indirect or secondary fertilization. 

Most lawn fertilizers are high in nitrogen. While trees use a lot of nitrogen to produce foliage, they may not require as much as lawns do. In this case, over-fertilizing is a risk. 

So, when planting in grass, choose a tree that isn’t sensitive to nitrogen. The same applies to planting trees around ornamentals or shade plants that have different NPK needs.

Different Types of Fertilizers

Fertilizers come with a number of different application methods, both organic and inorganic. Liquid options come either concentrated (that need to be diluted) or pre-diluted and ready to use.

Powders are water-soluble and should be mixed with water before using as a soil saturator or foliar spray. Some less potent powders can be mixed straight into your soil and activated when watered in, without overwhelming your plants. 

Even more convenient are granules and spikes. These slow-releasing options can last anywhere from three months at a time to one application for the whole season.

But, what are the benefits of each type and what will they do for your plants?

Fast Vs Slow-Release Fertilizer

Fast-acting, liquid nutrients are quickly carried to the roots, allowing for immediate uptake. They can keep crucial pH levels balanced, too. But, caution is recommended, as over-feeding could lead to root burn.

Slow-release fertilizers are heavily concentrated and provide a consistent flow of nutrients for several months at a time, saving you time and effort.

N-P-K Ratio

Every fertilizer label reflects an NPK ratio (three numbers, separated by dashes). Indicating its proportions of nitrogen, phosphorus, and potassium. For example, a 12-4-8 NPK fertilizer contains 12% nitrogen, 4% phosphorus and 8% potassium. 

Each of these macronutrients plays a vital role in cell formation, photosynthesis, nutrient distribution, and flower and fruit production. All working in tandem for optimal plant health. When applied properly, they can also increase soil fertility and beneficial bacteria activity.

Numbers on Fertilizer Packaging Explained in 60 Seconds

Tree & Shrub Fertilizer Ratios

Each tree and shrub variety will have its own unique nutritional needs. How each type responds to fertilization largely depends on the macronutrient ratio or NPK. Let’s take a look at the best NPK ratios for the most commonly planted trees.

Calculating Area & Fertilizer

So, how do you know how much fertilizer to give each of these tree varieties? As a general rule, younger trees and shrubs, both evergreen and flowering, require additional fertilizer in the first 2-4 years after planting. 

Root systems grow faster during these years as the trees move through their life cycles toward maturity. Once maturity is reached, they need less fertilizing, if any. How do you know how far out the roots grow, in order to properly fertilize them? By calculating the radius of the tree’s root spread as it relates to its drip line.

Finding The Drip Line Of A Tree

A tree’s dripline is simply the distance between the outer layer of foliage on one side of the tree to the outer layer on the opposite side.

This will be the farthest point from the trunk where rain drips off. Thus, the ‘drip line’, or the crown radius.

Naturally, the dripline of smaller trees grown in pots will be limited to the radius of the pot.

Establishing the Tree Root Zone

Calculating a tree’s root zone is easier than you might think.

The root zone will be roughly 1.5 times the width of the crown radius, also known as the drip line. This area is a circular pattern, from the point of the trunk.  

For best results, apply fertilizer evenly across the root zone, stopping 6” short of the trunk.

Fertilizer should never be applied to the trunk, as it could result in permanent damage and vulnerability to pests and disease

Root Zone Are to Fertilize

Amount of Fertilizer to Apply

For 2 – 4-year-old trees in lawns, follow the same application rate and frequency as you would for your lawn fertilization schedule. The lawn fertilizer will usually do a good job of providing enough nutrients for your tree.

For older trees, or trees in open areas, it is recommended to use 3lbs of nitrogen per 1,000sq.ft. You can calculate the square footage of your Root Zone by multiplying the Crown radius by x 3.14.

Alternatively, if you purchase tree fertilizer spikes or another purpose-made tree fertilizer the manufacturer will provide suitable direction on the packaging for fertilizer applicate rates.

Methods of Tree & Shrub Fertilization

Plants and shrubs in lawns can be fertilized either directly or indirectly. In both cases, the entire root zone should receive equal amounts so as not to cause roots to bunch up in one or two places. Densely packed, those roots would no longer be able to absorb what they need.

Because of beneficially high concentrations of oxygen near the surface, most tree roots will remain within the top 10-14 inches of soil. The use of either quick-release liquids or slow-release granules makes easy work of evenly distributed fertilization via rainfall or irrigation. 

Tree Fertilizer Spikes

Spikes work best on potted trees and shrubs. Nutrients break down and spread around the pot for full root ball access. Spikes used on large trees only fertilize the roots immediately around them, leaving the rest of the root ball unfed. Luckily, spikes can easily be crumbled up and broadcast evenly around the root zone.

Surface Top Dressing

Topdressing root zones with compost and fresh topsoil is another way to fertilize trees and shrubs. These materials give plants what they need, in a completely natural form, for optimal growth and health. This also works well on lawns, potted plants, garden borders, and vegetable beds.

Liquid Root Feed

Most liquid fertilizers are applied via an irrigation system, hose attachment, or by hand using a watering can. And since the feeder roots of most trees lay within the top 10” of soil, within the tree’s dripline, nutrients don’t have to go very far to reach them.

Foliar Spray

When tree foliage starts to show signs of nutrient deficiency, like yellowing or curling, a foliar spray can be an effective, albeit temporary, way to re-introduce adequate nutrients to those leaves. However, the nutrient deficiency in the soil still needs to be addressed to prevent further foliage maladies.

Deep Root Fertilizer Myths

Deep root fertilization can be an effective process for over-sized trees that are severely nutrient deficient. The amount of nourishment in standard feeds may not provide enough. But, for smaller trees, by hand, broadcast methods are equally as effective and less costly than professional subsurface fertilization.

Liquid Injection

Fertilizing small to medium-sized trees is manageable enough. But, when older, larger trees, with massive root systems, are showing signs of decline, arborists often utilize a deep root fertilization process. Whereby liquid nutrients are injected into the soil, evenly across the root zone, to nourish the roots directly.

Drilling Holes

Similar to deep root fertilization, holes can be drilled into compacted soil around small to medium-sized trees in lawns. Into which liquid or granular fertilizer can be poured. This process makes nourishment accessible to tree roots, when compacted soil prevents it, and away from those of your lawn. 

Tree Fertilization Final Thoughts

A proper (and not-so-complicated) tree fertilization plan can result in years of oxygen-giving, climate-moderating beauty across your landscape. Effective shrub fertilization can not only increase your fruit yield but fill your garden with fragrant, colorful flowers.

For those grown in the grass, remember that trees can indirectly benefit from quality lawn care. If not, making sure that the entire root zone is nourished will result in vibrant foliage and healthy trees.