Trees are a lot like people. Give them too little nutrition, and they struggle. Give them too much, and they can have problems, too.
If you’ve noticed your tree looking a little lackluster, you might be wondering if fertilizer is the answer. The truth is, tree fertilization isn’t a one-size-fits-all solution. While fertilizer can be incredibly beneficial under the right conditions, applying it at the wrong time, or when it isn’t needed, can do more harm than good.
Let’s dig into what tree fertilization actually does, when your trees need it, and how Colorado’s unique growing conditions affect the process.
Table of Contents
TL;DR: Do trees really need fertilizer?
Answer:
What Is Tree Fertilization?
Trees make their own food through photosynthesis by combining sunlight, water, and carbon dioxide. Fertilizer simply provides essential nutrients that may be missing from the soil, helping support healthy growth and important biological functions.
The three primary nutrients found in most fertilizers are:
- Nitrogen (N):
Promotes healthy leaves, needle, and canopy growth. - Phosphorus (P):
Supports root development and energy transfer.
- Potassium (K):
Improves overall tree health, disease resistance, and stress tolerance.
Trees also require smaller amounts of nutrients like iron, manganese, magnesium, zinc, and calcium. In Colorado, deficiencies in iron are especially common because our alkaline soils make it difficult for trees to absorb the nutrients already present in the ground.
The goal of tree fertilization isn’t to force rapid growth. It’s to supply nutrients that allow trees to develop naturally and remain healthy over the long term.
Do Trees Really Need Fertilizer?
The short answer? Not always.
Healthy, mature trees growing in nutrient-rich soil often receive everything they need naturally through decomposing leaves, organic matter, and healthy soil biology.
However, many commercial and residential landscapes don’t offer ideal growing conditions.
Trees may benefit from fertilization if they are growing in:
- Newly constructed neighborhoods where topsoil was removed
- Compacted soil caused by construction or heavy foot traffic
- Rocky or nutrient-poor Colorado soils
- Areas with limited organic matter
- Lawns where grass competes with trees for nutrients
- Sites recovering from drought or other environmental stress
Fertilizer can help restore vigor, but it’s important to remember that it isn’t a cure-all.
If a tree is declining because of improper watering, insect damage, disease, planting depth, root damage, or soil compaction, fertilizer alone won’t solve the problem. In some cases, it can actually increase stress.
That’s why professional diagnosis is one of the most valuable parts of any fertilization program. If you’re unsure whether your trees need fertilizer, call Front Range Arborists today or request a free estimate to have one of our certified arborists evaluate your trees and recommend the right solution for your landscape.
Signs Your Tree May Need Fertilization
Trees often give subtle clues when nutrients are lacking. Some common signs include:
- Smaller-than-normal leaves
- Pale green or yellow foliage
- Thin or sparse canopy
- Slower annual growth
- Premature leaf drop
- Dead branch tips
- Reduced flowering or fruit production
- Poor recovery after drought or storm damage
These symptoms can indicate that your tree isn’t getting the nutrients it needs, especially when multiple signs appear at the same time. Because similar symptoms can also be caused by drought, pests, disease, or other environmental stresses, it’s best to have your tree evaluated by a certified arborist before beginning a fertilization program. Front Range Arborists’ Plant Health Care services can identify the underlying cause of your tree’s symptoms and recommend the right treatment to restore its health.
Tree Fertilization: When Is the Best Time?
Timing can make all the difference, especially in Colorado’s climate. While trees can be fertilized at different times of the year, some seasons offer greater benefits than others. Here’s how the seasons rank, from best to least ideal, for tree fertilization.
Fall (Best)
Fall fertilization can be the most effective fertilization timeframe – occurring after leaves begin dropping but before the ground freezes. During this period, roots continue growing even though the canopy has stopped producing leaves. Because the tree isn’t currently supporting new leaf or shoot growth, more resources can go towards nutrient storage and root growth.
Spring
Early spring can be a good time for tree fertilization. As trees emerge from dormancy, they’re preparing for a season of active growth and can efficiently use available nutrients. However because the growing season in Colorado tends to be warm and dry, trees here often become stressed by early summer.
Summer
Summer fertilization is usually reserved for trees with documented nutrient deficiencies or those under professional care. Fertilizing during periods of extreme heat or drought can place additional stress on already struggling trees.
Winter
Frozen soil prevents roots from absorbing nutrients efficiently, making winter a poor time for fertilization.
Choosing the Right Fertilizer For Your Colorado Trees
Walk through any garden center, and you’ll find shelves lined with fertilizers promising bigger, greener, healthier plants. With so many options available, choosing the right fertilizer for your trees can feel overwhelming.
For trees, the best choice is usually much simpler.
Slow-release fertilizers gradually provide nutrients over time, reducing waste and lowering the risk of fertilizer burn.
In Colorado, nitrogen (N) is often the nutrient trees benefit from most. Many soils already contain adequate levels of phosphorus (P) and potassium (K), making balanced fertilizers unnecessary unless a soil test indicates otherwise.
Rather than guessing which nutrients your trees need or which fertilizer is best, professional arborists evaluate your soil conditions, tree health, and growing environment to determine whether fertilization is needed and, if so, which fertilizer will provide the greatest benefit.
Front Range Arborists currently offers two different proprietary fertilizer blends – a Spring Blend to promote new, health growth above ground and a Fall Blend to promote strong root systems below ground. These have been formulated with tree nutrition professionals to address many of the known nutrient deficiencies and soil issues specific to the Pikes Peak Region.
How Trees Are Professionally Fertilized
Professional tree fertilization goes far beyond placing fertilizer spikes next to the trunk or scattering fertilizer around the base of a tree.
In fact, a tree’s absorbing roots, the tiny feeder roots responsible for taking up water and nutrients, are usually located throughout the root zone and often extend well beyond the edge of the canopy, sometimes reaching two to three times farther than the tree’s branches. That’s why effective fertilization targets the entire root system, not just the area immediately surrounding the trunk.
Before recommending a fertilization plan, certified arborists evaluate the tree’s species, age, health, soil conditions, and growing environment. This helps determine not only if fertilization is needed, but also which nutrients will provide the greatest benefit.
Depending on the tree and site conditions, professional tree fertilization may include one or more of the following methods.
Granular Slow-Release Fertilizer
Granular slow-release fertilizers are evenly applied across the tree’s root zone, allowing nutrients to gradually become available over several months. This steady release more closely matches how trees naturally absorb nutrients, reducing waste, minimizing runoff, and lowering the risk of root burn.
This method works well for many established landscape trees growing in healthy, uncompacted soils.
Deep Root Fertilization
When trees are growing in compacted soils or showing signs of stress, arborists may recommend deep root fertilization.
Using specialized equipment, nutrients are injected several inches below the soil surface directly into the root zone. This process not only delivers nutrients where feeder roots can access them most effectively but also helps break up compacted soil, improving oxygen movement and water infiltration.
Deep root fertilization is especially beneficial for mature trees in urban landscapes where years of foot traffic, construction, or heavy clay soils have reduced the amount of air and water reaching the roots.
This method works well for many established landscape trees growing in healthy, uncompacted soils.
A Customized Approach
No two trees have identical nutritional needs. A newly planted maple, a mature blue spruce, and a stressed ash tree may all require different fertilization strategies.
That’s why professional arborists customize fertilization plans based on the tree’s condition rather than following a one-size-fits-all schedule. In many cases, improving irrigation, correcting soil problems, or treating insects or disease may be just as important as adding nutrients.
Common Tree Fertilization Mistakes
Even with good intentions, it’s easy to overdo it. Avoid these common mistakes:
- Mistake: Fertilizing Every Year Automatically
- Not every tree needs annual fertilization. Applying fertilizer without evaluating your tree’s health can create nutrient imbalances, encourage weak or excessive growth, and place unnecessary stress on the tree. Fertilizer should be applied based on the tree’s specific needs, not on a yearly schedule.
- Mistake: Using Lawn Fertilizer
- Grass and trees have different nutritional needs. Lawn fertilizers are formulated to promote rapid, shallow growth in turfgrass, while trees require nutrients that support long-term root development and overall health. Using the wrong fertilizer can provide an improper nutrient balance and may not benefit your tree as intended.
- Mistake: Fertilizing During a Drought
- Trees under drought stress are focused on survival, not growth. If the soil is dry and roots aren’t actively absorbing water, they won’t efficiently absorb nutrients either. Restoring proper soil moisture should always come first, followed by fertilization only after the tree has recovered enough to benefit from the added nutrients.
- Mistake: Applying Too Much
- More fertilizer doesn’t produce a healthier tree. Excess nutrients can burn roots, encourage weak, rapid growth, and increase susceptibility to insects and diseases. Overfertilization can also contribute to nutrient runoff, wasting fertilizer and potentially impacting nearby plants and waterways. Following the proper application rate is just as important as choosing the right fertilizer.
- Mistake: Ignoring Other Problems
- Many declining trees are actually suffering from poor watering practices, compacted soil, root damage, insects, or disease rather than nutrient deficiencies. Fertilizer won’t fix these underlying issues and, in some cases, may place additional stress on an already struggling tree. Identifying the root cause of the problem is the first step toward choosing the right treatment.
Why Colorado Trees Have Unique Fertilization Needs
Colorado presents challenges many other parts of the country don’t.
Our trees often grow in:
- Alkaline soils
- Heavy clay
- Rocky ground
- Less organically dense areas
- High elevations
- Dry climates
- Areas affected by construction
These conditions can make nutrients less available, even when they’re technically present in the soil.
Iron chlorosis, which causes leaves to turn yellow while veins remain green, is one of the most common nutrient-related problems seen in Colorado landscapes.
Combined with drought stress and compacted soils, many trees simply need extra support to stay healthy.
Should You Fertilize Trees Yourself or Call an Arborist?
Applying fertilizer isn't difficult. Knowing whether your tree actually needs it is the challenging part.
A certified arborist can evaluate factors such as:
- Tree species
- Age and maturity
- Soil conditions
- Nutrient deficiencies
- Signs of insects or disease
- Watering practices
- Environmental stress
Sometimes the best recommendation is to fertilize. Other times, the better solution is adjusting irrigation, improving soil conditions, pruning, or treating pests and diseases. Healthy trees don’t always need fertilizer, but they always benefit from the right care at the right time.
For homeowners looking for a proactive approach to tree health, Front Range Arborists offers TreeGuard, a comprehensive tree care program designed to help identify and address problems before they become major issues. TreeGuard includes regular tree health evaluations, monitoring for insects and diseases, and customized recommendations such as fertilization, pruning, or other treatments based on your trees’ specific needs.