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Is Fertilizing Your Trees Actually Bad For Them?

Lucas Capachin - Board Certified Master Arborist

Is fertilizing your trees actually bad for them?
Short answer: in Colorado, yes usually

I recently took a call from a fellow tree company owner. He was asking if I offer tree fertilization and offered me a commission on any sales I referred to his company. I told him I do not and then asked what the benefits of fertilizing trees were. I was curious if he knew what he was selling. The answer confirmed my suspicions that he doesn't actually understand the effects of fertilizing trees in Colorado. I decided to take 30-45 minutes out of my day to explain why fertilization is bad for trees in Colorado 99% of the time, and I also gave him alternative strategies that actually can help trees.

 I've often heard about fertilization from other companies as a great way to get more money per tree and a source of recurring revenue. Just upsell the customer on fertilization because “its good for the tree." I then decided to do some research, which confirmed my belief that in Colorado fertilizing trees generally leads to more adverse than positive effects. In this article I’ll go through the primary impacts that fertilization has on trees and why most of them are not beneficial for trees in Colorado. 

Micro vs Macro 

The first thing we need to do is make a distinction between micronutrient and macronutrient fertilization. Micronutrients are needed in very small amounts. Usually deficits in micronutrients leads to chlorosis, which should be addressed and you can read our article on that for more information. Macro nutrients are needed in much greater amounts and are what most companies mean when they refer to fertilization. Macronutrient fertilizers are primarily nitrogen, phosphorus, and potassium (N,P,K). In the sources linked below you will see them refer to nitrogen fertilization, which is the primary chemical in most of these fertilizers.



What effects do macronutrient fertilizers have?


Below are quotes published by universities or the national park service - because they say it best.


Trees will “shift more resources into canopy growth and correspondingly less into root growth...Fertilization may push canopy growth that a limited rooting system cannot support in summer heat and wind, leading to early decline”. (CSU)


Fertilizing trees will “stimulate excessive leaf growth at the expense of root growth, leaving the new tree susceptible to drought stress”. (Mizzou -University of Missouri)


“Too much nitrogen can cause trees to grow branches rather than roots, store less food, become structurally weaker, and more prone to insect attack.” (NPS)


“Nitrogen fertilization can produce long succulent shoots that are attractive to various sucking insect pests, like aphids.” (UMD)



Colorado specific tree issues that fertilizer makes worse


The majority of trees that need to be cut down in Colorado are those that have suffered from drought stress or storm damage. There are a few tree specific diseases that are problems, such as emerald ash borer for ashes or chlorosis in maples, but other than that the biggest reason why our trees have issues is storm damage or drought stress. This relates to fertilization because both of those issues are made worse by having more canopy surface area. Fertilization makes leaves larger and increases the amount of leaves. More leaves and larger leaves increase the water needs of a tree, making it so that the roots will have a harder time providing adequate water to the tree and resulting in more pest problems. Denser and larger canopies also have more surface area for wind and snow to impart force onto, making storm damage more likely. The bottom line is that the primary effects of fertilization actually make your trees less likely to have a long happy life. Fertilization will also make it so you will have to prune trees more often, which is another reason why I suspect many tree companies push it. 


So what can you do? 


Watering is the most important thing in Colorado, if you look at open spaces you can see what grows in Colorado naturally. You will see tall grass and cottonwoods near sources of water and that’s about it. Without supplemental watering trees would not be here. This year was the driest on record so read our other article on watering to understand what you can do.


FoothillsArborists.com/Watering


Application of mulch or other organic material will help trees and the soil be as healthy as possible. Organic matter leads to healthier soils that will lead to healthier trees. Even if you have a lawn there is a product that I really like. It can be used as a natural fertilizer and pretty much every nursery or garden store in the area stocks it. It’s called “Soil Pep” and it’s partially composted conifer bark. The best thing about it is that it’s particle size is small enough that you can spread it on grass and then rake it in and it will disappear under the grass. One major reason companies give for pushing fertilization is because we collect grass clippings and rake up leaves. This practice does interrupt natural nutrient cycling, but spreading chemical fertilizers isn’t the solution. If you can’t let your grass and leaves decompose, naturally fertilizing your yard by introducing plant material back is the best option.The pine bark in Soil Pep is also slightly acidic, which will help to adjust the pH of the alkaline clay soil that covers essentially all prairies of Colorado. 


A new product that does pretty much the opposite of fertilizer is growth regulator, and I have been using it increasingly more as it provides so many benefits. The growth regulator inhibits production of a hormone produced in the roots of trees. This causes the tree to think that it needs to relocate resources towards root growth. This product is not always a good option, but especially in Colorado less canopy growth and more root growth is always a good thing. We have an article on these as well if you want more information.


FoothillsArborists.com/TGRs


I’ll finish off this article by saying that there are some cases where fertilization can be beneficial, but it’s almost always not a good option. Soil tests and foliage tests can be used to determine if there is actually a deficit of some sort that needs to be addressed. But the majority of issues trees have in Colorado can be solved by just making sure they are watered properly. With the exception of chlorosis, trees in Colorado rarely need supplemental nutrients.


I would always recommend looking for information on these topics from universities and similar institutions such as CSU extension, rather than from a tree services website. There is too much financial motivation for tree services to provide inaccurate information, so anything you get from their articles or from what they say should be verified before you believe it. The reality is that most tree companies providing fertilization haven’t taken the time to understand the impact that fertilization has on trees and they start and end their research on “best fertilizer for trees”. This differs from my approach, which involves pursuing education and research to make sure what I’m offering is actually good for the trees and good for achieving the goals of the customer. 

Trees I Recommend Planting

By Lucas Capachin - Board Certified Master Arborist • October 16, 2024
Watering newly planted trees The key is to make sure the tree gets enough water, but not too much. Typically you want to water a newly planted tree every 3-5 days, depending on how hot it is and how much rain there has been recently. The best way to know if you should water more or less is to just stick a finger into the dirt by the base of the tree before watering, if it feels very dry water more often, if its still very wet, water less often. Whoever sells the tree will have more advice on exactly how much each species needs and how often, but its very important to make sure its not getting overwatered or under watered. Every planting site is different, some have more runoff from gutter or other hard surfaces like concrete patios, others have better drainage. You just need to monitor the soil and get a feel for the specific tree's water needs in that spot. Large trees Autumn blaze maple - Maples can be prone to chlorosis but it can be managed. Its pretty much the only tree on the list that grows fast at all. Catalpa - I have almost never cut out a broken branch from one and don’t know if I've ever seen a dead one. Some people object to the giant seed pods. Kentucky coffeetree - very rarely break in storms. Burr oak - very rarely break in storms. Honeylocust - very rarely break in storms, thyronectria canker can be an issue on larger trees but generally isn’t an issue for the first 30-40 years Hackberry - Good tree, only issue is they are covered in hackberry nipple gall (funny name, I know). It doesn’t impact the health of tree at all, it's just an aesthetic issue. Small Smoke bush - Royal purple variety looks great and is a nice contrast in green yards. Flowers look cool. Redbud - Beautiful pink flowers in the spring. Prefers shaded areas, no direct sun all day. Good to plant with a larger tree that will give it shade for 1/2-1/3 of the day, relatively fast growing for being a small tree. Japanese tree lilac - Yellow flowers in the spring, I've never seen a dead one so they clearly do well in Colorado. Service berry - Native shrub to Colorado with nice spring flowers. Small berries attract birds in fall, awesome fall color. Amur maple - Grows fast (almost too fast sometimes) Awesome fall color. Peach - Don’t plant if you don’t like peaches! (If you don’t like them your neighbors probably do though). Grows very fast, in years where they produce fruit it can be excessive amount and can even cause damage to the tree. The weight of fruit will break branches as they ripen so be aware in the fall. Grows very fast for a small tree. For those looking for a really thorough discussion on trees from CSU extension, this link is a list of literally every tree in Colorado. It's got far more information than almost anyone needs but it does rank trees as recommended or not. https://planttalk.colostate.edu/wp-content/uploads/2024/04/2024-Front-Range-Tree-List.pdf

Early Fall Color Means Your Tree Has A Problem

By Lucas Capachin - Board Certified Master Arborist • October 16, 2024
This linden isn’t getting enough water. Lindens typically need much more water than trees of other species in the same area. You can often identify the issue earlier in the year when you see browning along the edges of the leaf during the warmer months. Watering during the winter isn't always needed but it can give a tree like this the best chance to recover. Watering Article Winter Watering Article

Summer Watering

By Lucas Capachin- Board Certified Master Arborist • July 11, 2024
Do My Trees Need Water? A tree's roots will generally spread at least 2-3 times the width of the canopy of the tree. In Colorado landscapes that means that much of the roots exist in areas that do not get enough water. Watering should be focused within the drip line of the tree. The drip line is the entire circumference of the canopy of the tree where water would drip from the leaves and branches. Even if there is a garden bed that has some irrigation for plants planted in that area it is usually not enough for the trees, these systems are designed to give just enough water for the plants there and they rarely supply enough water for the tree. Any area within the drip line of the tree that has soil underneath should get supplemental watering including rock beds and even your deck. Yes, your neighbors may look at you strange when you are watering your deck but that is a small price to pay for healthy trees.

Apple Trees

By Lucas Capachin - Board Certified Master Arborist • March 9, 2023
The key is to prune the trees before the snows come as this will mitigate the damage from heavy snow. Although pruning cannot guarantee that there won’t be any broken branches - it certainly does help. The issue is apples can only be pruned while there are no leaves on them because ((fireblight link)) can spread if they are pruned at the wrong time of year. Don’t forget to learn to identify fireblight and manage it early to prevent serious issues, our article has many photos and information to help. So feel free to contact us about your apples if you want to have them pruned before they start to leaf out. If you don’t want to prune them don’t forget to knock the snow off if we get a storm. If you want to try your hand at pruning youself this video gives a bunch of good advice!

Early Spring

By Lucas Capachin - Board Certified Master Arborist • March 22, 2022
Frost Damage Trees prepare for the cold winter months in many ways to prevent damage to their cells. If trees start to reverse this process too early, a hard freeze can cause ice crystals to form in their cells. This can cause branch tips to die, or sometimes even entire branches or trees will die from frost damage. More information on frost damage can be found in a previous article I wrote found here . Trees that typically suffer the most from these cold snaps are stone fruit trees (all trees in the genus Prunus) and elms. A few years ago many pears were also severely affected. Unfortunately there isn’t much that can be done to prevent frost damage, although shaking snow off of trees and preventing the branches from dropping in temperature too much can help.
Trees, Pesticides, and Pollinators

Trees, Pesticides, and Pollinators

By Lucas Capachin - Board Certified Master Arborist • March 20, 2021
Pesticides are often overused or misused and the negative consequences can be devastating to non target organisms
Tree Growth Regulators and their surprising benefits to tree health

Tree Growth Regulators and Their Surprising Benefits to Tree Health

By Lucas Capachin - Board Certified Master Arborist • March 12, 2021
From increased drought tolerance to decreased need for pruning PBZ is a valuable tool for arborists

The Truth About Winter Pruning

By Lucas Capachin - Board Certified Master Arborist • March 4, 2021
I often hear “winter is the best time to prune trees”. This, like many over generalizations is not always correct. When to prune is often dependent upon the goal you wish to achieve for your trees. We are actually fortunate in Colorado - most trees can be pruned any time of the year. With winter pruning we can see the structure of the tree without it’s leaves. For trees that are weak, pruning in winter helps them retain vigor through the growing season. Preventing storm damage In Colorado one of the major causes of tree issues is from our severe storms. We can have both high winds and heavy, unseasonable snow here. A more structurally sound, recently pruned tree canopy will eliminate weak limbs and decrease the surface area for the snow and wind to act upon. Thereby reducing the forces imparted on the tree and limiting the chance of storm damage. Disease prevention There is one group of trees that is best pruned in winter - species which are susceptible to fire blight. In Colorado there are four main trees that are susceptible, they are mountain ash (Sorbus), apples/crabapples (Malus), hawthorn (Crataegus), and pear (Pyrus). Creating new wounds from pruning during the growing season will increase the chances of it contracting fire blight, so that’s why we try to prune these trees in winter. Here is a link to our article on fire blight. FoothillsArborists.com/Fire-blight Resource allocation In spring trees use a large amount of water and nutrients to create new leaves. Trees grow fastest in early spring, this can be seen by looking at growth rings, the larger lighter colored rings are formed in early spring, the darker skinny sections in the later months of the growing season. Spring is also when new shoots appear and growth occurs. If pruning is done before this flush of growth it means that the tree will push out more new growth on the remaining branches. It will fill out quicker than if it were pruned after the flush of new spring growth. If done later in the season there will be less growth in response to the old cuts. This will decrease the new growth on the tree. Therefore one drawback of winter pruning is that pruning may need to be done again sooner as new spring growth will occur more quickly than with later pruning. If a tree is severely stressed it may be advisable to only prune in late winter to minimize stress and retain it’s resources for new growth in the spring on the remaining branches. Wound compartmentalization Wounds can come from pruning, nature’s forces breaking branches, or pests and disease. Trees do not heal wounds like we do when a cut in our skin is healed with new tissue growth replacing the damaged tissue. They can only seal off the exposed inner wood to prevent damage to nearby healthy tissue. The tree forms barriers to prevent wood-decay fungus from progressing into the heartwood of the tree. If large cuts are needed, for instance to remove a large broken limb, pruning in the winter will give it a head start on closing that wound. Pruning before the growing season will give your tree a head start on covering up the wounds. Although this is one of the main reasons given for winter pruning, it usually will not make a significant difference if pruning is done right - as only small cuts should generally be made and these are much easier for the tree to seal off. When people ask me “When is the best time to prune my trees?” I usually respond by saying “Before they are damaged in a storm”. The extreme weather we get in Colorado is hard on trees, and very few of the trees we plant evolved in this climate. The trees we have brought to Colorado and nurture here are not prepared for the extreme wind or early/late snowstorms when they are leafed out. Our main goal with pruning is to prevent this sort of damage from occurring. In the best case, loss of major branches leaves a less attractive tree. In the worst case, it can lead to the decline and death of the entire tree. Fortunately most trees can be pruned any time of the year, and that is why I rarely recommend delaying pruning. You never know when the next tree damaging storm will hit. There is nothing worse than showing up to prune a tree and seeing that it has been recently damaged by a storm, so it is almost always better to prune as soon as possible.
Winter watering

Winter watering

By Lucas Capachin - Board Certified Master Arborist • February 25, 2021
Even while dormant trees continue to grow and respire so its important to give them the water they need.

Why is My Tree Yellow?

By Lucas Capachin - Board Certified Master Arborist • July 29, 2020
Chlorosis is a general term for foliage that is not producing adequate chlorophyll. This leads to a yellow or pale appearance. The lack of chlorophyll means that the plant's most important process is being interrupted. If not corrected, chlorosis will lead to dieback of branches and eventually the entire tree. Chlorosis generally is caused by a lack of iron, but it may also be caused by a deficit in other micronutrients as well. Iron is present in most Colorado soils but due to low pH it cannot be absorbed by the trees. How Can I Identify Chlorosis in My Trees? Chlorosis is a major problem with maple and oak trees in Colorado, although many other species can also be affected. The signs of chlorosis are any leaves or branches in the tree with inconsistent color. Parts of the tree may appear lime green while other parts are a deeper green. In advanced stages leaves will turn yellow and almost white at times. If you look at a leaf and the veins are darker green but the rest of the leaf is lighter in color - that is chlorosis.
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