Most homeowners assume that fertilizers and pesticides are always beneficial when it comes to maintaining a healthy landscape. After all, these products are designed to promote growth, eliminate pests, and improve the appearance of lawns and gardens.

But when used incorrectly, or even when used properly but too close to trees, these same chemicals can cause serious and sometimes long-lasting damage to trees.

In the St. Louis area, where many residential properties combine turf grass, ornamental plantings, and mature shade trees, it is especially important to understand how fertilizers and pesticides interact with tree health. What helps your lawn may not always help your trees.

Trees and Lawns Don’t Always Share the Same Needs

One of the most common issues in residential landscaping is treating the lawn and trees as a single system. While they share soil space, their nutritional and environmental needs are very different.

Lawns are often treated with:

High-nitrogen fertilizers for rapid green growth
Herbicides for weed control
Insecticides for turf pests

Trees, however, require:

Slow, deep nutrient absorption
Balanced soil chemistry
Stable root zone conditions
Minimal disturbance around the base

When lawn chemicals are applied heavily or repeatedly, trees can be unintentionally exposed to conditions they do not tolerate well.

Over-Fertilization Can Stress or Damage Trees

More fertilizer does not always mean healthier trees. In fact, excessive nitrogen is one of the most common causes of chemical stress in trees.

Over-fertilization can lead to:

Excessive, weak foliage growth
Reduced root development
Increased susceptibility to disease
Leaf burn or discoloration, similar to that caused by sustained, intense sunlight
Disruption of natural soil biology

Excessive nitrogen application can disrupt soil balance and negatively impact long-term plant health. Tree roots are especially sensitive to changes in soil chemistry, and repeated heavy fertilization in surrounding turf can gradually affect the entire root zone of a tree, and not always in a positive manner.

Herbicides and “Weed and Feed” Products Can Drift or Absorb

Another major risk comes from herbicides used to control weeds in lawns. These products are often absorbed through leaves or roots and can affect nearby trees if not applied carefully.

Even when applied to grass, chemicals can:

Drift onto tree leaves during spraying
Wash into the root zone during rainfall or irrigation
Be absorbed through shallow roots near the soil surface

Young trees are particularly vulnerable, but even mature trees can show signs of stress after repeated exposure.

Common symptoms include:

Twisted or curled leaves
Yellowing or scorched edges
Stunted new growth
Premature leaf drop

Pesticides Can Disrupt Beneficial Soil Life

Healthy soil is not just dirt, it is a living ecosystem filled with beneficial fungi, bacteria, and organisms that support tree health. Healthy soil packed with beneficial “ingredients” equals healthy trees.

Pesticides designed to eliminate insects in lawns can also unintentionally harm beneficial organisms in the soil around trees. This disruption can weaken a tree’s natural defenses and reduce its ability to absorb nutrients effectively.

Over time, this can make trees more vulnerable to:

Root disease
Fungal infections
Environmental stress such as drought or heat

Chemical Burn and Root Zone Damage

Trees can also suffer from what is commonly referred to as “chemical burn.” This occurs when fertilizer or pesticide concentrations are too strong or applied too close to a tree’s trunk or main root zone.

Because a large portion of a tree’s absorbing roots are located near the surface and extend well beyond the drip line, chemicals applied in surrounding turf can still affect them.

Damage is often slow to appear but can include:

Declining canopy density
Branch dieback
Reduced leaf size
Overall thinning of the tree

The Importance of Proper Application Around Trees

Proper lawn care near trees requires a more careful approach than open turf areas. Best practices include:

Avoiding direct application of chemicals near the trunk
Reducing fertilizer intensity in tree root zones
Using slow-release or tree-safe formulations when appropriate
Limiting herbicide use near established trees
Watering appropriately to prevent chemical concentration buildup

In many cases, simply adjusting application methods can significantly reduce risk to trees.

Signs Your Trees May Be Affected by Chemicals

Homeowners may not immediately connect tree decline to lawn treatments, but there are warning signs to watch for:

Sudden changes in leaf color or shape
Reduced canopy density over time
Unusual leaf curling or spotting
Decline that does not match seasonal expectations
Visible stress following lawn treatment applications

If these symptoms appear after fertilization or spraying, chemical exposure may be a contributing factor.

Professional Tree Care Helps Identify and Prevent Damage

When trees begin to show signs of stress, early diagnosis is critical. Professional arborists can evaluate soil conditions, inspect root zones, and determine whether chemical exposure is a factor.

At Hansen’s Tree Service, we help homeowners throughout the St. Louis area identify tree health issues, mitigate environmental stress, and protect valuable mature trees from long-term damage. Our certified arborists can evaluate the health of your trees, spot problems caused by fertilizers, herbicides and pesticides and help you take the correct steps necessary to protect your trees.

In short, proper tree care often includes not only trimming or removal, but also guidance on how surrounding landscape practices affect tree health.

Protecting Trees Means Understanding the Whole Landscape

Healthy trees depend on a balanced environment. While fertilizers and pesticides play a role in modern lawn care, they must be used thoughtfully and with awareness of their impact on surrounding vegetation.

With the right approach, it is possible to maintain both a healthy lawn and strong, thriving trees.

If you are concerned about the health of your trees or want guidance on safe landscaping practices, Hansen’s Tree Service is here to help. Contact us today for expert tree care, evaluation, and service throughout the St. Louis area.

Blog, Disease & Pest Prevention, Home Gardening Tips, Home Tree Care, Plant Health Care

Protect Your Trees: Fertilizers and Pesticides That Can Harm Trees