How to Apply Herbicides to Avoid Damage to Trees

Herbicides are powerful tools in managing unwanted vegetation, controlling invasive species, and maintaining healthy landscapes. However, when applied improperly, herbicides can cause unintended damage to desirable plants, particularly trees. Trees are vital components of ecosystems and urban environments, providing shade, improving air quality, supporting wildlife, and enhancing aesthetic appeal. Protecting trees from herbicide injury is essential for sustaining these benefits.

This article explains how to apply herbicides safely around trees, minimizing the risk of damage while effectively controlling unwanted plants.

Understanding Herbicide Types and Their Effects on Trees

Before applying herbicides near trees, it’s crucial to understand the types of herbicides available and how they interact with tree physiology.

Selective vs. Non-Selective Herbicides

Selective Herbicides target specific types of plants. For example, some act only on broadleaf weeds or grasses. These are generally safer near trees if used according to label instructions.
Non-Selective Herbicides kill or injure all plants they contact. Glyphosate and glufosinate are common examples. While effective for clearing large areas, they can easily harm or kill trees if misapplied.

Systemic vs. Contact Herbicides

Systemic Herbicides are absorbed by leaves or roots and transported throughout the plant’s vascular system, killing it entirely. This characteristic makes them effective against perennial weeds but poses a risk to trees if drift or root uptake occurs.
Contact Herbicides kill plant tissue they touch but do not move within the plant. Damage is often limited to treated areas but can still harm tree foliage or bark if contacted.

Herbicide Modes of Action Affecting Trees

Different herbicides affect various physiological processes:

Growth regulators (e.g., 2,4-D) disrupt hormone balance; broadleaf trees may be vulnerable.
Photosynthesis inhibitors affect energy production; usually more harmful to annual weeds but can impact tree seedlings.

Understanding the herbicide’s mode of action helps predict potential risks to nearby trees.

Common Ways Trees Can Be Damaged by Herbicides

Trees can be harmed through several pathways during herbicide application:

Foliar Contact

Accidental spray drift onto leaves or bark can cause leaf burn, discoloration, defoliation, or dieback. Young trees with thin bark are especially vulnerable.

Root Uptake

Herbicides applied to soil can be absorbed through roots by both target weeds and nearby trees. This is more common when herbicides persist in soil or when tree roots extend into treated zones.

Translocation via Grafting

Some tree species naturally graft their roots together underground. If one tree absorbs a systemic herbicide, it may transfer chemicals to adjacent trees through these grafts.

Bark Absorption

Herbicide contact with thin or damaged bark can lead to absorption and injury to cambium tissues vital for nutrient transport.

Understanding these damage routes enables better planning and precautions during herbicide use.

Best Practices for Applying Herbicides Near Trees

To avoid harming trees when applying herbicides nearby, follow a comprehensive set of best practices:

1. Identify Tree Species and Sensitivities

Different tree species have varied sensitivities to specific herbicides. For example:

Many broadleaf trees are sensitive to growth regulator herbicides such as 2,4-D.
Conifers are often more tolerant but still vulnerable at high concentrations.
Young seedlings and recently planted trees have less chemical tolerance than mature trees.

Consult extension services or arborists for species-specific recommendations before application.

2. Read and Follow Label Instructions Carefully

The herbicide label is the legal document that provides critical information on application rates, timing, protective equipment, mixing instructions, and restrictions related to proximity to desirable plants like trees.

Ignoring label directions increases risk of damage and legal liability.

3. Establish Buffer Zones Around Trees

Maintain an untreated buffer zone around tree trunks and critical root zones (generally the area beneath the canopy drip line). This helps prevent accidental root uptake or direct foliar contact.

Buffer distances depend on:

Tree size
Herbicide mobility
Application method
Soil type

Common buffer distances range from 5 to 20 feet but verify with label instructions or local regulations.

4. Use Targeted Application Techniques

Carefully control where and how the herbicide is applied:

Spot Treatment: Apply small quantities directly onto weeds with a hand sprayer or wipe-on applicator rather than broadcast spraying.
Shielded Sprayers: Use equipment with shields or wipers that minimize spray drift onto non-target plants including tree foliage.
Low Pressure/Coarse Droplets: Adjust spray nozzles for coarse droplets that reduce drift potential.
Avoid Spraying on Windy Days: Wind increases drift risk dramatically; apply only under calm conditions (wind speeds below 5 mph).

5. Avoid Soil Applications Near Trees Unless Approved

Soil-applied residual herbicides can leach into root zones and cause damage. Only use soil treatments near trees if specified safe on the label for that purpose.

For non-root-zone applications like mulch beds immediately surrounding trunks, avoid incorporation into soil unless directed otherwise.

6. Time Applications Appropriately

Applying herbicides during periods when trees are less active reduces risk:

Avoid spraying during active leaf-out or flowering when foliage is more susceptible.
Late summer or early fall applications may be safer for some species after growth slows.
Avoid applications before heavy rainfall that could wash chemicals into root zones.

7. Consider Alternative Weed Control Methods Near Trees

Reducing reliance on chemical control near trees can further minimize risk:

Mulching around bases suppresses weed growth physically without chemicals.
Manual removal of weeds reduces chemical exposure.
Use landscape fabric barriers under mulch beds.
Employ grazing animals in rural settings for vegetation control.

Integrated weed management combining these methods with carefully targeted herbicide use provides safer outcomes for trees.

Monitoring and Managing Potential Herbicide Injury

Despite precautions, sometimes unintended injury occurs. Early detection and intervention improve tree survival chances.

Signs of Herbicide Damage in Trees

Look for symptoms such as:

Leaf curling, cupping, or distortion
Chlorosis (yellowing)
Necrosis (brown spots or edges)
Premature leaf drop
Twig dieback
Reduced growth rate
Resin exudation on bark (in conifers)

These symptoms may appear days to weeks after exposure depending on the chemical involved.

Steps to Take if Injury Suspected

Identify Cause: Confirm herbicide injury versus disease or environmental stress through observation and consultation with arborists or extension agents.
Reduce Additional Stress: Provide adequate water during dry periods; avoid pruning damaged branches immediately unless necessary.
Avoid Further Chemical Applications: Halt any further spraying until recovery is confirmed.
Apply Fertilization Judiciously: Fertilize if soil tests indicate deficiency but avoid excessive fertilization that stresses damaged roots.
Consider Protective Measures: In severe cases, physical barriers might be installed around base roots before future applications.
Document Incidents: Record application details including date, product used, weather conditions — useful for future reference or claims if applicable.

Conclusion

Applying herbicides near trees requires knowledge about chemical properties, careful planning, precise application techniques, and ongoing vigilance for potential injury signs. By understanding which chemicals pose risks to specific tree species and following recommended safety practices such as maintaining buffer zones, using targeted applications, avoiding spraying in windy conditions, and timing treatments appropriately, landowners and landscapers can effectively control weeds without compromising tree health.

Integrating cultural methods like mulching and manual weed removal further enhances protection efforts. Should injury occur despite precautions, prompt identification and responsive care increase the likelihood of recovery and long-term tree vitality.

Protecting our valuable trees from inadvertent herbicide damage safeguards not only individual plants but also broader ecological functions and community well-being supported by healthy urban forests and natural landscapes. With conscientious application practices grounded in science and best management principles, effective weed control harmonizes with sustainable tree stewardship.