The Summer of Triclopyr: Mounting evidence for off-target damage

There have been a significant number of questions and problems regarding triclopyr that have come across my desk this growing season. Several of these problems have come through the Purdue Plant and Pest Diagnostic Laboratory (https://ag.purdue.edu/department/btny/ppdl/index.html) from landscapers, nurseries, and property managers.

General Information about Triclopyr

Triclopyr is a selective systemic herbicide belonging to the pyridine class. It is primarily used to manage woody plants, vines, and some broadleaf weeds. Unlike glyphosate, a broad-spectrum herbicide, triclopyr specifically targets the growth hormones in dicotyledonous plants, making it effective in controlling species that are difficult to manage with other herbicides. It works by mimicking plant hormones, disrupting normal growth processes, and leading to the death of the treated plant (Fig. 1).

What Triclopyr Controls

Triclopyr is utilized in various settings to control unwanted vegetation. It is effective against species such as:

• Woody Plants: Includes trees like tree of heaven and callery pear and shrubs like autumn olive and honeysuckle (Fig. 2).
• Vines: Particularly those that can outcompete native flora, such as kudzu or English ivy (Fig. 3).
• Broadleaf Weeds: Some herbaceous plants that pose ecological or agricultural problems.

Where and When Triclopyr is Used

Triclopyr is applied in a range of environments, including:

• Forestry: To clear invasive species and manage forest regeneration.
• Agriculture: To control weeds and shrubs that interfere with crop production.
• Aquatic Areas: Formulations that are safe for aquatic environments help manage invasive species near water bodies.
• Rights-of-Way: To maintain clear paths and prevent the spread of invasive plants.

Timing of application is crucial. Triclopyr is most effective when applied during the growing season when plants are actively taking up nutrients and can transport the herbicide throughout their system.

Movement in the Plant

Triclopyr is absorbed through foliage and roots and is translocated systematically within the plant. Once inside, it moves upward through the xylem and downward through the phloem (ambimobile). This systemic movement allows triclopyr to affect not just the treated areas but also parts of the plant that are not directly sprayed. This attribute makes it effective for managing large, established plants, specifically through hack and squirt treatments (Fig. 4).

Phytotoxicity to Off-Target Plants

Many applicators have experienced off-target damage when they report minimal drift. Most often, this damage has been primarily thought to be either volatilization or root exudation, (or via root grafts) (Figs. 5 and 6).

Volatilization Potential

Triclopyr has a relatively high volatility compared to some other herbicides, especially the ester formulation. This characteristic can increase the risk of volatility when certain environmental factors are present, such as increased temperatures and lower humidity.

Off-Target Damage Due to Root Exudation

A significant concern with triclopyr is its potential for damage through root exudation but previously was only speculation. Recent research has shown that off-target damage can occur via root exudation (Oberweger et.al., 2023 and Graziano et.al., 2022). These studies found that triclopyr can be excreted from the roots of treated plants into the soil, where it can affect nearby vegetation (Figure 7).

Root exudation may lead to:

• Soil Contamination: Residual triclopyr in the soil can impact plant growth and health in the vicinity.
• Non-Target Plant Damage: Plants growing near the treated area might suffer from root absorption of triclopyr, leading to stunted growth or even death.

Studies have shown that while triclopyr’s mobility in soil is relatively low, persistence in the rhizosphere can still impact sensitive species. There is still some speculation that damage could occur via root grafting and/or mycorrhizae between plants, but research is still unclear at this time.

Symptoms of Triclopyr Damage

There are a few symptoms that occur when triclopyr damage is suspected, which are similar to other growth regulator herbicides. Symptoms can be seen on small weeds in the sun in as little as two hours, while symptoms on larger plants or in shade can take 48 hours or more (Figs. 8a, 8b, and 9).

• Epinasty: Twisting, curling, and cupping of leaves and petioles are a common symptom of triclopyr damage (Figure 10).

• Strapping: Strapping is when a leaf elongates and decreases in width. The leaves can also have leaf margins that are significantly different from a non-affected leaf (Figure 11).

• Water-soaked appearance: Leaves can become darkened and wilted (Figure 12).

• Necrosis: Blackening (death) of leaves is usually first seen in new growth.
• Spiraling Damage on Evergreens: Due to the differences in vascular tissue between deciduous and evergreens, spiraling damage can be found on evergreens with systemic herbicides (Figure 13).

Conclusion

Triclopyr is a valuable tool for managing invasive and woody plants, with its systemic action making it effective for large and stubborn species. However, its use requires careful management to mitigate potential risks to non-target plants and ecosystems. Understanding its movement within plants, potential for volatilization, and impacts through root exudation can help minimize unintended damage and ensuring that its benefits are maximized while its drawbacks are minimized.

References:

1. Graziano G, Tomco P, Seefeldt S, Mulder CPH, Redman Z (2022). Herbicides in unexpected places: non-target impacts from tree root exudation of aminopyralid and triclopyr following basal bark treatments of invasive chokecherry (Prunus padus) in Alaska. Weed Sci. 70: 706–714.doi: 10.1017/wsc.2022.61
2. Oberweger CA, Enloe SF, Wilson PC, Prince CM, Sperry BP, Hinz FO (2024) Physical and physiological pathways of off target triclopyr movement and associated nontarget injury following basal bark application. Weed Sci. 72: 68–75. doi: 10.1017/wsc.2023.62