Herbicide Incision Point Injection for Woody Plant Control Saves Time and Resources

Invasive woody brush and trees such as albizia (Falcateria moluccana), Brazillian pepper (Schinus terebinthifolius), octopus umbrella tree (Schefflera actinophylla), and poison devil’s pepper (Rauvolfia vomitoria) have significant impacts on Pacific Island ecosystems. These exotic woody plants have extreme growth rates and form monotypic stands that displace other native and indigenous vegetation types.

Physical removal alone or in combination with herbicides is often used to control invasive woody vegetation in Hawai'i. The cost of physically removing trees ranges from $100 to more than $1,000 per tree, limiting the use of this method in most natural areas. Herbicide treatments on large trees are primarily basal trunk (bark) or frill cut applications with an herbicide diluted in an oil carrier. Although this method is effective when treating scattered invasive trees, it is impractical when working in large landscapes across difficult terrain characteristic of natural areas.

Studies were initiated by the University of Hawai’i Cooperative Extension Service Invasive Weed Management Program to find a method to improve individual plant herbicide application techniques and determine efficacy of various herbicides applied as undiluted formulations.


The herbicide incision point injection technique is a refinement of the more traditional ‘frill cut’ or ‘hack-and-squirt’ application methods by minimizing cuts to small incisions around the base of the tree at equidistant points, and delivering calibrated herbicide doses to these points. The basic concept of the injection method is to make a shallow cut at a 45˚ angle with a sharp implement (i.e., hatchet/machete) at the base of the trunk within three feet (1 meter) of the soil interface. The cut is made in the shape of a small trough, just deep enough to expose the cambium and be able to retain up to 1 ml of the undiluted herbicide formulation.

In these studies, four equidistant shallow cuts were placed around the circumference of a tree, with the exception of umbrella octopus tree, which received two incision points. The distance interval between cuts varied based on the size of the tree. The cut dimensions typically had a depth of about 2 inches (5 centimeters [cm]) and a width of four inches (10 cm).

Figure 1.  A full girdle, which is an improper technique for administering herbicide injections (A) compared to proper incision technique made with a cane knife on an angle to create a clean, intact trough where the herbicide can be retained and absorbed into the vascular system.

Milestone®, Garlon® 4 Ultra, glyphosate, and imazapyr were applied as undiluted herbicide at the rate of 0.5 ml per cut (total of 2 ml per tree). Applications were made with a metered draw-off syringe (adjustment range 0.5 to 2.0 cubic centimeters) originally designed for large animal vaccinations.

Visual observations were made of the tree leaf canopy. The canopy was visually divided into four quadrants, with each tree receiving four efficacy evaluation ratings (one for each quadrat). This rating system was designed for rapid and efficient characterization of herbicide efficacy.

Rating System

Canopy defoliation rating sheet


1 = no intact leaf canopy (100 percent defoliation)
2 = canopy defoliated from 50 to 99 percent (average 75 percent defoliation)
3 = canopy defoliation of less than 50 percent
4 = no observable symptoms of defoliation (0% defoliation)

Trees included in the study were similar in height and diameter within species, with three to four trees per herbicide treatment. Table 1 shows mean basal circumference for trees included in the study. Data were analyzed by analyses of variance (ANOVA) with quadrant data serving as a sample set within each treatment (n=4) and treatment sets blocked according to tree size when necessary.

Results for field efficacy trials

The injection technique used in these field trials demonstrated very rapid defoliation on target trees compared to other methods of application (i.e., foliar broadcast) possibly due to immediate herbicide uptake through the vascular system. Incision points placed equidistant around the circumference improved consistency of results by ensuring complete translocation throughout the tree canopy.

Table 1. Mean basal circumference of trees in study.

Data show that Milestone provided good to excellent defoliation of albizia at the rate of 0.5 ml of undiluted herbicide per 12 inches (30 cm) of circumference (Table 2). High levels of suppression/control and defoliation were also demonstrated for lead tree, Brazilian pepper, umbrella octopus tree and poison devil’s pepper at an application rate of 0.5 ml per four inches (10 cm) of circumference.

Table 2. Percent defoliation of invasive trees with application of undiluted herbicide by incision point injection method.

Garlon 4 Ultra was also effective on albizia in this study. However, in previous studies, Garlon 4 Ultra was determined to be ineffective on albizia, indicating the herbicide is inconsistent and typically requires higher application rates.


Results of the study show excellent efficacy of Milestone for these invasive tropical hardwood species. There are also several other woody species shown to be highly sensitive to Milestone®, which is proving to have greater utility in natural area weed management than other traditional herbicides. The syringe application device allows for a well-calibrated delivery system that is unique to other individual plant treatment methods. The effective Milestone rates identified in this study translates into treating over 100 large-class trees per acre without exceeding the maximum spot treatment rate of 14 fluid ounces per acre.

The herbicide injection technique has an established history in weed management and is the most efficient application method (and possibly the most hygienic) since the target plant takes up 100 percent of the herbicide. The injection technique is also a very surgical approach that promotes accurate calibration, and reduces applicator exposure with a metered, low-pressure milli-dose delivery system. Another important benefit for operations is weight reduction carried into the field. In Hawai’i and American Samoa, extreme topography is a major impediment to weed management, making diluted high-volume foliar applications impractical and unsafe. The simple surgical methodology to meter an exact dose with the smallest volume increment is a critical step to establishing new paradigms in natural area weed management. A quart of Milestone® (ca. 2 pounds of bulk weight) can extend from a full day to entire week of field operations eliminating hundreds of invasive tree targets and protecting thousands of acres.

As a disclaimer, this injection technique is designed for efficient and effective elimination of large woody species in natural areas and should not be used in the vicinity of public access and infrastructure where injury or damage may occur from decomposing dead snags. In these cases, a certified arborist may need to be consulted for contracting physical removal.

Additional details regarding this work can be found by contacting Dr. James Leary at leary@hawaii.edu.

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® Trademark of Dow Chemical Company ("Dow") or an affiliated company of Dow. Milestone is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Consult label for full details. State restrictions on the sale and use of Garlon 4 Ultra apply. Always read and follow label instructions.