Herbicide Alternatives Research - Executive Office of Transportation

included side-by-side comparisons, effects of increased concentrations, effects of repeat
applications, as well as effects of preparation of test plots by mowing.
• Clove oil or citric acid and acetic acid formulations had little effectiveness throughout the
season, weed masses in these plots approached or equaled that of untreated plots. As these
herbicides are defoliants, they do not kill crowns and therefore do not prevent plants from
sending up new growth.
• Pelargonic acid showed strong suppression soon after application, but its effectiveness
dissipated by the fall. As with the other acids, this was attributed to inability to kill crowns
or seedlings emerging after application.
• Clove oil, pelargonic acid, and glyphosate were tested on sumac and vines (grape and
poison ivy). All treatments demonstrated effective suppression of sumac by the end of the
season. Clove oil generally was unsatisfactory on the vines by the end of the season.
Pelargonic acid initially appeared to be effective on the vines, yet its effectiveness started to
diminish by the end of the season. By comparison, the conventional herbicide, glyphosate
was initially ineffective but very effective by the end of the season.
• Doubling the concentrations of clove oil, pelargonic acid and limonene demonstrated
negligible benefit relative to the recommended concentration rate.
• Plots mowed prior to treatment demonstrated no additional suppression relative to the unmowed
plots. In fact, mowing seemed to suppress, rather than enhance, the efficacy of the
herbicides. The investigators speculated that the longer shoots in the un-mowed plots may
have provided more area of plant exposure to the herbicides, thus increasing their efficacy.
• Repeat applications (two or three applications) of alternative herbicide over the growing
season were necessary to achieve vegetation control comparable to that achieved by a single
application of conventional herbicide.
• Corn gluten meal applied at varying rates on plots prepared with a weed torch or glyphosate
demonstrated little suppressing effect on weed growth by itself. Pre-burned plots showed
vegetation increasing with increased applications of the meal. Plots pretreated with
glyphosate showed continued control of vegetation throughout the season, but the corn
gluten meal did not suppress growth. Weed mass from the corn gluten meal plots was the
largest of any treatment, and plants in these plots were growing vigorously at the final date
with few signs of senescence, most likely due to the fact that corn gluten meal is a nitrogen
fertilizer.
Mechanical Methods
• Mulches (bark, woodchip) applied at 2 to 3 inch thickness after preparation (burning was
used as preparation for this research) gave season-long control of vegetation and had the
least end-of-season weed mass of all of the treatments. Mulches did not need to be reapplied
for the second year of research and were considered to have controlled vegetation
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