Testing was conducted from fall 2001 through spring 2003 to evaluate the efficacy of several herbicides, timing of application (foliar vs. dormant), and an alternative application method (Burch Wet Blade) for control of Japanese knotweed along highway rights-of-way.
Foliar treatments with glyphosate (Rodeo) in late spring, fall, or in the fall following a single spring mowing did not produce apparent kill in the season in which it was applied but essentially no greenup of treated plants occurred in the following spring. Triclopyr amine (Garlon 3A) gave rapid top kill of leaves and stems within a week or two, which persisted until the end of the growing season. When evaluated during the following spring, there was no evidence in the triclopyr plots that any treatment had occurred; triclopyr-treated plots all were green and healthy. Clopyralid (Transline) and dicamba DGA (Vanquish) caused minor, but temporary leaf curling and slight discoloration; metsulfuron methyl (Escort) produced essentially no visible symptoms. All plots treated with clopyralid, dicamba DGA, or metsulfuron-methyl were green and healthy the following spring. Combinations (tank mixing) of herbicides did not improve knotweed control. When glyphosate was combined with triclopyr amine, control was poorer than with glyphosate alone. This suggested possible interference of triclopyr amine with glyphosate.
Glyphosate (Rodeo) was applied at 0.5 to 2 gal/acre, with a retreatment of half of each plot in the summer. At the end of the growing season, only minor yellowing of leaves was apparent, in spite of the treatment rate or whether or not re-treatment occurred. In the spring, there was essentially complete kill in all but the 0.5 gal/acre treatments, and there were no obvious differences between areas receiving a single treatment vs. two treatments.
Preliminary evaluation of triclopyr ester (Garlon 4) and imazapyr (Arsenal) were conducted at two sites at the fall treatment dates. Garlon 4 did not produce any obvious kill that carried over into the following growing season. There was slight, but not acceptable reduction in the following spring of shoot heights of knotweed that had received fall treatment with imazapyr.
The use of the Burch Wet Blade to apply glyphosate (Rodeo), triclopyr amine (Garlon 3A), or imazapyr (Arsenal) in the summer did not produce any lasting effects on the knotweed. Substantial regrowth occurred throughout all of the treated plots by the following spring.