Abstract:
Japanese knotweed is an invasive perennial shrub that dominates riparian
ecosystems. Effective management techniques are currently limited to repeated annual
herbicide applications and there is little science-based information about which control
tactics result in the greatest management success. Restoration of invaded sites to a
functioning riparian plant community is needed to prevent re-infestation of Japanese
knotweed or other invasive weed species. Field and greenhouse experiments were
initiated in fall of 2007 to generate this information on Japanese knotweed management.
A greenhouse experiment was conducted to evaluate the efficacy of the experimental
herbicide aminocyclopyrachlor methyl ester for Japanese knotweed control. Treated
plants were unable to produce new shoots from underground rhizomes indicating that
aminocyclopyrachlor is an effective control. This level of Japanese knotweed suppression
is comparable to imazapyr and surpasses the level of control that glyphosate provided
under greenhouse conditions. A field experiment was initiated in the Nehalem River
watershed in western Oregon to evaluate the integration of chemical weed management
with restoration of the site to a diverse native grass plant community. We documented
that native grasses could be established at this site, but the long-term survival of these grasses was poor. This result indicates that methods for simultaneous chemical control of
invasive knotweed and restoration of sites need to be investigated further. The most
effective chemical treatment in terms of visual injury and reduced Japanese knotweed
biomass and cost of application was glyphosate applied at a rate of 4.21 kg ae/ha for a
cost of $160/ha. A secondary experiment evaluating herbicide tank mixtures of
glyphosate with imazapyr, imazapyr with aminopyralid, and triclopyr with 2,4-D was
conducted at the same site. The most effective chemical treatment in terms of Japanese
knotweed control and cost of application for the tank mixture experiment was imazapyr
with aminopyralid applied at rates of 1.12 kg ae/ha (imazapyr) and 0.12 kg ae/ha (aminopyralid) for a cost of $346/ha.
