Herbicide Effects on Density and Biomass of Russian Knapweed and
Associated Plant Species
Stephen M. Laufenberg and Roger L. Sheley
Introduction
Non-native invasive plants can
reduce wildlife habitat, increase
soil erosion and stream sedimentation, and decrease plant species
diversity. One such invasive species
of concern is Russian knapweed
(Acroptilon repens), a rhizomatous
perennial forb that is difficult to
control and considered to be the
most persistent of the knapweeds.
Infestations of Russian knapweed
can displace vegetation through a
combination of competition and
allelopathy, which reduces the
health and productivity of the land.
It has become clear that controlling
Russian knapweed is paramount
to recovering and maintaining the
plant communities that it infests.
Previous research involving herbicide suppression of Russian
knapweed has focused primarily on
controlling the weed, with limited
regard to the effects on the existing plant community. The objective
of this study was to determine the
influence of clopyralid plus 2,4-D
(Curtail®), glyphosate (Roundup®),
and fosamine (Krenite®), at different application rates and timings, on
Russian knapweed and associated
existing plant groups, based on species density and biomass.
(3 herbicides, 3 rates of application, 3 application timings, untreated
control) were applied June through
August 2000. The herbicides (clopyralid plus 2,4-D, glyphosate, and
fosamine) were applied in June
(spring rosette stage of Russian
knapweed), July (bud to bloom
stage), and August (flowering stage).
Low, medium, and high rates of each
herbicide were applied at each application date. Density was recorded
for all existing plant species and
Russian knapweed during June and
August of 2001 and 2002. Biomass
of all species and Russian knapweed
was collected in August 2001 and
2002 using a 4.74-ft2 hoop randomly
placed once within each plot.
Results and Discussion
Of the herbicides tested in this
study, clopyralid plus 2,4-D provided the best control of Russian knapweed. Russian knapweed biomass
was reduced from 1,116 lb/acre to
about 223 lb/acre using clopyralid
plus 2,4-D, irrespective of rate or
timing of application (Fig. 1). Also,
density of Russian knapweed was
reduced by about 70 percent for
clopyralid plus 2,4-D compared
with the untreated control (Fig.
2). Grass density and biomass was
either maintained (nonnative grass
understory) or increased (native
grass understory) using clopyralid
at medium or high rates. Neither
glyphosate nor fosamine provided
Experimental Protocol
Two study sites were selected
in north-central Montana about
170 miles north of Lewiston,
Montana on the Charles M. Russell
National Wildlife Refuge along the
Missouri River riparian corridor.
In a randomized complete block
design at both sites, 28 treatments
Figure 1. The effect of application timing by herbicide on Russian knapweed
biomass. Control is no herbicide treatment, Clop. + 2,4-D is clopyralid plus 2,4-D
(Curtail®), glyphosate (Roundup®), and fosamine (Krenite®).
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substantial Russian knapweed
control or increases in grasses. No
herbicides increased native forbs,
which are particularly important to
the sustainability of the plant community.
Management Implications
Only increases in grasses were
detected in this study, which demonstrates that the rehabilitation of
the plant community’s structure was
not successful. Without sufficient
community structure and competition from other critical plant groups,
Russian knapweed will most likely
recover from suppression treatments. Therefore, herbicides alone
are inadequate for the restoration
or rehabilitation of desirable plant
communities infested with Russian
knapweed. Although revegetation
is expensive and has a high risk
of failure, this study indicates that
herbicides must be combined with
revegetation in areas lacking a diverse mixture of species capable of
occupying the newly opened niches.
Figure 2. The effect of herbicide on Russian knapweed density at four sampling
dates. Control is no herbicide treatment, Clop. + 2,4-D is clopyralid plus 2,4-D
(Curtail®), glyphosate (Roundup®), and fosamine (Krenite®).
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