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Krenite (fosamine ammonium) is a foliar herbicide that primarily targets woody plant species, however formal evaluations of its efficacy and potential for non-target impacts are scarce in the literature. The few tests that exist of Krenite focus primarily on its use in open environments, and the value of Krenite in controlling invasive understory shrubs is unclear. Here, we test the impact of Krenite on invasive Common Buckthorn ( Rhamnus cathartica L.) and co-occurring herbaceous plants across six forest sites in Minnesota, USA. Buckthorn treated with Krenite had a 95% mortality rate, indicating high efficacy of Krenite for use against buckthorn. Non-target impacts varied between forbs and graminoids such that forb cover was reduced by up to 85%, depending on site, whereas graminoid cover was sparse and impacts of Krenite on graminoids were unclear. These results indicate that while Krenite can provide effective control of buckthorn and other understory shrubs, there is potential for significant non-target impacts following its use. We therefore suggest that land managers carefully consider the timing, rate, and application method of Krenite to achieve desired target and non-target impacts.
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Invasive Plant Science and
Management
www.cambridge.org/inp
Note
Cite this article: Schuster MJ, Bockenstedt P,
Wragg PD, and Reich PB (2020) Fosamine
ammonium impacts on the targeted invasive
shrub Rhamnus cathartica and non-target
herbs. Invasive Plant Sci. Manag. doi: 10.1017/
inp.2020.17
Received: 19 December 2019
Revised: 30 April 2020
Accepted: 28 May 2020
Associate Editor:
Rob J. Richardson, North Carolina State
University
Keywords:
Buckthorn; bud inhibitor; efficacy; Krenite®;
non-target effects; herbicide; understory
Author for correspondence:
Michael J. Schuster, Department of Forest
Resources, University of Minnesota, St Paul, MN
55108. (Email: Schuster@umn.edu)
© Weed Science Society of America, 2020.
Fosamine ammonium impacts on the
targeted invasive shrub Rhamnus cathartica
and non-target herbs
Michael J. Schuster1, Paul Bockenstedt2, Peter D. Wragg1and Peter B. Reich3
1Postdoctoral Associate, Department of Forest Resources, University of Minnesota, St Paul, MN, USA; 2Project
Manager, Stantec Consulting Services Inc, Minneapolis, MN, USA and 3Professor, Department of Forest
Resources, University of Minnesota, St Paul, MN, USA; and Hawkesbury Institute for the Environment, Western
Sydney University, Penrith, NSW 2751, Australia
Abstract
Fosamine ammonium (Krenite®) is a foliar herbicide that primarily targets woody plant species;
however, formal evaluations of its efficacy and potential for non-target impacts are scarce in the
literature. The few tests of fosamine ammonium that exist focus primarily on its use in open
environments, and the value of fosamine ammonium in controlling invasive understory shrubs
is unclear. Here, we test the impact of fosamine ammonium on invasive common buckthorn
(Rhamnus cathartica L.) and co-occurring herbaceous plants across six forest sites in
Minnesota, USA. Rhamnus cathartica treated with fosamine ammonium had a 95% mortality
rate, indicating high efficacy of fosamine ammonium for use against R. cathartica. Non-target
impacts varied between forbs and graminoids such that forb cover was reduced by up to 85%,
depending on site, whereas graminoid cover was sparse and impacts of fosamine ammonium on
graminoids were unclear. These results indicate that while fosamine ammonium can provide
effective control of R. cathartica and other understory shrubs, there is potential for significant
non-target impacts following its use. We therefore suggest that land managers carefully
consider the timing, rate, and application method of fosamine ammonium to achieve desired
target and non-target impacts.
Introduction
The control of invasive woody plants is a common management goal in temperate forests, and
many herbicides can be employed to remove undesired species from the understory (Delanoy
and Archibold 2007; Ghassemi et al. 1982). Foliar spraying is commonly the application method
of choice for managing large areas of invasive woody species at relatively low cost (Power et al.
2013), because foliar spraying is not reliant on mechanical removal or on targeted application,
unlike cut-stump or basal bark methods. Foliar spraying is also often used as a follow-up treat-
ment after mechanical removal operations to control resprouts or plants missed by the initial
removal. Non-target impacts of foliar spraying can be reduced by spraying late in the season
when many native species are less vulnerable to herbicide (Caplan et al. 2018), and non-target
impacts can be further reduced by using herbicides with higher target specificity (Luken et al.
1994). One such herbicide is fosamine ammonium (Krenite®, Albaugh, Ankeny, IA), which
targets woody plant species (Marrs 1984). Fosamine ammonium can be used at similar cost
to land mangers as other foliar sprays (e.g., tricylopyr; P Bockenstedt, personal observation),
but the efficacy and non-target impacts of fosamine ammonium are rarely reported in research
literature, and the value of fosamine ammonium for management of invasive woody plants is
not well understood.
The mechanism by which fosamine ammonium affects perennial plant growth is poorly
described (Beste 1983; Marrs 1985; Naylor and British Crop Protection Council 2002), but
plants sprayed with fosamine ammonium exhibit inhibited apical and vascular meristem growth
(Morey and Dahl 1980). Similarly, bud formation in deciduous species is prevented if
leaves received full coverage the previous season. Thus, fosamine ammonium is an effective
bud inhibitorthat can starve plants of carbon if they are unable to develop enough leaves.
Most of what is publicly known about fosamine ammonium is based on the manufacturers
label, which states that when mixed with water and applied to the leaves of plants up to
2 mo before senescence, fosamine ammonium offers effective control of more than 45 woody
species or genera, many of which are invasive in at least some regions. There is a limited set of
scientific studies published evaluating the use of fosamine ammonium. These studies have been
primarily conducted in open environments (Knezevic et al. 2018; Marrs 1985; Milbauer et al.
2003), but also in some woodlands (Nature Conservancy et al. 2001; Stein 1999). They report
mixed results, with studies targeting woody species more frequently reporting high efficacy of
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fosamine ammonium (Derr 2008; Luken et al. 1994; Marrs 1984,
1985), and those targeting herbaceous perennial species reporting
low to no impact of fosamine ammonium (Knezevic et al. 2018;
Laufenberg et al. 2005; Shaw and Mack 1991). This disparity is
consistent with theorized modes of action (Coupland and
Peabody 1981; Morey and Dahl 1980), such that woody plants that
maintain tissues sprayed with fosamine ammonium should be
more sensitive to the chemical than perennial herbs that undergo
complete aboveground senescence and lose affected tissues.
Despite this, there are still qualitative reports of significant non-
target effects of fosamine ammonium on herbaceous species and
bracken ferns [Pteridium aquilinium (L.) Kuhn] (Marrs 1984,
1985; Nature Conservancy et al. 2001). Thus, impacts of fosamine
ammonium are highly variable in the literature. In addition to
differences between speciesphysiology, it is likely that the timing
of fosamine ammonium application influences its efficacy on both
target and non-target species, and that differences in the timing of
application between studies at least partially explains some of this
variability (Derr 2008; Luken et al. 1994). Notably, to the best of
our knowledge, the utility of fosamine ammonium for managing
invasive understory shrubs is untested.
Common buckthorn (also called European buckthorn;
Rhamnus cathartica L.) is a common invader of temperate forests
of eastern North America (Kurylo et al. 2007), with extensive
impacts on ecosystem structure and function (Knight et al.
2007). These impactsincluding reduced plant and animal
diversity, reduced habitat quality, accelerated decomposition and
nutrient cycling, arrested forest regeneration, and stimulated
agricultural pests and pathogens (Heimpel et al. 2010; Knight
et al. 2007)are largely the result of R. cathartica forming dense,
monospecific stands that outcompete native understory plants for
light (Archibold et al. 1997). Thus, native plants that are able to
persist under R. cathartica invasion are essential to maintaining
and restoring understory biodiversity (Roth 2015), and non-target
effects of R. cathartica management should be avoided as much as
possible when forest restoration is a goal of management. Rhamnus
cathartica may be a particularly good candidate for control with
fosamine ammonium due to its extended phenology, wherein
R. cathartica retains its leaves into late autumn or early winter
(Fridley 2012; Harrington et al. 1989; Knight 2006), allowing
R. cathartica to potentially be treated at a time when other
understory plants are inactive (Caplan et al. 2018). Yet, to what
extent fosamine ammonium controls R. cathartica and how much
it affects non-target species is unknown.
Here, we evaluate the efficacy of fosamine ammonium on
R. cathartica and the potential for non-target effects in a forest
restoration experiment replicated across six sites. Specifically,
we hypothesized that foliar application of fosamine ammonium
to R. cathartica resprouts would result in high mortality of
R. cathartica in treated areas and that herbaceous cover would
remain unaffected.
Methods
We evaluated the efficacy of fosamine ammonium (41.5%
fosamine ammonium) for R. cathartica control and impacts on
non-target species using an ongoing restoration experiment in
Minnesota, USA. We selected six sites where mature R. cathartica
had been chemically (cut-stump or basal bark application of
Garlon®4 [Dow Agrosciences, Indianapolis, IN, USA] mixed with
oil) or mechanically (forestry mower) removed in the past year and
were in need of follow-up management to prevent reestablishment
of R. cathartica from seed and resprout (Table 1). All sites were
temperate deciduous forests characterized by canopies composed
primarily of Quercus,Acer, and Populus. Rhamnus cathartica was
the most abundant understory species, but creeping charlie
(Glechoma hederacea L.), wood nettle [Laportea canadensis (L.)
Weddell], enchanters nightshade (Circaea lutetiana L.), bedstraw
(Galium spp.), woodbine [Parthenocissus quinquefolia (L.)
Planch.], and white snakeroot [Ageratina altissima (L.) R.M.
King & H. Rob.] were also common.
Experimental Design
As part of a larger restoration experiment, we established three to
six replicate blocks in each site in February 2017. Each block con-
tained a pair of plots, one assigned to receive foliar fosamine
ammonium and another to serve as a control. Most plots were
12 by 30 m, but plots at Circle Pines and Chaska sites were half
as wide (6 by 30 m) to accommodate space constraints and areas
containing large stacks of cut stems (i.e., slash piles). We also iden-
tified four 1 by 1 m2subplots within each plot (spaced 5 m apart
along a 20-m transect) for use in sampling. If subplots could not be
feasibly placed along the transect (e.g., due to intervening tree
trunks), they were shifted the minimum distance required to allow
representative sampling.
All sites were in need of follow-up herbicide treatments to
remove residual R. cathartica resulting from stump resprouts
and seed germination. On July 2527, 2017, plots assigned to
the fosamine ammonium treatment were sprayed using a combi-
nation of application techniques. Because sites varied in removal
method and initial conditions (Table 1), R. cathartica abundance
was variable between sites. Following common management
practices, we varied the application method between sites based
on the amount of R. cathartica to be sprayed. Sites with dense
R. cathartica regeneration were treated at 300 L ha1using pistol
grip sprayers fed from a vehicle-mounted tank containing 3.5%
fosamine ammonium mixed with water (4.2 L ha1fosamine
Management Implications
The management of Rhamnus cathartica (common buckthorn)
often includes the use of foliar herbicides. Fosamine ammonium
(Krenite®) is a bud inhibitor that is thought to primarily target woody
plant species and may be an effective tool in controlling R. cathartica
and other invasive understory shrubs. At six sites in Minnesota,
USA, we found foliar treatments using fosamine ammonium caused
almost complete mortality of R. cathartica stems (>40-cm tall).
However, the use of fosamine ammonium was also associated with
severe reductions in forb cover in some sites, particularly those
treated using high-volume pistol grip sprayers. Graminoid cover
was low across all sites, but tended toward similar negative responses
to fosamine ammonium. Our findings suggest that fosamine ammo-
nium can exert strong control over R. cathartica, but that land man-
agers should carefully consider potential non-target impacts when
using fosamine ammonium. Delaying restoration seeding until after
fosamine ammonium applications have concluded or timing fos-
amine ammonium application to occur after the senescence of
understory herbs may mitigate non-target impacts of fosamine
ammonium while maintaining comparable levels of R. cathartica
control.
2 Schuster et al.: Fosamine ammonium on Rhamnus
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ammonium). Sites with sparse R. cathartica regeneration were
treated at 100 L ha1using backpack sprayers containing 7%
fosamine ammonium mixed with water (2.9 L ha1fosamine
ammonium). These application rates are below the manufacturers
recommended minimum for fosamine ammonium (5.8 L ha1
fosamine ammonium), but we have observed effective control
of R. cathartica with these application rates in the past
(P Bockenstedt, personal observation). Thus, our treatments pro-
vide a conservative metric of fosamine ammonium efficacy. Using
foliar sprays, especially a bud inhibitor like fosamine ammonium,
is dependent on complete coverage of a target plants leaves. Leaves
that are not sprayed are prone to return the following spring and
allow for continued growth, severely limiting the efficacy of the
treatment overall and requiring additional treatments at added
cost. For this reason, it is common to use foliar sprays subsequent
to mechanical removal. Once R. cathartica stems have been cut,
they resprout, and these resprouts are sufficiently small to allow
for complete coverage by foliar sprays. Use of either mechanical
removal or fosamine ammonium in isolation is only likely to be
effective in very young stands that are short and lack stored carbo-
hydrate reserves. Therefore, we performed our study within the
context of stands that had already undergone some R. cathartica
management.
Efficacy of fosamine ammonium against R. cathartica was
measured by assessing the health of all standing R. cathartica
stems >40-cm tall in May 2018. Stems were identified by any
remnant leaves and by bark characteristics. Stems with green leaves
or active buds were determined to be alive, while those that lacked
leaves or active buds and were brittle to the touch were determined
to be dead. The total number of live and dead R. cathartica was
recorded for each subplot.
Effects on community composition were measured by
visually estimating cover in July 2018. Within each subplot, we vis-
ually estimated total woody cover (including R. cathartica), total
graminoid cover, and total forb cover.
Statistical Analyses
First, we characterized inherent differences between sites by ana-
lyzing the total number of R. cathartica surveyed and graminoid
and forb cover as functions of site, using data only from control
plots. We then tested our hypothesis using a series of generalized
mixed models. We evaluated R. cathartica mortality as the
proportion of total R. cathartica stems that were dead and evalu-
ated community composition by considering the cover of woody
species, graminoids, and forbs. Mean R. cathartica mortality within
a plot and mean woody cover, mean graminoid cover, and mean
forb cover within a plot were analyzed based on site, whether the
plot had received fosamine ammonium, and the interaction
between site and fosamine ammonium. Rhamnus cathartica
mortality was modeled in a general linear mixed model using
PROC MIXED in SAS v. 9.4 (SAS Institute, Cary, NC) using the
Satterthwaite approximation for degrees of freedom and with
block as a random factor nested within site. All other models were
performed using PROC GLIMMIX, assuming a Poisson distribu-
tion and with block as a random factor nested within site. We also
tested post hoc whether sites with different fosamine ammonium
application techniques differed in impacts on target and non-target
species.
Results and Discussion
Experimental sites differed greatly in their baseline conditions as
indicated by control plots (Figure 1). The St Croix site had the
greatest baseline abundance of R. cathartica stems (numbers in
Figure 1A; Table 2), and the Circle Pines and St Croix sites had
similarly high woody cover (Figure 1B). The Elk River site had
the lowest R. cathartica abundance but also had the greatest base-
line forb cover (Table 2). In contrast, the Chaska site had the lowest
forb cover. Baseline graminoid cover was consistently low across all
sites (Table 2).
Mortality of R. cathartica greater than 40-cm tall was highly
elevated following fosamine ammonium application (Table 2).
Across sites, 95% of all R. cathartica that had been sprayed were
considered dead at the time of survey. Mortality rates in sprayed
plots were near 100% at four sites, but were 80% and 88% at the
Circle Pines and St Croix sites, respectively (Figure 1A). In
comparison, ambient mortality rates in the absence of fosamine
ammonium (i.e., in control plots) were 0% at all sites except
Chaska, where mortality that may have occurred in prior years
due to basal bark herbicide application might have been included
in our survey. Rhamnus cathartica mortality did not vary signifi-
cantly by site, and there was no interaction between site and
fosamine ammonium (Table 2). In accordance with the high
mortality rate we observed during the spring, cover of woody
species (including R. cathartica) in summer was 74% lower in plots
treated with fosamine ammonium (Figure 1B). Post hoc tests of
woody cover showed a trend toward high-volume pistol grip
sprayers resulting in lower woody cover compared with
backpack sprayers, but this was not statistically significant
(Tukey HSD, P =0.06). Although species aside from R. cathartica
(e.g., pin cherry, Prunus pensylvanica L.) contributed to woody
cover estimates, R. cathartica remained the most common woody
species across the experiment, even after fosamine ammonium
Table 1. Characteristics of the six experimental sites in Minnesota, USA.
Site CoordinatesanbRemoval methodcCanopydSoil texture
Fosamine ammonium
applicatione
Savage 44.715398°N, 93.333028°W 3 Cut-and-treat Quercus Sandy loam Backpack
Elk River 45.303173°N, 93.579193°W 6 Forestry mower Quercus Loamy sand Pistol grip
Chaska 44.852885°N, 93.608456°W 3 Basal bark Quercus Sandy loam Backpack
Circle Pines 45.110445°N, 93.180008°W 3 Cut-and-treat Quercus Loamy sand Backpack
St Croix 45.171437°N, 92.765094°W 5 Forestry mower Quercus Sandy loam Pistol grip
Hastings 44.757154°N, 92.869074°W 6 Cut-and-treat Populus Loamy sand Pistol grip
aGPS coordinates of each site.
bn, number of replicate blocks at each site.
cMethod used to remove Rhamnus cathartica initially, before the start of the experimental period.
dDominant tree canopy at the site.
eMethod by which fosamine ammonium was applied to the site.
Invasive Plant Science and Management 3
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treatment. The disparity between our observations of high mortal-
ity and some residual woody cover following fosamine ammonium
treatment is most likely due to individuals that did not receive full
fosamine ammonium coverage or otherwise avoided being
sprayed. It may be that increasing the application rate would result
in more complete control of woody species, but the reality of
imperfect coverage likely requires managers to repeat treatments
for several seasons, despite the high chemical efficacy of fosamine
ammonium.
Non-target effects of fosamine ammonium varied by site and by
growth form. Graminoid cover was not consistently affected by
fosamine ammonium, and varied between sites (Table 2). The
Elk River site had the greatest graminoid cover overall compared
with other sites (Figure 1C). Two of three sites with detectable
graminoid cover (the two with most cover) did show marked
reductions in fosamine ammoniumtreated plots, but with
high variability in responses. Forb cover also varied by site and
was adversely affected by fosamine ammonium application
(Table 2); forb cover in fosamine ammoniumtreated plots was
only 37% of what it was in control plots on average across all sites.
However, there were also strong differences in the impact of
fosamine ammonium on forbs between sites (Table 2). For
example, forb cover was reduced by 85%, 73%, and 50% at the
Elk River, Hastings, and Savage sites, respectively, whereas there
was no consistent different in forb cover between control and
fosamine ammoniumtreated plots in the other sites
(Figure 1D). The sites with the greatest reductions in forb cover
(Elk River and Hastings) were also those that were treated with
high-volume pistol grip sprayers instead of backpack sprayers
(Tukey HSD, P <0.01). Similar differences between pistol grip
sprayers and backpack sprayers were not apparent in grami-
noid cover.
Reductions in forb cover with fosamine ammonium were
associated with shifts in species composition as well. In the Elk
River site, control plots were characterized by large components
of L. canadensis,A. altissima, and G. hederacea. In contrast, fos-
amine ammoniumtreated plots at Elk River were characterized
by different forbs: exotic garlic mustard [Alliaria petiolata
(M. Bieb) Cavara & Grande], native clearweed [Pilea pumila (L.)
A. Gray], and native wood-sorrels (Oxalis spp.). These shifts in
forb composition were not observed in sites like Chaska, where
there were no discernable differences in forb cover (Figure 1D).
The variability in forb responses may also be tied to resident com-
munity composition in a way that sites comprising species other
than what were found in our study would respond differently to
fosamine ammonium treatment. For example, species with earlier
phenology (e.g., spring ephemerals) would likely be less affected by
fosamine ammonium spraying during dormancy.
Following intensive fosamine ammonium application across
six forest sites, we found consistent evidence of high efficacy
of fosamine ammonium against R. cathartica. However, we
also observed markedly reduced forb cover as an unintended
consequence of fosamine ammonium, particularly in sites treated
with high-volume pistol grip sprayers. Our findings indicate
that fosamine ammonium can be used to effectively control
R. cathartica, but also highlight the need for careful planning
and evaluation of non-target risk by land managers.
Figure 1. Mean (±SE) Rhamnus cathartica mortality as a proportion of all
R. cathartica stems located in May 2018 (A). Also presented are mean woody
(B), graminoid (C), and forb (D) cover in July 2018 in control plots (white) and those
treated with fosamine ammonium (gray). Sites were treated using either pistol grip
sprayers (open bars) or backpack sprayers (hashed bars). Numbers in A indicate
the mean density of R. cathartica >40-cm tall (m2) at each site. Sites are organized
in order of increasing R. cathartica abundance.
4 Schuster et al.: Fosamine ammonium on Rhamnus
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To the best of our knowledge, this is the most comprehensive
test to date of fosamine ammonium efficacy and non-target
impacts in forest understories. However, it is important to inter-
pret our findings only within the context of our experiment. We
likely overestimate efficacy by only considering stems taller than
40 cm. Stems shorter than this could have been obscured by taller
vegetation or gone undetected during spraying, and therefore
survived the treatment by avoiding contact with fosamine ammo-
nium. Even in plots where we detected 100% mortality of stems
taller than 40 cm, some smaller R. cathartica persisted to contrib-
ute to woody cover in the following summer (Figure 1B; M
Schuster and P Wragg, personal observation). These remaining
R. cathartica warrant continued monitoring over the next growing
season to evaluate whether additional management is necessary. In
terms of biological response, our estimate of high efficacy is likely
accurateR. cathartica that were exposed to fosamine ammonium
had much lower survival ratesbut in terms of practical manage-
ment, effective mortality rate would be lower, as some R. cathartica
escaped treatment.
The non-target effects we observed may have been brought on
by the timing of our fosamine ammonium application in late July.
It is recommended by the manufacturer that fosamine ammonium
be applied within 2 mo of plant senescence. This is a challenging
criterion to follow, given that woody plant phenology is highly
variable across species and years (Fan et al. 2019; Fridley 2012;
Zettlemoyer et al. 2019). Therefore, it is not always possible to dis-
cern the optimal time for fosamine ammonium application, and
land managers may apply fosamine ammonium earlier than
necessary to ensure sufficient coverage of target species. Because
fosamine ammonium must be applied to green leaf tissues, delay-
ing application later into autumn risks reduced efficacy as leaves
are lost throughout the season. However, many herbaceous species
are also still active during these earlier application windows
compared with later ones. Therefore, the leaves of these non-target
herbaceous species can still be affected by fosamine ammonium,
especially when it is applied with nondiscriminate, high-volume
pistol grip sprayers. Therefore, applying fosamine ammonium in
late summer, as we did here, likely leads to high efficacy against
R. cathartica, but also exposes herbaceous species to considerable
risk. Moreover, for species like R. cathartica that do not undergo
distinct autumn senescence in nonnative North American
territory, the optimal application date might be in autumn. That
is because R. cathartica will commonly hold a large portion of their
leaves fully or remain nearly fully green well past first frost and only
become fully defoliated as leaves are lost to necrosis (Harrington
et al. 1989; Knight et al. 2007; Pretorius 2015) when low temper-
atures reach approximately 12 C (personal observation).
We speculate that a later application date would have reduced
non-target impacts with only minor differences in target efficacy.
Further research is needed on how the timing of fosamine ammo-
nium applications affects both target and non-target species.
Rhamnus cathartica treatment utilizing fosamine ammonium
herbicide can be effective. However, potential non-target impacts
must be considered when planning and executing treatment and an
overall restoration approach. Observations in the field at multiple
sites, including the plots that are part of this study, indicate that the
application tools, timing, and methods can all influence the type
and level of non-target impacts. Our observations indicate that
particular types of vegetation can be negatively impacted by
bud-inhibitor application, including herbaceous seedlings and
juvenile plants, as well as mature plants with woody or semi-woody
roots.
There are likely several strategies that would minimize non-
target damage in restoration areas, including carefully choosing
application tools and timing to conduct foliar applications, as well
as delay of native seeding for restoration. We have observed that a
treatment approach that minimizes non-target impacts is to focus
on foliar treatment of brush under 1-m tall, allowing the applicator
to hold a spray wand tip over the plant rather than needing to spray
upward to reach the uppermost stems and leaves of taller brush,
which often results in overspray (P Bockenstedt, personal observa-
tion). As would be expected, high-volume applications by boom
and pistol resulted in the most frequently observed non-target
impacts, while spot treatment using backpacks typically resulted
in the least non-target impacts.
Our results also have relevance when establishing timelines for
R. cathartica management. Although avoiding non-target impacts
is a common goal of management, control of invasive or weedy
herbaceous species in addition to R. cathartica may be an
advantageousif unintendedconsequence of using fosamine
ammonium. Lower cover of highly competitive species like
A. petiolata can allow for the reestablishment of desired native spe-
cies and facilitate more diverse understory communities (Stinson
et al. 2006,2007). Additionally, it may be that mature perennial
plants are less sensitive to herbicide than first-year seedlings due
to differences in stored reserves (Marrs et al. 1991), suggesting that
non-target impacts may be particularly harmful to plants establish-
ing from seed as part of revegetation efforts (Wagner and Nelson
2014). Thus, for sites where a significant portion of an area is
expected to be foliar sprayed to treat abundant R. cathartica
resprouts and/or seedlings in the first growing season after initial
invasive brush management, delaying restoration seeding for one
growing season can be an effective approach for developing long-
term native ground cover and mitigating non-target impacts
(Luken et al. 1994).
Acknowledgments. Partial funding was provided by the Minnesota Invasive
Terrestrial Plants and Pests Center through the Minnesota Environment and
Natural Resources Trust Fund. Carl Anfang and Gabriele Menomin contributed
significantly to fieldwork. No conflicts of interest have been declared.
References
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shrub European Buckthorn, Rhamnus cathartica L., near Saskatoon,
Saskatchewan. Can Field-Nat 111:617
Table 2. Statistical results for analyses of Rhamnus cathartica mortality, woody cover, graminoid cover, and forb cover.
Mortality Woody cover Graminoid cover Forb cover
df F Pdf F Pdf F Pdf F P
Fosamine ammonium 1, 13 536.47 <0.01 1, 20 65.78 <0.01 1, 20 1.95 0.18 1, 20 80.92 <0.01
Site 5, 13 1.31 0.32 5, 20 2.62 0.06 5, 20 3.74 0.01 5, 20 4.08 0.01
Fosamine ammonium x site 5, 13 0.87 0.53 5, 20 2.19 0.10 5, 20 0.46 0.80 5, 20 29.14 <0.01
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6 Schuster et al.: Fosamine ammonium on Rhamnus
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... Yet, native species are rarely able to exploit these abundant resources (Ibáñez et al., 2021) due to legacies of buckthorn invasion (Lamb et al., 2022). Buckthorn readily resprouts from the stem, allowing a plant to rapidly regrow and outcompete newly-establishing plants if it has been cut or non-lethally treated with herbicide (Delanoy and Archibold, 2007;Schuster et al., 2020a). Buckthorn also produces a prolific amount of seed that can result in continuous cover of seedlings (Delanoy and Archibold, 2007;Qaderi et al., 2009). ...
... The types of revegetation considered in this study will often require additional management to fully prevent buckthorn re-establishment. Most importantly, existing buckthorn and their resprouts must be thoroughly controlled (e.g., via foliar herbicide, see Schuster et al., 2020a) since revegetated plants cannot establish quickly enough to compete against the rapid growth of resprouts (> 2 m in a single growing season for large trees; MS personal observation) . Additionally, while some of our treatments often resulted in the complete exclusion of buckthorn (zero buckthorn seeds survived in the majority of replicates of the tree and shrub plantings), buckthorn responses to revegetation were variable. ...
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Woody invaders of temperate forest understories reduce native diversity worldwide. Common buckthorn Rhamnus cathartica, is among the most widespread of such invaders in North America. Invaded communities often have seedbanks largely comprised of the dominant invader - with few native species remaining - and therefore lack the capacity to build biotic resistance against re-invasion following invader removal. Consequently, invaders, including buckthorn, often quickly re-establish in the absence of continued management. We investigated the capacity of native plant revegetation to inhibit buckthorn re-establishment from seedbanks in the understories of three forests of Minnesota, USA. Specifically, we established experimental plots subjected to seeding of 35 native species, planting of Pennsylvania sedge (Carex pensylvanica) plugs, or bare-root plantings of either mixed shrubs (Sambucus canadensis, Sambucus racemosa, Corylus americana, and Cornus racemosa) or mixed trees (Abies balsamea and Acer saccharum). We then measured buckthorn germinant establishment, growth, and survival for the following four growing seasons. We observed consistent impacts of revegetation on ground-level light availability and associated buckthorn performance. Compared to unplanted understory controls beneath the mature tree canopy, shrub plantings were the most impactful. Shrubs reduced light availability to buckthorn seedlings by 67% relative to unplanted controls (to <2% total light by the third year) and led to 51% lower year-over-year survival of buckthorn by the end of the experiment. Revegetation also suppressed buckthorn seedling growth. After four years, shrub plantings resulted in buckthorn that were 53% shorter and had 38% fewer leaves than buckthorn grown in unplanted controls. Considering the combined impacts on survival and growth, planted shrubs, trees, and sedges reduced buckthorn invasion by 89%, 81%, and 66%, respectively; and seeding alone reduced invasion by 51%. Our findings indicate that revegetating forests, particularly with shrubs and trees, can greatly reduce invasion by buckthorn and potentially other species. Greater adoption of revegetation by land managers may therefore increase native biodiversity, reduce herbicide applications, and improve the overall health and value of forests.
... Follow-up herbicide may be having non-target effects on newly seeded plants. Exploring ways to reduce non-target effects, such as waiting to seed until after the first year or two of follow-up herbicide, using more woodyselective herbicides such as bud inhibitors (Schuster et al. 2020), and using graminoid-only revegetation mixes that are less affected by broadleaf-selective herbicides may reveal how to combine seeding and follow-up herbicide for greater benefit than either alone. This underlines the value of covariates in retrospective studies for generating new hypotheses and research directions. ...
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(1) Birch is a problem weed on many lowland heath nature reserves. In order to devise control methods suitable for conservation use, an experiment was carried out to compare the effects of (1) mechanical control methods, and (2) foliar sprays of three selective herbicides (fosamine ammonium, 2,4,5-T and triclopyr) singly or in combination with cutting. The effect of these treatments on the target species (birch: Betula pendula Roth. and Betula pubescens Ehrh.) was assessed as well as their effects on a non-target species (Calluna vulgaris). Particular attention was paid to damage and recovery of the Calluna population. (2) Of the mechanical means of controlling birch, cutting was not as successful as the labour intensive and time-consuming pulling, primarily because birch regenerated from the cut stumps. Cutting in successive years increased the degree of control. Mechanical treatments did not adversely affect the Calluna understorey, and these treatments tended to increase the number of regenerating Calluna seedlings. (3) All three herbicides, whether applied to birch scrub or to regrowth after cutting, gave better control than cutting treatments without herbicide application. Applying triclopyr, but not fosamine ammonium or 2,4,5-T, decreased the number of birch seedlings. (4) Fosamine ammonium and 2,4,5-T damaged some, and killed a few Calluna plants, but recovery was rapid. Triclopyr on the other hand reduced the number of Calluna plants and seedlings. (5) The role of selective herbicides for the control of birch by foliar spray for practical conservation purposes is discussed in relation to efficacy of birch kill, and damage to Calluna. On balance it appears that fosamine ammonium and 2,4,5-T are the most suitable herbicides for this purpose.