SPOTTED KNAPWEED'S RESPONSE TO VARIOUS MANAGEMENT
TREATMENTS IN LAC DU BOIS: A SEED BANK APPROACH

Morgan Robinson

UREAP
Final Report

In the Department of
NATURAL RESOURCE SCIENCES
FACULTY OF SCIENCE
at
THOMPSON RIVERS UNIVERSITY

May 31, 2024

Abstract
Invasive plants pose a significant threat worldwide, with far-reaching impacts on
landscapes. In Canada, grasslands are among the most endangered ecosystems, covering
less than 1% of British Columbia's (BC) land area. Lac du Bois, the second-largest
protected area in BC dedicated to grassland conservation, faces numerous threats despite
its protected status, primarily due to its proximity to the city of Kamloops. One major
invasive plant of concern in Lac du Bois is Centaurea stoebe spp. micranthos, commonly
known as spotted knapweed, which can severely impact grassland ecosystems.
Understanding effective control methods for spotted knapweed is crucial for preserving
native grasslands. While methods to control spotted knapweed have been well-researched
and trialed, there is limited information on the composition of the seed bank following
treatments. In 2022, the Grassland Conservation Council initiated demonstration trials on
Red Hill within the Lac du Bois protected area to study the effects of various knapweed
treatments on the seed bank. The primary objective of this independent study was to
assess the abundance of knapweed in the seed bank compared to native species and to
evaluate the potential success of restoration efforts. Seven treatments were conducted: a
control, hand pulling knapweed in years one and two, weed whacking the entire plot in
year one, weed whacking the entire plot in years one and two, spraying Milestone at high
or low concentrations when knapweed bolts, and spraying Milestone in the fall at a high
concentration. Soil samples from the twenty-four treated plots underwent a twelve-week
growth period in a growing chamber. As seedlings emerged, they were identified,
counted, and removed. After 12 weeks, the germination experiment was terminated, and
soil seed bank density was calculated and converted to the number of seeds per square
meter for each identified plant. A total of eleven different species were detected: six
native to BC and five introduced. The residual activity of Milestone effectively controlled
spotted knapweed in the sprayed treatments but also restricted the growth of other
species, including grasses and forbs. The hand-pulled and weed-whacked soil samples
had much higher densities of both non-native and native forbs. Across all treatment types,
very few grasses emerged from the seed bank. This study provides valuable information
for grassland restoration efforts.

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Table of Contents
Abstract ............................................................................................................. ii
List of Figures ................................................................................................... iv
List of Tables ...................................................................................................... v
Acknowledgement ............................................................................................. vi
Introduction ...................................................................................................... 1
Objectives ...................................................................................................... 4
Methods ............................................................................................................ 4
Site Description .............................................................................................. 4
Data Collection .............................................................................................. 5
Greenhouse Experiment and Data Analysis ...................................................... 7
Results .............................................................................................................. 8
Discussion ........................................................................................................ 13
Spotted Knapweed ........................................................................................ 13
Forbs............................................................................................................ 13
Grasses ......................................................................................................... 14
Limitations ................................................................................................... 15
Conclusion ....................................................................................................... 15
References ........................................................................................................ 17

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List of Figures
Figure 1. Location of the Red Hill study site in Lac du Bois Grasslands Protected Area
near the city of Kamloops, in the interior of BC (Represented by red diamond). Map
source: iMapBC 2024. ......................................................................................... 5
Figure 2. The randomized block design used to collect cover data at Red Hill in the Lac
du Bois Grasslands Protected Area in the interior of BC. ........................................... 6
Figure 3. The plot layout demonstrating where soil samples were collected out in the
field at Red Hill in the Lac du Bois Grasslands Protected Area in the interior of BC. ..... 7
Figure 4. The germination trays in the growing chamber and an example of some of the
forbs and grasses that emerged. ............................................................................. 8
Figure 5. The average density (# seeds/m2) of spotted knapweed recorded in the soil
samples collected from treated plots at Red Hill in Lac du Bois Grasslands Protected
Area. The treatments consisted of a control C, hand pull year one and year two HP2,
spray Milestone at a high concentration in the fall SF, spray Milestone at a high or low
concentration when knapweed bolts SH SL, weed whack year one WW1, and weed
whack year one and two WW2 (error bars represent 1 standard deviation). ................. 10
Figure 6. The average density (# seeds/m2) of native forbs recorded in the soil samples
collected from treated plots at Red Hill in Lac du Bois Grasslands Protected Area. The
treatments consisted of a control C, hand pull year one and year two HP2, spray
Milestone at a high concentration in the fall SF, spray Milestone at a high or low
concentration when knapweed bolts SH SL, weed whack year one WW1, and weed
whack year one and two WW2 (error bars represent 1 standard deviation). ................. 11
Figure 7. The average density (# seeds/m2) of non-native forbs recorded in the soil
samples collected from treated plots at Red Hill in Lac du Bois Grasslands Protected
Area. The treatments consisted of a control C, hand pull year one and year two HP2,
spray Milestone at a high concentration in the fall SF, spray Milestone at a high or low
concentration when knapweed bolts SH SL, weed whack year one WW1, and weed
whack year one and two WW2 (error bars represent 1 standard deviation). ................. 11
Figure 8. The average density (# seeds/m2) of native grass recorded in the soil samples
collected from treated plots at Red Hill in Lac du Bois Grasslands Protected Area. The
treatments consisted of a control C, hand pull year one and year two HP2, spray
Milestone at a high concentration in the fall SF, spray Milestone at a high or low
concentration when knapweed bolts SH SL, weed whack year one WW1, and weed
whack year one and two WW2 (error bars represent 1 standard deviation). ................. 12
Figure 9. The average density (# seeds/m2) of non-native grass recorded in the soil
samples collected from treated plots at Red Hill in Lac du Bois Grasslands Protected
Area. The treatments consisted of a control C, hand pull year one and year two HP2,
spray Milestone at a high concentration in the fall SF, spray Milestone at a high or low

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concentration when knapweed bolts SH SL, weed whack year one WW1, and weed
whack year one and two WW2 (error bars represent 1 standard deviation). ................. 12

List of Tables
Table 1. Soil seed bank composition after the control, hand pull, herbicide, and weed
whack treatments at Red Hill in the Lac du Bois Grasslands Protected Area. The average
seed density (#seeds/m2) was estimated for each identified species with a minimum to
maximum range in parentheses………………………………………………………….10

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Acknowledgement
This project would not have been possible without my support system. I would first like
to thank the Grassland Conservation Council for allowing me to utilize their field data
and knowledge, specifically Dennis Lloyd and Kristi Gordon, who created the research
design, completed field work, and will continue to look after the demonstration area into
the future. I would like to send more gratitude to Kristi and my supervisor Wendy
Gardner, who offered their time and knowledge throughout the whole process. I have
truly learned so much from this research experience. Lastly, I am also very grateful for
Peggy Broad helping me out with plant identification.

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Introduction
Invasive plants are an ongoing issue in British Columbia (BC) causing significant impacts on the
landscape. Invasives are specifically threatening to the native grasslands that cover less than 1%
of BC’s land base (GCC 2017a). Within this category, Lac du Bois is the second-largest
protected area in BC contributing to grassland conservation, safeguarding nearly 25% of the total
bunchgrasses in the province (BC Parks 2017). Even though it is a protected area, Lac du Bois is
adjacent to the city of Kamloops and faces threats from recreation, natural disturbance,
overgrazing, climate change, tree encroachment, and the invasion of non-native plants.

There have been several non-native plants introduced to Lac du Bois that are threatening to the
natural communities. A plant of major concern is Centaurea stoebe spp. micranthos, spotted
knapweed, a member of the Asteraceae family. This plant was introduced in the 1890s from
Europe and it can now be found all over North America (Marrs et al. 2008). Spotted knapweed is
a noxious forb regulated under the BC Weed Control Act and it can be found throughout the
lower and middle elevational grasslands across the province (GCC 2017b).

Spotted knapweed has many resilient traits that allow for easy dispersal and establishment. It is a
biennial or a perennial plant that can have four life history stages as a seed, seedling, rosette, or
flowering plant (Jacobs 2012). Spotted knapweed disperses by seed and each individual plant can
produce thousands of seeds that can remain viable in the soil for up to eight years (Sheley et al.
1998). The seeds can be further spread by people, animals, water, and wind. Seedling
germination occurs in the fall or spring and than the plant has a few options for the growing
season. Spotted knapweed can remain as a seedling, grow and persist as a rosette, or develop into
a flowering plant all in one growing season (Jacobs 2012). Mature plants flower throughout the
summer and disperse seeds in the fall. The next growing season, an individual plant can either
continue to bolt and flower or revert to a vegetative adult rosette (Emery and Gross 2005).
According to Emery and Goss (2005), spotted knapweed can live up to eight years and older
plants can have multiple rosettes and bolting stems. The low growing rosette stage benefits
knapweed primarily because it allows the plant to establish a strong root system and conserve
resources before it has to transition into a reproductive stage. Some other physical characteristics
that help this plant succeed are a deep tap root, spiny bracts that deter grazing animals, and the
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ability to form large monocultures. Overall, it is not a desirable forage species in the rosette stage
or when fully mature (Sheley et al. 1998). Some researchers believe that knapweed is
allelopathic and can outcompete surrounding plants, by releasing chemicals like catechin into the
soil (Coghill 2020).

Spotted knapweed is capable of colonizing native grasslands and the effects can be detrimental.
For example, a study completed in Glacier National Park suggested that spotted knapweed can
successfully establish on fescue grasslands due to adjacent roadside knapweed populations and
rapid seed dispersal (Tyser and Key 1988). Some of the natural biotic features of grasslands
facilitate the spread of spotted knapweed such as natural breaks in cover, a warm-dry climate,
mammal burrowing activities, and overgrazing by ungulates/livestock (Tyser and Key 1988).
Disturbed areas are at high risk of knapweed invasion, especially in the dry interior of BC, as
spotted knapweed can tolerate a wide range of environmental conditions (Watson and Renney
1974). Some reported negative impacts from spotted knapweed infestations include decreased
species richness, reduced forage, loss of habitat, and increased soil erosion (Sheley et al. 2001,
Tyser and Key 1988). Spotted knapweed is a successful invader and could continue to spread in
Lac du Bois if there is no management in place.

There are a range of methods that have been used to control spotted knapweed, including
manual, biological, cultural, and chemical treatments. The treatments completed at the Red Hill
Demonstration area in Lac du Bois consisted of herbicide, hand pulling, and weed whacking.

Hand pulling is commonly used for small infestations to manually remove individual plants,
including the roots. This treatment is most effective when the site is revisited, the plant is pulled
before it sets seed, and the soil has enough moisture to allow the entire plant to be removed
(Sheley et al. 1998). The key to effective hand pulling is to leave no root fragments behind so
there is no potential of re-sprouting and minimize soil disturbance so re-invasion can’t occur (Tu
et al. 2001). Mowing or weed whacking is a technique that involves removing the aboveground
biomass to reduce knapweed’s density and ability to produce seeds. A disadvantage with
mowing is that the plant can continue to grow and produce flowering parts over the growing
season if conditions are favorable (Story et al. 2010). It is important to remember that manual
2

and mechanical treatments can disturb the soil and provide prime conditions for re-invasion by
weedy species (Tu et al. 2001). Hand pulling and weed whacking treatments are often beneficial
if used in conjunction with other control methods like herbicide.

Herbicide is often necessary in controlling large infestations of spotted knapweed, and broad-leaf
herbicides are normally used so native grasses aren’t affected (Sheley et al. 2000). Picloram,
dicamba, clopyralid, 2,4-D, aminopyralid, and aminocyclopyrachlor are all common herbicides
used to control knapweed (USDA 2014). A study completed in Montana found that various
herbicide treatments increased perennial grass biomass, with picloram being the most effective
and persistent in decreasing spotted knapweed over time (Sheley et al. 2000). Milestone was the
chosen herbicide to use at the Red Hill demonstration area because it can offer preemergence and
postemergence control (DOW date unknown). The active ingredient in Milestone is
aminopyralid, which is structurally similar to picloram. They are both auxin-type herbicides that
control broadleaf weeds and provide residual weed control (Fast et al. 2010). The only difference
is aminopyralid has a shorter half-life than picloram suggesting it has a lower environmental
impact than picloram (Fast et al. 2010). Fast et al. (2010) analyzed soil sorption, concluding that
soil sorption of aminopyralid is greater than picloram, meaning there is less potential for offtarget movement of aminopyralid. The most effective time to spray spotted knapweed is either in
the fall during the seedling to early rosette stage or in the spring/early summer to target plants
before they flower (USDA 2014). The residual activity of Milestone may offer control all season
long or well into the season after application, depending on the weed species. (DOW date
unknown).

Understanding how the spotted knapweed treatments discussed may impact the seed bank is
crucial for successful restoration efforts. The seed bank represents a reservoir of plant diversity
and potential future vegetation, playing a pivotal role in ecosystem regeneration and resilience.
Intensive control measures, such as herbicide application or mechanical removal, can
inadvertently alter the composition, density, and viability of seeds within the seed bank. For
instance, while targeting invasive species, these treatments may inadvertently affect the viability
of desirable native seeds or promote the dominance of other invasive species with persistent seed

3

banks. Moreover, disturbances caused by control methods can create favorable conditions for the
germination and establishment of certain weed species, perpetuating the cycle of invasion.
Objectives
The main objective of this project is to determine how different knapweed control treatments (a
control, hand pulling, weed whacking, spraying Milestone at a high concentration or a low
concentration when knapweed bolts, and spraying Milestone in the fall at a high concentration)
impact the seedbank. Specifically, observing how each treatment affected the density of
knapweed, native, and non-native plants.

Methods
Site Description
There are eight distinctive grasslands regions that can be found in BC, with our study focusing
on the Thompson-Pavilion. This grassland region follows the valley bottoms and lower slopes of
the Thompson River valley, Hat Creek valley, and the Fraser River north of Lillooet (GCC
2017a). The climate is affected by the rain shadow from the coastal mountains with dry air from
the south in the summer and cold air from the arctic in the winter. The Thompson-Pavilion
grassland region covers a total of 58,265 hectares, with the Thomspon Basin covering a large
portion of those hectares at 39,999, Lac du Bois would be a large contribution to those hectares
(GCC 2017a) (Figure 1).
Treatments were conducted at Red Hill, a protected area in Lac du Bois in the Bunchgrass Nicola
Very Dry Warm (BG xw1) biogeoclimatic (BEC) zone. The BG xw1 represents the middle
grasslands, a transitional area between the lower and upper grasslands with an elevational range
of 700-1000m (GCC 2017a). A moister climate with cooler temperatures results in fewer sage
brush (Artemisia tridentata) and more bluebunch wheatgrass (Pseudoroegneria spicata) and
rough fescue (Festuca campestris) (GCC 2017a). Sandberg’s bluegrass (Poa secunda) and other
flowering plants like thread-leaved fleabane (Erigeron filifolius), mariposa lily (Calochortus
macrocarpus), yarrow (Achillea millefolium), and arrow-leaved balsamroot (Balsamorhiza
sagittata) are commonly found in the middle grasslands (GCC 2017a).

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Red Hill is known for its reddish-orange colored dirt and rock giving it a red appearance from
afar. Before the protected area was created in Lac du Bois, Red Hill was damaged by motorized
vehicles. Protecting the area and prohibiting motorized vehicles has provided time for the
vegetation to heal but now spotted knapweed has established on the hill. Lac du Bois provides
grazing for cattle starting in the spring and Red Hill is within the Halston pasture. The desired
plant community is late seral with a goal of bluebunch wheatgrass and rough fescue with a mix
of forbs.

Figure 1. Location of the Red Hill study site in Lac du Bois Grasslands Protected Area near
the city of Kamloops, in the interior of BC (Represented by red diamond). Map source:
iMapBC 2024.

Data Collection
The plots for the Red Hill project were established in the summer of 2022, all initial treatments
were conducted on June 27, 2022. The slope chosen for the project had relatively constant cover
of knapweed over the whole area. The demonstration site was purposely split into two, so the
defined strips of seeded alfalfa covering old, motorized trails could be avoided. A randomized
block design was used and replicated three times with each block containing eight plots to
compare the six main treatments, with a total of twenty-four plots (Figure 2). The six treatments
included a control C, spray Milestone at high concentration as the plant bolts SH, spray
Milestone at low concentration as the plant bolts SL, spray milestone at high concentration in the
5

fall SF, hand pull HP, and weed whack WW. On July 19th, 2023, all of the six hand pull plots and
three of the weed whack plots were retreated. Therefore, the WW treatments are split into 2 –
WW1 where weed whacking only occurred in year 1 and WW2 where weed whacking occurred
in year 1 and again in year 2. All the hand pull treatments are labelled as HP2 because they were
pulled in both years. For the seedbank study the result is seven treatments (C, SH, SL, SF, HP2,
WW1, WW2) all with 3 replicates expect HP2 which has 6 replicates.

Figure 2. The randomized block design used to collect cover data at Red Hill in the Lac du Bois
Grasslands Protected Area in the interior of BC.

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Soil samples were collected on October 5, 2023. Four soil cores (5 cm depth, 6 cm diameter)
were collected from each of the twenty-four 7.5 m x 7.5 m treated plots. To avoid any
disturbance within the permanent center 2 m x 2 m plant sampling plot that will be used for
future monitoring, the cores were taken 1 m away from each of the four corners of the center plot
(Figure 3). The four soil cores taken from each plot were mixed in a Ziplock bag labelled with
block number and treatment type.

Figure 3. The plot layout demonstrating where soil samples were collected out in the field at
Red Hill in the Lac du Bois Grasslands Protected Area in the interior of BC.

Greenhouse Experiment and Data Analysis
The germination experiment was conducted for a total of twelve weeks, beginning on January 5th,
2024, and ending on March 28th, 2024. Soil seed bank samples were placed in a freezer for three
months after field collection to experience a cold stratification period and then were prepared for
the greenhouse chamber. Soil samples were air dried for two days and then sieved with a 4 mm
mesh to remove any larger debris and plant material. Aluminum foil pans were used as
germination trays with drainage holes, landscape fabric, and a 2 cm layer of sterile sand. Each
soil sample was placed in a labelled germination tray, and these were randomly placed in the
growing chamber (Figure 4). The germination trays were exposed to 16 hours of light on and 8
hours of light off to imitate a normal day-night regime. The temperature ranged from 20-25℃.
7

Beakers of water were placed on each level to regulate relative humidity. The germination trays
were in the growing chamber for a total of twelve weeks and were watered accordingly.
The seedling emergence method was used to analyze the composition of the soil seed bank, this
consisted of identifying, counting, and removing any seedlings that emerged. Plants that were
difficult to identify were transplanted into potting soil and left to grow. If no seeds germinated
for two weeks, the soil in that tray was gently turned over with a plastic fork. After the
greenhouse experiment was terminated, the mean seed density for each identified plant species
within each treatment type was calculated.

Figure 4. The germination trays in the growing chamber and an example of some of the forbs and
grasses that emerged.

Results
A total of eleven species were detected from the seed bank study (Table 1). Of those species, six
were considered native to BC and five species were introduced (Table 1). There were three
species that are known to be invasive in BC including spotted knapweed, corn gromwell, and
Japanese brome. The hand pull, control, and weed whacking treatments (HP2, C, WW1, WW2)
had the most seeds germinate while the herbicide treatments had the lowest amount (SH, SF,
SL).

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Table 1. Soil seed bank composition after the control, hand pull, herbicide, and weed whack treatments at Red Hill in
the Lac du Bois Grasslands Protected Area. The average seed density (#seeds/m2) was estimated for each identified
species with a minimum to maximum range in parentheses.

9

The hand pull treatments (HP2) had the highest density of spotted knapweed at approximately
360 seeds/m2 (Figure 5). The density of knapweed was lower in the control at approximately 130
seeds/m2 and in the weed whack once treatment (WW1) at approximately 43 seeds/m2.

Figure 5. The average density (# seeds/m2) of spotted knapweed recorded in the soil samples collected
from treated plots at Red Hill in Lac du Bois Grasslands Protected Area. The treatments consisted of a
control C, hand pull year one and year two HP2, spray Milestone at a high concentration in the fall SF,
spray Milestone at a high or low concentration when knapweed bolts SH SL, weed whack year one WW1,
and weed whack year one and two WW2 (error bars represent 1 standard deviation).

Native forbs were recorded in the control, hand pull, (HP2), weed whack in year one and two
(WW2), and one of the spraying treatments (SL) (Figure 6). No native forbs emerged from the
soil that was treated with a high concentration of Milestone (SH, SF). The weed whacking
treatment had the highest density of native forbs at approximately 130 seeds/m2. Non-native
forbs were very dominant in the seed bank study and were recorded in all of the treated soil (C,
HP2, SH, SL, SF, WW1, WW2) (Figure 7). The hand pull (HP2) and weed whacking treatments
(WW1, WW2) had a high density of non-native forbs, all above 4800 seeds/m2.

10

Figure 7. The average density (# seeds/m2) of native forbs recorded in the soil samples collected from
treated plots at Red Hill in Lac du Bois Grasslands Protected Area. The treatments consisted of a control
C, hand pull year one and year two HP2, spray Milestone at a high concentration in the fall SF, spray
Milestone at a high or low concentration when knapweed bolts SH SL, weed whack year one WW1, and
weed whack year one and two WW2 (error bars represent 1 standard deviation).

Figure 6. The average density (# seeds/m2) of non-native forbs recorded in the soil samples collected
from treated plots at Red Hill in Lac du Bois Grasslands Protected Area. The treatments consisted of a
control C, hand pull year one and year two HP2, spray Milestone at a high concentration in the fall SF,
spray Milestone at a high or low concentration when knapweed bolts SH SL, weed whack year one
WW1, and weed whack year one and two WW2 (error bars represent 1 standard deviation).

There was a low density of grasses overall in the seed bank study (Figure 6 and 7). Native grass
was recorded in the control, hand pull (HP2), spray fall (SF), and weed whack twice (WW2)
treatments, with all densities lower than 45 seeds/m2 (Figure 8).
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Non-native grasses were recorded in the hand pull (HP2), spray low concentration (SL), and the
weed whack twice (WW2) treatments, again all below 45 seeds/m2 (Figure 9).

Figure 8. The average density (# seeds/m2) of native grass recorded in the soil samples collected from
treated plots at Red Hill in Lac du Bois Grasslands Protected Area. The treatments consisted of a control
C, hand pull year one and year two HP2, spray Milestone at a high concentration in the fall SF, spray
Milestone at a high or low concentration when knapweed bolts SH SL, weed whack year one WW1, and
weed whack year one and two WW2 (error bars represent 1 standard deviation).

Figure 9. The average density (# seeds/m2) of non-native grass recorded in the soil samples collected
from treated plots at Red Hill in Lac du Bois Grasslands Protected Area. The treatments consisted of a
control C, hand pull year one and year two HP2, spray Milestone at a high concentration in the fall SF,
spray Milestone at a high or low concentration when knapweed bolts SH SL, weed whack year one WW1,
and weed whack year one and two WW2 (error bars represent 1 standard deviation).

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Discussion
Spotted Knapweed
The seed bank study results indicate that the spray treatments (SH, SL, SF) have effectively
controlled spotted knapweed. Zero knapweed seedlings emerged after a single application of
Milestone that was completed on June 27, 2022. This aligns with the post treatment vegetation
cover data that was collected in June of 2023, where zero knapweed cover was recorded.
Interestingly, spraying Milestone at a low concentration was just as effective as the high
concentration treatments. The fall treatment was also just as effective as the summer treatments.
Milestone supposedly can control weedy species at much lower concentration rates than other
herbicides (DOW date unknown). Additionally, spotted knapweed is believed to be more
susceptible to lower rates of certain herbicides (Rice et al. 1997). Other studies have shown
success with controlling knapweed after a single application of Milestone. For example,
Milestone at the maximum label rate effectively reduced spotted knapweed density after a single
treatment applied in either May or June on grasslands at Camp Ridley Military Training Site in
Minnesota (Malone 2015; Jacobs 2017).

Knapweed seedlings were the first seedlings to emerge in the greenhouse study within ten days
of the growing period. Seedlings were recorded in the control (C), hand pull plots that were
treated twice (HP2), and weed whacking plots that were treated once (WW1). Other studies
demonstrate that the effectiveness of hand pulling and weed whacking improves when treatments
are repeated or used in combination with other treatments (Rinella et al. 2001; MacDonald et al.
2013; Martin et al. 2014; Hindley 2018). Even though the hand pulled plots were treated twice,
they had the highest density of knapweed, even compared to the control. The action of hand
pulling can cause soil disturbance and this could have allowed more knapweed seedlings to
germinate. On the other hand, the plots that were weed whacked twice did not have any
knapweed emerge, so repetition of weed whacking is showing to be successful.
Forbs
The hand pull (HP2) and weed whacking (WW1, WW2) treatments had a high density of forbs.
There was a higher density of non-native forbs versus native forbs recorded and spotted
knapweed was not included in this category. Treatments such as hand pulling or weed whacking
can help reduce competition from weedy species which can allow other forbs to thrive (Williams
13

et al. 2007; Skurski et al. 2013; Martin et al. 2014). It is important to consider that mowing has
been shown to actually benefit some non-native forbs that are low-statured or have a rosette
stage like knapweed (Prevéy et al. 2014). Corn gromwell, thyme-leaved sandwort, and spring
draba were the most dominant forbs in the seed bank and they are all non-native to Lac du Bois.
The vegetation cover data collected in June of 2023 did not indicate any presence of corn
gromwell. These three plants have a few advantageous traits that make them competitive weedy
species. They are adapted to disturbed environments and an annual life cycle allows these plants
to complete their life cycle quicky so they can rapidly establish and utilize resources
(Klinkenberg 2023).

A downside of spray treatments is the reduction in non-targeted forbs. Any forbs that emerged
from the spray treated soil (SH, SL, SF) appeared stressed and died right away. This was an
expected result as Milestone targets pre or post emerging broadleaves and residual activity can
last up to a season or longer (DOW date unknown). It is possible that native forbs will increase
over time, as other studies have found that it takes up to two-three years for forbs to recover
following spray treatments (Rice et al. 1997; Davis et al. 2016). Monitoring and revegetation
efforts are important to consider post spraying, to assure that there isn’t a shift to non-native
species such as corn gromwell that was quite prevalent in the seed bank.
Grasses
Overall, there was a low number of grasses recorded in the seed bank study. Two grass species
were recorded, rough fescue and Japanese brome. One of the major concerns with spraying
treatments is the possible increase of disturbance-based species. A return of native grasses seems
to be a more uncommon result, as in other studies, the broadleaf herbicide was highly effective at
reducing knapweed but caused a significant increase in non-native grass cover, primarily
cheatgrass (Bromus tectorum) (Skurski et al. 2013; Whitehouse 2021). Jacobs (2017) found
similar results, where Milestone decreased both knapweed and native grass species but increased
non-native grasses and forbs. Jacobs (2017) took it a step further by completing a supplemental
greenhouse experiment for ten weeks to show that Milestone can negatively affect native grass
seedlings. This could be why not many grass seedlings successfully grew. It is concerning that
Japanese brome was found in the seedbank and the Red Hill site should continue to be
monitored. Japanese brome is considered an invasive and can successfully outcompete native
14

plants. Annual bromes like Japanese and cheatgrass have the ability to germinate in the fall and
can initiate early spring growth, giving them a head start in competing for moisture and nutrients
with native species (Symstad et al. 2021). This could greatly alter the dynamics of a native
bunchgrass ecosystem.
Limitations
It is essential to recognize some of the limitations that could have affected the results of this
study. A short twelve-week growing period could have been insufficient for many plant species.
This timeframe may have led to an overrepresentation of fast-germinating species, which could
skew the understanding of the actual seed bank composition. The number of soil samples was
also quite low, considering the size of the treated area, which could provide an incomplete
picture of the actual seed bank diversity. Lastly, identifying plants at early stages was difficult
and misidentification could have occurred.

Conclusion
The results indicate that a single application of Milestone effectively controlled spotted
knapweed, as zero seedlings emerged from the seed bank. This aligns with the vegetation cover
data, which recorded no knapweed presence a year after treatment. The effectiveness of
Milestone at lower concentrations is particularly noteworthy and could be a cost-effective
management option. However, the herbicide's residual activity also impacted non-target forbs,
stressing the need for careful monitoring and potential revegetation efforts to ensure ecosystem
recovery.
Hand pulling and weed whacking showed varied results; there was a high density of knapweed
after hand pulling year one and year two, suggesting that seeds are being pulled up from soil
disturbance and this is something to carefully consider when completing hand pull treatments.
Weed whacking year one and two seemed to be more successful as zero knapweed seedlings
emerged, suggesting that mechanical treatments, particularly when repeated, can be a viable part
of a management strategy. Even though both treatments had high forb germination, there was a
high density of non-native forbs, pointing to the need for ongoing management to support native
species recovery.

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Grasses recorded in the seed bank study were few, with rough fescue and the invasive Japanese
brome noted. The presence of Japanese brome is concerning due to its competitive nature and
potential to dominate native species. This study underscores a common challenge in grassland
restoration: the increase of disturbance-based species post treatment. Milestone, while effective
against knapweed, also may have led to reduced native grass seedling success and increased nonnative grasses, reflecting findings from similar studies.
For successful grassland restoration, an integrated management approach is recommended. This
includes not only the initial herbicide application to control invasive species but also following
up with mechanical treatments to supplement, while continuing to monitor and conduct
revegetation if necessary to promote native species recovery. The integration of these methods
could help restore and enhance the resiliency of the grassland ecosystem.

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