Impact of Canada Thistle Cover on Plant Community Structure in Early Stage Prairie Restoration

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By Byron B. Sleugh , Mary B. Halstvedt, Roger L. Becker, and Paul Bockenstedt

LAND MANAGERS’ ABILITY TO RECONSTRUCT OR RESTORE PRAIRIES HAS ADVANCED GREATLY THE LAST TEN YEARS. Although much has been learned, restoring mixed wildflower (forb)-grass prairie landscapes is often compromised by the presence of invasive plants such as Canada thistle (Cirsium arvense L.). 

A field study was conducted near Renville, Minnesota to determine if there was a threshold of Canada thistle cover that would impact desirable plant community structure. Objectives of the study were to: 1) measure the effect of Canada thistle cover on seeded species in a two-year old prairie restoration; and 2) determine the threshold of Canada thistle cover that would impact the desirable plant community. 

Study Site

The study site encompassed 74 acres of former cropland that was converted to permanent prairie habitat as part of a Ducks Unlimited acquisition. The area was cultivated and seeded with 10 native prairie grasses and 31 native forbs in May 2011 (Figure 1). Seedling rate averaged about 6 pounds (lbs) of pure live seed per acre (PLS/A) for all grasses combined, and 1.5 lbs of PLS/A for all forbs combined. Five days following seeding, the site was treated with glyphosate herbicide to remove undesirable plants prior to emergence of desirable seeded species. 

Desirable seeded species successfully established one year following planting; however, within two years Canada thistle began to invade the restored prairie (Figure 2). There was concern that Canada thistle could out-compete desirable species in the restoration, which precipitated the field study. 

Materials and Methods

Canada thistle in the study area was mapped and the acreage and perimeter of infestations recorded with a hand-held GPS unit. Mapping was conducted annually in mid-summer from 2013 through 2015. 

The plant community was characterized by measuring percent cover of individual species along permanent transects established within Canada thistle infestations. Cover data were collected in 25, 0.25 meter squared (m2) quadrats per transect. Quadrats were measured annually in mid-summer from 2013 through 2015. A total of 925 quadrats were measured during the three-year study. Individual species were grouped into botanical categories including: native grasses, non-native grasses, native forbs, Canada thistle, and other non-native forbs. The percent bare ground and litter was also recorded. The study area was mowed once in September 2013 after transects were established.

Data from all quadrats were pooled to develop mean percent cover by botanical category in each year of the study. In addition, data from quadrats were sorted and grouped by Canada thistle cover classes, and a mean was calculated to measure the interaction of Canada thistle cover on each botanical category. A Oneway Analysis of Variance (ANOVA) test was performed to compare native forb percentage across five categories of Canada thistle growth. Comparison of means was conducted with a Student’s t-test. In addition, correlation coefficients were estimated by the REML (Restricted Maximum Likelihood) method. 

Results and Discussion

Canada thistle populations expanded from less than one acre in 2013 to greater than three acres in 2014. Individual patches of Canada thistle were dynamic over the three-year study, with small low-density patches disappearing and others coalescing into larger, better defined populations during the three year study (Figure 3). 

FIGURE 3. LOCATION AND SIZE OF CANADA THISTLE POPULATIONS ON THE RESTORED PRAIRIE STUDY SITE IN 2013 (LEFT) AND 2014 (RIGHT).

FIGURE 3. LOCATION AND SIZE OF CANADA THISTLE POPULATIONS ON THE RESTORED PRAIRIE STUDY SITE IN 2013 (LEFT) AND 2014 (RIGHT).

Table 1. Change in percent cover by botanical grouping from 2013 through 2015 on the study site (all sample quadrats combined in each year)

Table 1. Change in percent cover by botanical grouping from 2013 through 2015 on the study site (all sample quadrats combined in each year)

Data collected from sample quadrats in 2013, two growing seasons following seeding, showed native grass and forb cover was about 54 and 31 percent respectively (Table 1). Dominant native grasses included big bluestem (Andropogon gerardii) and switchgrass (Panicum virgatum). Common native forbs included: Canada milk vetch (Astragalus Canadensis), common ox-eye (Heliopsis helianthoides), golden alexander (Zizia aurea) and yellow coneflower (Ratibida pinnata). The remaining 15 percent cover was predominately Canada thistle (9%), other non-native forbs (4%) and bare ground or litter.  

Bare ground and litter cover increased from 0.5 percent in 2013 to more than 21 percent in 2014 (Table 1). Native forb cover declined from about 31 percent to 14 percent during this same time period. Native grass cover also declined slightly between 2013 and 2014. However, cover of Canada wildrye (Elymus Canadensis), slender wheatgrass (Elytrigia trachycaulum), and Virginia wildrye (Elymus virginicus) was significantly reduced. The increase in bare ground is attributed to mowing in September 2013, which had a negative impact on desirable forb and grass cover.  

Vegetative data collected from sample quadrats were grouped into five Canada thistle cover classes: no Canada thistle, less than 10 percent, 11 to 19 percent, 20 to 30 percent, and greater than 30 percent Canada thistle. The percent cover of each botanical group was compared within the five cover classes. Results showed that as Canada thistle cover increased, the percent cover of native forbs and some grasses declined (Figure 4). The greatest impact to native forbs occurred at Canada thistle cover of 20 percent or greater (Figures 4 and 5).

Management Implications

Results from this study have important implications for land managers restoring native prairie. Managers must recognize that Canada thistle populations in grasslands and prairies can be dynamic. Infestations should be monitored and management efforts implemented on areas with Canada thistle cover greater than 20 percent. 

A control and containment strategy is recommended, which includes mapping and documenting noxious weed infestations and determining the best management option for a target plant. 

Considerations include: 

  • Focus management efforts on areas where invasive plants are having the most impact on plant community. 
  • Utilize current information to understand the benefit and risk of an herbicide treatment to desirable forbs or grasses, and tolerance of desirable plants to the herbicide selected.
  • Choose an appropriate herbicide rate and application timing to create a mosaic of greater forb diversity.
  • Timing, frequency, and height of mowing may impact desirable prairie vegetation. 
  • Mowing to reduce competition from weedy species is not recommended on restored prairies after the first growing season.