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.
