Population growth rate (λ) of knapweed species in each of the seven sites per year. Growth rates are shown for an August to August transition for a 2016–2017 b 2017–2018; and c 2018–2019. An asterisk shows population growth rates across all plots significantly greater than 1.0 (P < 0.05), with the dotted line indicating stable populations (λ = 1).