Perennial pepperweed (Lepidium latifolium) is a widespread invasive plant in North America. This yearlong field study at a pasture peatland infested with L. latifolium demonstrates that these plants are large emitters, on a per area basis, of methyl chloride (CH3Cl) and methyl bromide (CH3Br), compounds that contribute to the destruction of stratospheric ozone. Annually averaged net emission rates were 9.0 ± 11.8 µmol m−2 d−1 for CH3Cl and 460 ± 430 nmol m−2 d−1 for CH3Br, comparable to observed coastal salt marsh emission rates. A stable isotope tracer technique was used to distinguish between simultaneous production and consumption processes for CH3Cl and CH3Br. Over the course of the year, gross production rates for methyl halides varied widely over different life cycle stages, with the highest fluxes surprisingly occurring at senescence. Maximum emissions of CH3Cl and CH3Br occurred at midday, the time of highest solar radiation. During the growing season, methyl halide gross production rates were positively correlated with temperature and live biomass, suggesting that the production of methyl halides from L. latifolium at this time was mostly biotic. During plant senescence, the peak emissions of CH3Cl and CH3Br are unexplained but may be released from L. latifolium associated with transitions in biochemistry. Overall, these flux measurements show expected diel and growing season trends based on plant production for most of the year, overlaid by a high emission peak during senescence, suggesting that both plant biochemical shifts and varying environmental factors need to be considered as important internal and external controls on emissions.