2014). Influence of cutting time and stubble height on regrowth capacity of Juncus effusus and Juncus conglomeratus. Weed Research 54, 603-613.Fig. 1 Regrowth of shoots during the season 2009 from two ecotypes (western and eastern Norway) of Juncus effusus and J. conglomeratus, measured as dry weight of shoots per unit, in a growth chamber at 17.5°C. Each point represents shoot dry weight for one unit. The lines show quadratic functions for the given ecotype, species and stubble heights, based on the fitted model. © 2014 European Weed Research Society 54, 603-613 J. conglomeratus -West Norway (2010) J. effusus -West Norway (2010) J. effusus -East Norway (2010) J. conglomeratus -East Norway (2010) Fig. 2 Regrowth of shoots during the season 2010 from two ecotypes (western and eastern Norway) of Juncus effusus and J. conglomeratus at 17.5°C. Each point represents shoot dry weight for one unit. The lines show quadratic functions for the given ecotype, species and stubble heights, based on the fitted model.

SummaryDuring the past two decades, significant spread of the perennial weeds Juncus effusus (soft rush) and Juncus conglomeratus (compact rush) in coastal parts of Norway seems to have coincided with an observed rise in winter temperatures. This study investigated the frost tolerance (LT 50 ) and effects of moderate frost exposure on rush plant regrowth over time during the period late November to late winter/spring, and photosynthetic activity in late winter/spring. Juncus effusus and J. conglomeratus of physiologically young age (seedlings) displayed similar high frost tolerance (LT 50 ) and did not differ significantly in regenerative ability following prolonged frost exposure. Regrowth capacity generally increased during winter and when stress conditions increased, shoot formation was prioritised over total biomass production. Maximum quantum efficiency of photosystem II (F v /F m ) and performance index of photosystem II (PI) were high in late winter/ spring, with J. effusus showing higher values than J. conglomeratus. Green, photosynthetically active shoots, which facilitate accumulation of carbohydrates during autumn and even in winter, may provide Juncus spp. with substantial competitiveness in late winter and spring. The results revealed that the dominance of J. effusus over J. conglomeratus in pastures and leys is not due to major differences in winter survival parameters, but probably the higher photosynthetic efficiency observed in J. effusus. Generally higher temperatures during winter and lower frost kill may be contributing to the current increase in rush infestation.