1. Effect of frost on regrowth ability and frost tolerance of rush (Jûncus spp.) Folkestad J. 1, Østrem L. 1 and Netland J. 2 Norwegian Institute for Agricultural and Environmental Research (Bioforsk), 1 Fureneset, 6967 Hellevik i Fjaler, Norway, 2 Plant Health and Plant Protection, 1430 Ås, Norway Introduction Soft rush (Jûncus effûsus L.) and compact rush (J. conglomerâtus L.) have had an increasing success in coastal parts of Norway. One hypothesis is that temperature increase during winter might represent a driving force facilitating growth, spread and distribution of the weeds. The aim of this study was to test the effects of freezing temperatures on the rush species. Materials and methods Location and plant material. The experiments were conducted at Fureneset, Fjaler, West Norway (61°34 ′ N;5°21 ′ E, 10 m a.s.l.). Soft and compact rush were established in mid-May 2009, and the seedlings were placed outdoor in August to achieve natural hardening conditions. Freezing tests were conducted on 23 November 2009, 2 February and 12 April 2010. Frost tolerance (LT 50 ). Plants of both species were frozen to predetermined temperatures (−3 to − 21 °C ). After treatment plants were thawed, transplanted in plastic trays along with unfrozen controls, and transferred to a greenhouse (15 °C, 24 h light) for 28 days. The number of surviving plants were recorded and LT 50 (lethal temperature of 50% of the plants) was calculated. Effect of frost exposure on regrowth capasity. Plants of both species were placed in a freezer room at −8 to −10 °C, 2 parallels per species and length of treatment (unfrozen, 6 h, 12 h,,24 h, 36 h, 48 h and 72 h). Potted plants were insulated to prevent low temperature damage to roots during the frost treatment. Plants were thawed at 2 °C for 24 h, and transferred to a greenhouse (15 °C, 24 h light) along with controls. After 4 weeks dry weight (DW) were recorded. Results and discussion Frost tolerance (LT 50 ). The estimated LT 50 values for soft and compact rush were in November −13.0 °C and − 13.7 °C, in February − 18.6 °C and −16.9 °C and in April − 10.8 °C and − 9.7 °C, respectively. Regrowth capacity. In general, exposure to low temperatures decreased the regrowth of both soft and compact rush, and the overall DW mass of aerial shoots was strongly affected by treatment duration (Fig 1). Figure 1. Regrowth of aerial shoots (DW per shoot ± SE) of soft and compact rush from freezing tests in November, February and April for frost treated plants (24 h, 48 h and 72 h) and non-treated plants (0 h). Soft rush (left) and compact rush (right) inflorescence. Mown meadow partly invaded by rush plants.A pasture owergrowned with soft rush plants. In November and April significant growth reduction was observed after 6 h of frost treatment for both species. A deviating pattern observed in February might partly be explained by the uncommon cold winter experienced in 2009-2010. In mid-winter both species seems to withstand frost treatment and had a high regrowth capacity for the whole treatment period (72 h) as well as high frost tolerance. In mid-April the rush plants were dehardened as shown with the reduced frost tolerance and their increased susceptibility for frost. However, the high regrowth ability (Fig. 1) indicates that the plants have large energy resources stored in their perennial roots or generated from substantial cell activity in early spring. Further investigations are needed for recording translocation of energy from late autumn to early spring. Conclusions The results have shown that frost affect regrowth ability of both soft and compact rush. The decline in DW regrowth already after 6 h of frost exposure indicates that rush species are susceptible to frost especially in late autumn and early spring when the plants are dehardened. The freezing tests used seems to be useful for investigating frost tolerance and regrowth capacities of the two rush species. The apparent high energy reserves and growth capacity in early spring surely give the rush species competitive advantage compared with commonly used grass species in leys and pastures. Acknowledgements. This study was funded by the Norwegian Research Council and Bioforsk as part of the project ‘Control of rush (Juncus spp.) – an expanding weed in grassland and pasture areas in Western Norway’ (2009-2012).