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In organic farming systems weed control is in general performed mechanically by soilinterventions such as mouldboard ploughing (Arshad 1999) and the main factordetermining minimum ploughing depth is control of especially perennial weeds(Kouwenhoven et al. 2002). In order to allow early sowing in spring, autumn ploughing isoften preferred. However, soil erosion risk and N-leaching losses increase with thenumber of tillage operations in autumn (Askegård et al. 2011) and the depth ofmouldboard ploughing is directly related to CO2 loss from the soil and increased use offuel (Reicosky and Archer 2007). Annual ploughing also negatively affects soil quality(Riley et al. 2007) and erosion is larger with deep than with shallow tillage (Lundekvamet al. 2003). In order to optimize the effect of mechanical and cultivation methods onweed regulation, it is important to understand the biology and growth pattern of thevarious weed species. Variation in growth activity throughout the season exist(Brandsæter et al. 2010), and should be accounted for when timing tillage operations .The depth of the roots or rhizomes varies with species. The root system of Cirsiumarvense extends to below 200 cm, with the highest root concentration between 20 and40 cm (Nadeau and Van den Born 1989), while rhizomes of Elymus repens and roots ofSonchus arvensis grow down to approximately 10 to 15 cm (Korsmo 1954). Followingtillage operations, root fragments of C. arvense will be present in the upper soil layer and
undisturbed roots below the tilled layer. Insights into the regeneration potential from root
fragments and undisturbed root systems are therefore important. The challenge is to
obtain an acceptable level of weed management and yield in combination with minimizing
the environmental impact. This topic will be discussed in the presentation
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