Jung LS, Eckstein RL, Otte A & Donath TW (2012). Above- and below-ground nutrient and alkaloid dynamics in Colchicum autumnale: indications for optimal mowing dates for population control or low hay toxicity. Weed Research52, 348-357. Summary In some Central European regions, the conservation of seminatural grasslands is jeopardised by management intensification or abandonment, caused by high densities of the toxic weed Colchicum autumnale. We investigated two possibilities to deal with C. autumnale: (i) reducing population densities by mowing when nutrient contents are high in leaves and capsules and low in the storage organs, that is, when the ratio between both is highest, or (ii) reducing alkaloid content in hay by mowing when alkaloid content of leaves and capsules of C. autumnale is low. To identify the optimal mowing point, we analysed the dynamics of nutrients, starch and alkaloids of naturally grown plants in two biogeographical regions. In the colder region, the maximum nutrient ratio between above-ground and storage organs, as well as alkaloid content in leaves and capsules, occurred significantly later. Compared with the common first mowing date (15 June), alkaloid content decreased significantly until 5 July in both regions. On both dates, it was on average 1.8 times higher in the colder region. Our results suggest the following time for the two management options: (i) mowing at about 25 cm plant height (late April/early May) to reduce C. autumnale densities or (ii) delayed mowing in late June/early July when the plant has turned brown and dry and alkaloid content has declined.