If dune slacks need mowing it should in the autumn, in the short window between the end of the growth period and the slacks starting to flood, making sure to remove the seed capsules of any rare species, e.g., fen orchid (Liparis loeselii) before mowing and replacing them after. The immediate effects of close mowing, i.e. the short sward, areas of open ground and mowing pattern (seen in the image here), remain visible until the next growth period, after which any obvious signs of mowing become difficult to detect.
