The invention provides a hydrophytic germination prevention agent, capable of preventing such as an emerging plant, a floating leaf plant, a submergent plant and a hydrophyte plant rooted into the bottom mud of the fresh water area. The approach for solving the technical problem is that: a magnesia powder with particle diameter of below 1mm is distributed in the bottom mud of the fresh water area.本發明之課題係提供不會使淡水水域之底泥硬化，且可防止挺水植物、浮葉植物及沉水植物等根系生根於淡水水域底泥中之水生植物發芽之方法。解決方法係於淡水水域之底泥上散佈粒徑為1mm以下之氧化鎂粉末。
