Recycling of the nutrient solution during cultivation of greenhouse crops reduces the total input of water and fertilizers and decreases emission of nutrients to the environment. However growers fear accumulation of harmful substances in the drain water. For this reason drain water in practice is regularly discharged into the environment. In this study drain water from a commercial rose grower in The Nether¬lands was tested with a phytotoxicity test (Phytotoxkit) during a period of one year. The Phytotoxkit was used to assess growth response of drain water without disinfec¬tion; drain water disinfected with UV; drain water disinfected with UV and peroxide. In this phytotoxicity test, garden cress (Lepidium sativum) and mustard (Sinapis alba) are grown in a Phytotoxkit container on a filter paper moistened with the drain water under analysis. After incubation for three days in a climate-controlled cabinet, root length is measured. The Phytotoxkit showed growth decrease caused by untreated drain water disappeared when using a UV disinfection of 100 MJ.cm-2 or UV disinfection of 100 MJ.cm-2 combined with 10-25 ppm peroxide. Untreated drain water caused growth responses of -20% root length in only 3 out of 10 sample moments. Control of drain water with the Phytotoxkit provides the grower information about the presence of phytotoxic substances in the drain water and the effectiveness of treatments. The Phytotoxkit thus can prevent unnecessary discharge of drain water to the environment.