The discs are black in colour and are placed inside the pot. The purpose of the Marix Disc is to prevent compost or soil from leaving the pot, keeping the system clean & the AQUAvalve free from debris. They will also provide minimum protection from escaping roots. Although the Marix Discs will last several seasons it is advisable to replace the discs at the beginning of each season, this will prevent any disease from being transferred if present.