Sold Individually. The Root Control Discs or RCD™s are designed to fit the 1Pot system, the easy2grow Kit & the 2Pot extension trays to extend the easy2grow Kit. These discs are black & gold in colour and are placed in the tray underneath the pot, gold face up. The purpose of the Root Control Disc is to prevent roots from leaving the pot & heading towards the AQUAvalve. They are very effective due to the special metallic gold coating. Please note that roots even though they are tender have the ability to go through concrete, therefore it is wise to occasionally lift the pot to see if any have managed to escape the barrier. If so you can simply cut away the roots with a pair of scissors. This will in no way harm the plant. Although the 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.