A water retaining cartridge (20) able to be incorporated in any growing medium (30) comprises a container (C) with open top able to let penetrate into the container to the roots of a plant in the growing medium, the container includes liq. retention medium (220,230) e.g. cotton, sponge, polymer foam or any such material, and a connection (24) to directly load the container with liq. The retention medium (230) is pref. a granule or powder polymer, swellable in water e.g. polyacrylic being an active retainer mixed with a passive retainer with a ratio of water retained of 25-50% for the passive material and 50-75% for the active material.
