A hygrostat suitable for use burying in the soil near a plant (for example, a pot-plant) which employs a hygroscopic element of a coherent polymer that is preferably arranged to be in direct contact with the soil and to have no lost motion as it expands or contracts so that soil cannot interpose itself at the ends of the element. A hygrostat system and method for delivering water to the surface of the soil at a fixed distance from a buried hygrostat unit are also disclosed. In the example shown in the figure, the hygrostat unit comprises: an upper chamber (12) shaped like an inverted cup which is closed by a diaphragm and which has water inlet/outlet spigots (16) extending from its top; a lower frame (18) that is shaped like an inverted stirrup having a lower arch (20) and an integrally-moulded upper snapring (22) which fits inside the rim of chamber (14); a strip-form polymeric hygroscopic element (24) which is clipped at its lower end to a key-stone protrusion (26) at the bottom of frame (18); and, a pair of guide rails (28) on each side of element (24) which serve to stiffen the element against buckling, the upper ends of rails (28) being joined together by a foot (30) to which the upper end of element (24) is clipped and which is itself attached to the diaphragm by 'pull-through' studs (32), the lower ends of rails (18) being free to slide over keystone (26) while being laterally located thereby.