A moisture - sensitive control valve and its use in methods of promoting plant growth are described. Such valves use a moisture sensitive swellable material (3) to control flow between an inlet (6) and an outlet (7) of the valve. The valve described here includes an additional component (4) operatively disposed between the moisture sensitive material and a fluid- impermeable diaphragm (5) and configured to promote sealing of the fluid- impermeable diaphragm upon a seating surface in the valve.
