An apparatus is provided for unattended watering of plants over an extended period of time utilizing a watering system that regulates a partial vacuum to control the discharge of water to the roots of the plants. A water reservoir is provided having an encapsulated interior cavity and an exterior exposed to ambient air, and having upper and lower ends. The reservoir is located above the root level of a plant to be watered. At least one drain opening is defined in the lower end of the reservoir. The upper end of the reservoir is closed and is provided with an adjustable valve, preferably a pinch valve, and an air conduit, preferably a flexible tube, that passes through the valve and extends between the interior and the exterior of the reservoir. The valve thereby interacts with the conduit to partially obstruct air flow through it, so as to maintain a partial vacuum in the upper end of the reservoir as water drains from the lower end of the reservoir.