Automatic watering in the greenhouse
There are several systems you can employ, some comparatively low-tech, others highly sophisticated.
Sand bench: The simplest is a tray of sand, connected to a water tank that feeds it via drip feed nozzles. Pots are set on the sand and take in water through their drainage holes.
Trickle irrigation: This consists of a plastic pipeline with nozzles that drip water into planters, or feed misting jets that moisten plants from overhead. The flow of water from a small tank is usually controlled by a hand-adjusted valve.
Photo-electric control: This system automatically switches on when the light reaches a certain intensity and runs for a set period. It can be used to run either of the systems above.