Cut the 2-by-4 posts into two 36 inch pieces. These will be the supports for the overhead water line. Drill a 3/4-inch hole in top end of each support. The 1/2-inch PVC pipe will run through these holes. Fasten supports to each end of your potting bench.
Place the PVC pipe through the support holes. Install the solenoid valve on one end of the PVC extension and cap off the end opposite of where you want the controller and water supply to be. A solenoid valve is an electro-mechanical device that controls the flow of liquids or gas with an electric current through a coil of wire around an iron core.
Fit nozzles into the PVC pipe above the potting bench about 3 to 4 feet apart according to the type of nozzle and the manufacturer's directions.
Hook up the water supply hose to the solenoid valve. Pay attention to the water flow arrow on the solenoid for proper connection.
Plug in the programmable controller to an electricity source. Connect the controller to the solenoid valve. Program the controller according to the requirement for your particular cuttings, seeds or seedlings.