Greenhouse - Automatic irrigation system

You need:

Rainwater container
Waterpump for garden pool
Digital timer
3/4" hose
5 mm. hose
20 mm. Rigid and even wire tube
Plug that fits 20 mm tube (plug from a bicycle handlebar fits perfectly)
16 mm. M5 Stainless steel bolts
Glue or silicone. I've used plastic roofing glue with great success.

The M5 bolt acts as a valve. Cut a notch as shown in the picture. You can then adjust the water flow from no water to fully open within a quarter turn. Drill out the hole in the printed part for the M5 bolt with a 4.5 mm drill. Then cut a M5 thread in it.

Remember to mount all outlets above the water level in the rainwater barrel. Otherwise, water will drain out of the lower outlets even if the pump is not running.