N.B. This is an indoor setup only. The water hose / pipe can go anywhere.
An inexpensive digital timer and a washing machine solenoid can control an automatic watering system.
Inexpensive digital timers could have many uses if they could switch an inductive load.
Unfortunately if you try to use one on an inductive load (an electric motor or a solenoid coil) you will damage the timer. I have tried to use rC filters and varistors to supress the pulse induced when the timer switches off without any success (the still connected but ineffective components are in the plastic bag).
The solution to this problem is available at almost no cost in every home. You can use a switch mode power supply to block the offending pulse. All of the phone chargers and DC power supplies I have seen in the last 5 years are switch mode power supplies. An older type linear power supply will not work. In the picture the DC power supply is plugged into the timer. The black lead from the 12V DC power supply is connected to a small 12V relay which switches the larger standard Octal relay.
Watering plants needs very little time I use 2 min/day and digital timers can go a low a 1 min. You can buy water drip kits in hardware stores or put holes in a length of hose depending on your needs.
Parts for automatic watering..
A washing machine water solenoid valve.
A redundant switch mode power supply and suitable small relay.
An optional larger relay.