I simply installed a water hose with a solenoid valve (I used a standard replacement valve for washing machines), and poked thin copper tubes into the hose, leading to each pot. Those plants that require less water get thinner copper tubes. And then I used the following timer circuit to control the valve. Problem solved. The plants are happy.
resolution schematic is here.
This circuit starts with a 555 timer connected as astable multivibrator, running at one cycle every several seconds. A CMOS ripple counter divides this time down to several hours. When the time is over, the counter triggers another 555 timer, which is a monostable multivibrator and can be set for times of a few seconds to about two minutes, controlling the solenoid valve via an optotriac (as shown) or a relay (like implemented later). A small regulated power supply feeds the circuit. Both the interval between activations, and the duration of the watering, are user-adjustable over a wide range. One LED blinks to show the heartbeat of the first timer, the other LED lights when the valve is activated.
After years of service, the valve I was using clogged with silt. Being sealed shut, it was not repairable, so I replaced it by a different one. This second valve was too inductive, and the optotriac proved incapable of properly controlling it! Instead of searching for a different valve, I opted for replacing the optotriac by a small relay. That's the version that is still running today at my home.
I also added a pushbutton from pin 2 of the second 555 to ground, for manually triggering the monostable for testing of the copper tube outlets.
Of course, watering plants is not the only application for a timer like
this. Use it for whatever you fancy!