I don't like to waste my time dragging a hose or watering can around the yard. Our summers are very dry, and irrigation is a must, so automating that is one of my priorities.
I've tried the little battery powered ball valves that go on a garden hose connection. They're garbage, and they're expensive. The mechanical parts are weak, the batteries die and / or corrode, etc.
A much better alternative is a "real" lawn sprinkler valve. They operate via a low-voltage signal, that controls a little solenoid, which in turn influences the distribution of pressure inside the valve, moving a big rubber diaphragm back and forth to control the water flow. It sounds complicated, but they're very reliable, can move a lot of water, and take a certain amount of abuse (like freezing).
Rainbird, Orbit, Hunter, and Toro all make fine valves. You can find them at Home Depot or the like, or Amazon. The standard signal is 24V AC, which you can supply in a couple of ways. The easiest is to just buy a sprinkler timer. A cheap one is about $20 and will control more than one valve. Another option is to use some other timer, and use it to control a 24V power supply. A cheap Intermatic 24h timer and a "wall wart" power supply from your scrap pile will work fine.
There are other control methods, too. When I was a kid, I fitted a misting system into my grandfather's small greenhouse. Instead of a timer, I took a mercury switch from an old thermostat, and built a see-saw type mechanism from brass rod and pvc pipe. One end of the see-saw had a cloth pad attached to it, and the other end had some weights. The mercury switch was attached to the arm and closed when it tipped away from the cloth. The contraption sat on the bench with the plants, and the idea was that whenever the pad dried out, the see-saw would tip, closing the mercury switch and providing power to the sprinkler valve. It worked great, watering more on hot days and less on cool days and basically keeping the moisture levels very constant.
