Joe,

Kill the ant nest and dig up the wiring and test for wire shorts. Let us know what you find.
Aloha

Do you have to turn off your main water supply to the yard to turn it off?

If so, you probably have a mechanical issue in the valve. If your wire to that zone is disconnected, or if the power to the timer is off and the valve is still running, you can then conclude that your problem is not electrical.

You may have debri under your diaphragm in the valve, may have a broken/faulty diaphragm in the valve, or have a cracked valve. These things would all make your diaphragm stay up, allowing water to flow freely.

If your valve has any bleed screws or on/off levers, be sure that they are set to OFF or screwed down tight. Flow control stays open always. You will need to repair the valve before turning your main water back on.

If, when you unplug a wire to that zone it turns off, then you can tell that you either have a bad solenoid, debri with the solenoid, or a bad circuit board terminal that has fried.

Hope this helps.