By the way, if the current set up has any kind of switch or button that turns it on/off, you might also be able to add the equivalent of a “robot finger” just to push that button or activate that switch. But I know not all garden fixtures would have this. if it does, post a picture of the on mechanism and we can recommend an appropriate actuator. This is a pretty common automation approach for devices which are on a different type of current than the regular house current, because then you don’t have to worry about wiring anything.
See option 4 in the following FAQ:
And, yes, for Terminology “mains power“ is the AC power you get from the outlets in your home, sometimes called “wall power“ for this reason. We used to call it “grid power“ but now with solar electricity that no longer applies.