FWIW I have built a remote sensor to trigger OpenSprinkler to trigger my drip irrigation system. I have strawberries planted in gutters along the perimeter of my deck and the soil dries out quickly. I used a Vegetronix VH400 soil moisture sensor to measure the soil in the gutters. When it gets dry, OS turns on the zone and waters it. You can see the charts here https://thingspeak.com/channels/239044
My setup involves a custom circuit and some programming but they also sell a controller board that might work for you. I’ve not used it but it looks pretty simple. Check out http://www.vegetronix.com/Products/VG-RELAY-AC/
Mike