Hi Peter,
If I understand your question correctly, I'm thinking the same as Gary with the 240v remote switch.
What is cool with this method is the unit is cheap to buy, you install it via wi fi on your router and it sounds like it has a web server already running on it. The remote switch has it's own free app.
You will need to setup your router so the device can be seen from outside world eg port forwarding or DMZ, both easily done in the router settings.
Assumptions include:
Your watering system has a plug pack that currently plugs into normal GPO 240v.
Your watering system is simple so when power is on, water flows, power off, water stops.
Or
Go the technical way and do it with an arduino, network shield, relay(s) to switch 240 or other better ways, your own web server which means that you don't need any special apps to remote into it, you just use the web browser on your phone and type in the IP address. Fun but time consuming
Let me know if I didn't understand your question correctly.
Cheers,
Damien.
EDIT:
Oops, I just read about the Kasa app, cloud based does suck! You want your own direct control. DIY