Detecting the wind speed is easy - as is creating a signal to 'switch' something. The hardest part (for someone with limited access to tech parts) is sending the signal via GSM or internet.
It would be easier to fit a clock (timer) and solenoid-activated valve such that the valve is opened if the wind is 'zero' AND the time is suitable (night).
As such, practically any 'fan' will generate a voltage when spun manually or via wind throughput and this voltage can be amplified to activate an electronic switch to apply power to the solenoid valve. If the solenoid power is, itself, switched via a timer (mechanical or electrical) then you have a signal that switches the water on 'at night' but only IF there is zero wind.
Practically off-the-shelf parts available anywhere......
My guess is that Braam doesn't want the spraying to start the first instant the wind speed registers as zero. I don't know how winds work there, but I think in most places, it dies gradually and vacillates, and a given location will reach zero speed a few times before it's actually done for the night. you don't want to be spraying into the dying gusts of the wind.
So I would suggest that the motor-turned-generator power a signal to keep a relay from starting a simply timing circuit. If the wind kicks up again, the timer is aborted/reset until the signal reaches zero again. Braam could best judge how long to set the timer for.
Timers can be made very simply from basic components and probably in your case, don't need to be super-accurate. It's been awhile but I've made simple ( crude but reliable) timer circuits from a couple of transistors, a capacitor, and a couple of resistors.