Could hack the Toro Xtra Smart Wireless Soil Sensor. It uses the Atmel AT86RF212 900MHz RF chip and costs $45 on eBay (including receiver that contains a PIC and a relay so it can act as a rain sensor for a traditional irrigation controller).
thanks for your input. I’d need one of these sensors for each zone which could quickly become expensive. There are cheaper moisture meters for maybe £2 or £3 each but as a total novice to coding / programming wouldn’t know where to start. Others have found solutions by using moisture sensors to accept or cancel a command sent by a timer. If the command isn’t cancelled (ie when the moisture content is below a low threshold), the timer activates a solenoid valve and irrigation commences.
I’m also looking for ways to automate the raising and lowering of a ventilation curtain – triggered by temp sensors. Maybe I’m trying to sprint before learning how to crawl!