I ended up getting rid of the weather-service. The weather station sends stuff out on a radio frequency that can be intercepted. I then dump it on an MQTT bus. Then I use some Telegraf service that dumps it into InfluxDB. Then I have a Python script that:
1. Queries InfluxDB for 24h data
2. Calculate Zimmerman
3. Use the API to set the watering level
I run this twice a day. All good, problem solved.