There are several services (Phant and ThingSpeak come to mind) that are purpose designed to allow you to record sensor data, and already include easy ways to query and graph said sensor data, while also allowing for future upgrades.

I have personally used thingspeak for the ESP8266/DHT22 in my daughter's room, and I have data going back to a few months after she was born that gets recorded every few minutes, and I can query and display that data in a million different days.

Jeff is our plant, we should make his data as accessible as possible.

If we can make this happen I'm willing to help with the effort to convert the existing moisture data into the service of choice so that there is no data loss.