Analog multiplexer or analog to digital converter to add analog inputs to NodeMCU?
I asked a similar question here:
Simple (low-performance) ADC multiplexer design

The final conclusion was that a multiplexer was dead easy to use out of the box. For watering, presumably, your sample rate will be in the minutes rather than milliseconds, so switching, stabilizing, and measuring, can easily happen without worrying about it taking too long. 

I used the CD4051 multiplexer, which worked well and cost silly cheap on amazon.