I found some esp32 stuff that should be useful with the 2nd one for an outdoor build and I'm also working on a solar powered dashboard with e-ink display for the indoor zone with the main set of plants that have to be watered.
I'll be tying the two plant sensors that aren't near the main dashboard back to the main dashboard with MQTT pub/sub stuff most likely while also having the main MQTT server tied into my home-assistant.io deployment so I can have the main home-assistant dashboard(s) I've come to <3 as well as a mini-dashboard by the plants so I don't lose track.
Debating if I go with 3xAA NiMH or 1x LiPO. There are pros/cons to both approaches. If I could find a water proof 18650 battery holder + leads I'd go that route as I could add a battery meter.
For waterproofing: I'm going to be ordering some of the moldable plastic play-doh stuff for wrapping the sensor + esp32 to make it water tight.
My current indoor build is coming along really well and I have the core pieces/parts sorted out for the firmware side. The indoor build is using an esp32 and >1 senor with an i2c mux so the outdoor one with a single sensor should be much easier to put together and cheaper as it won't have solar or display.
Given all my plants can see my WiFi at home I'm going to be going that route for now. If/when I need more distance I'll likely build a LoRa gateway for them. Unfortunately that opens at a much higher cost to do "right" and I'm not there yet for needs.